Transcript for:
A-Level Biology Complete Guide

hi everyone and welcome to miss estrick biology this video is going to be a mega one and so helpful for you to prepare for your exams because I'm going through everything you need to know for AQA a-level biology paper one that means it's going to be longer though so you can use times two speed to get through it quicker or skip to the time codes for the different topics that you need and if you do need any more extra help then check out the resources I have in my description that are there to boost your grades such as my edible notes or my flashcards but for now let's get into it so monomers are smaller units which can create larger molecules and the polymers are made from lots of monomers which are bonded together and the examples that you need to know are glucose amino acids and nucleotides so RNA or DNA nucleotides for the monomers and then the polymers which those monomers can create so glucose can create starch cellulose glycogen amino acids can make proteins DNA and RNA polymers Now to create these polymers it is a condensation reaction and this would be a two or three Mark definition so joining two molecules together would be the first Mark creating a chemical bond would be the second and removing water would be the third to hydrolyze which means to break apart or to split the monomers that would be a hydrolysis reaction and it's still three marks but the opposite three the breaking of a chemical bond between two molecules and involves the use of water so carbohydrates is the first biological molecule that you need to know and I've got an overview here of the three levels of size of the molecules you need to know so monosaccharide mono means one so one sugar units disaccharide die is two so that's when you have two sugar units joined together and the polysaccharides is when you have many joined together and for each of those carbohydrates there are three examples that you need to know glucose fructose and galactase are the three monosaccharides that you need to know the disaccharides the sucrose maltose and lactose and the polysaccharides starch cellulose and glycogen now for the monosaccharides the main thing you need to know is the structure of Alpha and beta glucose so alpha glucose we can see here this is the level of detail that you'd be expected to draw this for AQA biology and you would also need to know the formula so C6 h12o6 now I did say you need to know Alpha and beta and that's because glucose comes as two isomers which is when you have the same molecular formula but there is a different structure and the key differences I'll just highlight here so for alpha glucose on the carbon one which is the carbon that would be in this position you have the hydrogen atom on top and the hydroxyl group on the bottom for beta glucose the only difference is those swap around so the hydroxyl groups on top and the hydrogen atom is on the bottom the disaccharides then it's made up of two monosaccharides when those are joined together the chemical bond that forms is a glycosilic bond and they are created via a condensation reaction now I did name the three that you need to know but in addition to that for those three you would need to know the word equation and therefore which two monosaccharides they're made from so maltose is glucose plus glucose lactose is glucose plus galactose easier one to remember because there's lactose in the name and then sucrose is made up of glucose plus fructose and because all three are condensation reactions that is why one of the products is water because water is released now for the polysaccharides you need to know the structure and how that links to the function as well as some general other facts so here's a very basic summary starch and cellulose are both found in plants but they have different functions starch is a store of glucose so it can provide chemical energy and the cellulose is also in Plants but the function is structural strength in the cell wall glycogen is the only one found in animals and this is a store of glucose as well mainly found in the liver and the muscle cells so just to summarize everything you need to know about the carbohydrates polymer which is the polysaccharides you need to know which monomer they're made of now yes it is glucose for all three but which isomer is different starch and glycogen are both made from alpha glucose but cellulose is beta glucose they're all glycosidic bonds but they're different types and the thing that's different about them is the location and that's what these numbers refer to so a one to four glycosidic Bond means the bond forms between carbon one in one of the molecules and carbon four in another and those numbers just refer to the position of the carbon in the glucose ring so starch is made up of one to four and one to six and amylose which is one of the polysaccharides of starch only has one to four whereas amylopectin has both cellulose only has one to four glycogen also has both and it's the one to six glycosidic bonds that create a branched structure the one to four forms polymers in a straight line the function we've already said on the previous slide as have we said the location but a little bit more about the structure then and this again links to what we were just saying about the bonds so amylase is an unbranched polymer and it actually coils up to make a helix and that is really useful because if it coils up it can then be compacted to fit a lot in a small space amlopectin is Branched and the advantage of that is the branches create a larger surface area so more enzymes can attach to the end and hydrolyze to turn it back into glucose when the plants might need glucose all three have one feature in common they're polysaccharides which means they're large and because they are large they're insoluble that means they won't affect the water potential of the cell and therefore no impact on osmosis cellulite has got a very different structure and this is because it only contains one to four glycosidic bonds so the polymer forms long straight chains now those chains line up in parallel next to each other and hydrogen bonds join them together and we call that structure a fibril because there are so many hydrogen bonds holding These Chains together collectively they provide a lot of strength and that is why cellulose is a very strong molecule it's the large quantity of hydrogen bonds glycogen now this is actually very similar to starch in particular the amylopexin in starch the key difference is it has a higher proportion of one to six glycosidic bonds and for that reason it's even more branched and it can be even more readily hydrolyzed back into glucose and that's an advantage because we find in animals because animals move they will need more glucose the next molecule is the lipids and there's two lipids you need to know triglycerides and phospholipids so here is the level of detail you need to know their structure they both have a glycerol molecule and for the triglyceride there are three fatty acid chains that come off that so try for the three try meaning three phospholipids the key difference is one of those fatty acid chains is lost and instead it has a phosphate group attached to the glycerol molecule now how they form is the same but I'm actually just going to go through it with the triglycerides so it is a condensation reaction and it'll be three condensation reactions because there are three fatty acids that are going to be joining to the glycerol now although it's a condensation reaction that does not mean this is a polymer and it isn't a polymer because it's not many repeated units joining together so what we have is one water molecule is lost between each of the fatty acids and the part of the glycerol it's attaching to so in total that means we have three water molecules lost and three condensation reactions occurred and the bond that forms is called an ester Bond and we have three that's the bonds so that is the structure of a triglyceride a little bit more about the structure is the fatty acids you do need to know that they can be either saturated or unsaturated and a saturated fatty acid means that there are no double bonds between any of these carbon atoms so you only have single bonds between the carbon atoms and therefore it's fully saturated holding the maximum amount of hydrogen but this here would be your one Mark definition in an exam unsaturated fatty acids have at least one double bond between the carbon atoms but you do have to State between the carbon atoms in the exam to get the mark the properties then of the triglycerides so the function is as an energy store and the reason it stores lots of energy is because of the large ratio of the energy storing carbon to hydrogen bonds and that is compared to the number of carbon atoms so you have lots of energy stored in those bonds the second reason is due to the high ratio of hydrogen to oxygen atoms and that can actually act as a metabolic water source so triglycerides can release water if they are oxidized and that's actually what camels have in their hump they have lipids triglycerides they don't have water triglycerides also have a similar property to the polysaccharides in that they do not affect the water potential and therefore affect osmosis and this is because they are large and also hydrophobic which means they will repel water the last property is lipids have a very low mass compared to other types of tissue in your body for example muscles and that's an advantage because it means you can store a lot without increasing the mass as much as extra muscle would so you can store a lot of this for an energy storage without increasing the mass as much as other tissues would next then the phospholipids so we've already talked through the difference in the structure it's still made by condensation reactions but it would only be two condensation reactions because there's only two fatty acid chains still an ester Bond but we'd only have two Ester bonds and we can see up here that is where the phosphate group attaches to the glycerol now this phosphate group is what gives the phospholipids some very different properties to the triglycerides the phosphate group has a negative charge and due to that negative charge that bit so the hydrophilic head will actually attract water but it will repel lipids the fatty acid chain so the hydrophobic Tails they don't have any charges on them so they're actually described as hydrophobic meaning it repels water but they are able to mix with other fats or other lipids and that is why the phospholipids can form this bilayer we have these two charred regions and in water these phospholipids would position themselves so that the heads are on the outside exposed to the water because they are hydrophilic meaning they're attracted to the water but because the tails are hydrophobic meaning they are repelled or they repel water they would spin round and the Tails would face each other so they're not in contact with the water protein Zen is our next biological molecule and they are another example of polymers and the amino acids are the monomers that they're made up from you do need to know how to draw this General structure of an amino acid so it is one of the things you could be assessed on in the exam now a way to help you to remember it is to box it into these key groups you have a central carbon in the middle of the molecule there's a hydrogen atom that comes off and an R group that comes off the top now those could actually be either way round top or bottom the R Group represents the variable group so that changes for all 20 different amino acids the amine group or amino group that will always be present and that is nh2 and the carboxyl group that will also always be present C double bond o o h now to make a dipeptide which means two amino acids bonded together it would be a condensation reaction so water would be removed the bonds that would form would be a peptide bond to make a polypeptide that would be when you tap multiple amino acids joined together and multiple condensation reactions still all joined together by peptide bonds so that would create your primary structure of a protein but that primary structure gets modified into the secondary that gets modified into the tertiary or it could be a quarternary so we're going to go through what all of these four levels of organization or development of a protein look like and how they're held in place so the first level is the primary structure and this is what is made straight after translation in protein synthesis and the definition for this would be a one mark question it's the order or you could say the sequence of amino acids in a polypeptide chain so that's your polymer the secondary structure then is when that primary structure is folded or it could be modified by twisting so we can see here the alpha Helix but that would be the key marking point that we then have an alpha helix or a beta pleated sheet that's created and those are held in place by hydrogen bonds the secondary structure then gets modified further so it's further folded to create a unique 3D shape and that shape is held in place by ionic hydrogen and sometimes disulfide bonds and it's actually the primary structure that determines the location of these bonds the ionic hydrogen and disulfide bonds and it's the location of the bonds which determine how it folds and the 3D shape the final level of organization is the quarternary structure that's cut off slightly but that says structure there now that is still this unique 3D shape with the same bonds but the only difference is It's A protein that is made up of more than one polypeptide chain but it is still the basically the tertiary structure you just have more than one chain so we call it quaternary enzymes are an example of proteins that you need to know so an enzyme is a protein in the tertiary structure so that unique 3D shape and their function is that they catalyze reactions and they do this by lowering the activation energy of a reaction now every enzyme is specific and what that means is it can only catalyze one particular reaction and that is due to the unique shape of the active sites which this is an application of what we just said in the slide before that primary structure determines the locations of the bonds that determines the folding and the unique shape so this is why each enzyme can only catalyze one particular reaction and you get that unique or specific active site so in that way the act of sight is complementary in shape to a particular substrate now there's actually different models which explain how enzymes work and at GCSE you would have learned the lock and key model but the accepted model currently is the induced fit model so that is what you'd be expected to talk about at a level you wouldn't be expected to mention the lock and key method so the induced fit model is one that states that the enzymes active site is induced or it slightly changes shape to mold around the substrate so initially the substrate in active site are not completely complementary but as the substrate binds that causes the enzymes act to slightly active site to slightly change shape and mold around that moving around the substrate puts strain and tension on the bonds and therefore less energy is needed to break the bonds and that is how enzymes lower the activation energy which is the amount of energy needed for a reaction to occur there are five factors that you need to know that affect the rate of an enzyme-controlled reaction temperature pH substrate concentration enzyme concentration and Inhibitors so let's have a look at each one so for temperature if there is a lower temperature that would mean that the molecule so the enzyme and the substrate would have less kinetic energy therefore they won't have as many successful collisions and you'd have fewer enzyme substrate complexes that is why the rate is lower at colder temperatures above the optimum though there is now so much kinetic energy that it causes some of the bonds to break say for example the hydrogen bonds might break and that means that the protein loses its unique 3D shape the active sites change shape and therefore you won't have enzymes substrate complexes forming and the rate decreases for pH either side of the optimum pH which actually can vary depending on where the enzyme is found either side we have a very rapid denaturing of the enzyme and that's because either too high or too lower pH will interfere with the charges in the amino acids found at the active sites that can cause the hydrogen and the ionic bonds to break and again the loss of that tertiary structure and the active site changes shape so we describe that as the enzyme denaturing and again there'd be fewer enzyme substrate complexes and therefore the rate of reaction decreases substrate in enzyme concentration have a similar idea behind them there's no enzyme denaturing but if we have a look at this one first if there's insufficient substrates there will be fewer collisions between the substrate and the enzymes and that's why the rate of reaction is lower but if you add more and more substrates but no extra enzyme eventually you'll get to the point where the enzyme active sites are all in use or they're saturated so even if you add more substrate there's no more free enzymes so the reaction can't go any faster so the rate remains constant for the enzyme concentration if there's insufficient enzymes so at these low concentrations then the active sites will become saturated with whatever substrate is there and that's why if we don't add more enzyme the rate will stay low but as you add more enzyme the rate will increase however you'll get to a point though where if you keep adding more and more enzymes but don't add any more substrates you'll just have a surplus of enzymes and there isn't any extra substrate for those enzymes to bind to so the rate won't increase any further the last one was the enzyme Inhibitors now both type of inhibitor the competitive and the non-competitive both bind to an enzyme so in the exam you have to be specific and say which part of an enzyme they attach to to get the mark a competitive inhibitor binds to the active sites so we can see that here and if it combines the active sites that means this inhibitor must be the same shape or very similar in shape to the substrate and if the inhibitor is bound that will prevent enzyme substrate complexes forming so if you add more substrate for a competitive inhibitor the substrate will actually eventually be able to knock out the inhibitor take its place and therefore with a very very high concentration of substrates the effect of the inhibitor is no longer seen however a non-competitive inhibitor binds to the allosteric site and that is a part of the enzyme away from the active site so it doesn't bind to the active site but as it binds it causes the active site to change shape and for that reason the inhibitor has made it impossible for enzyme substrate complexes to occur because the substrate is no longer complementary to that active site and it can't bind so that is how these Inhibitors lower the rate of reaction and even if you add more substrates that won't help because the active site is a different shape now you could be asked for the biochemical test for all of those molecules that I've just gone through or some of them anyway the ones I'm going to go through so the first one is the biochemical test for starch this would be a two mark question to describe the method you add iodine and a positive test result would be the iodine goes from orangey Brown to blue black the test for reducing sugars you would add Benedict's reagent and you have to heat it so you need both of those ideas to get the mark the positive test result would be the original blue color would turn either green yellow orange or brick red and those different colors indicate the concentration of reducing sugar present so the more red it is the higher the concentration of reducing sugar the test for a non-reducing sugar test then you would first of all have to do your Benedict's test for reducing sugars and if it remains blue which means a negative test result you then go on to these stages you would add acid and boil you have to say boil because it does need to be above or at 100 for this to work you would then cool and neutralize the solution because you've cooled it you would then have to say heat and add Benedict's reagent again and this time the positive result would be the solution goes from Blue to either orange or brick red now it would always be orange or brick red this time because if it was a non-reducing sugar that means it was probably sucrose and sucrose is made up of glucose and fructose so when you hydrolyze it that means it goes to Two Sugars so glucose and fructose instead of just one sugar sucrose so you've now doubled the concentration of sugar present so it will always be orange or brick red the test for proteins is you add biorets which is blue in color and if you have a protein present it will go purple for lipids it would be a three mark question you have to dissolve your sample in ethanol and you would do this by adding ethanol and shaking and sometimes there is a mark for saying you have to shake once you've done that you then add distilled water and a positive test result would be white in color and we describe it as an Emulsion this sort of thick milky texture so the next biological molecule is DNA or deoxyribonucleic acid and this function is the case with the sequence of amino acids in the primary structure so it's very very essential it contains the genetic code and it can be passed on to make new cells but also it can be passed On to the Next Generation now DNA is a polymer and you get two polymer chains joining together to create a double helix so the monomer you would have to be able to draw this in this level of detail we have our phosphate group attached to a pentose sugar and the nitrogenous base but because this is a DNA nucleotide you would have to say that the pentose sugar is deoxyribose and the nitrogenous bases or nitrogen containing bases are guanine cytosine adenine and thymine so to make the polynucleotide which is the name for the polymer it would be a series of condensation reactions where water is removed and a phosphodiester bond forms between the deoxyribose and the phosphate group of another nucleotide and that makes this sugar phosphate backbone which is very very strong because of phosphodiester bond is a strong covalent bond between the two polymer chains we get hydrogen bonds and those form between the complementary base pairs which are cytosine in guanine and adenine and thymine RNA is the other nucleic acid that you need to know about and it's almost identical in shape or in structure the key differences are that instead of having deoxyribose as the pentose sugar it has ribose it doesn't have the nitrogen containing base thymine it has uracil now RNA is also in terms of the polymer it's much shorter than the DNA polymer and that's because mRNA is only a copy of one gene whereas DNA is all of the genes and also TRNA is relatively short also and all of the polymers are single stranded for RNA now the functions you actually learn in more detail in topic four but you do need to know about one particular RNA molecule called rrna that combines with proteins to make ribosomes so DNA replication then in order for new cells to be created every new cell needs their own copy of the entire genome which is all of the DNA so the DNA must replicate before a new cell can be created and the way DNA replicates is described as semi-conservative replication and what that means is one of the original strands of DNA combines with one newly synthesized strand to create the new molecule so how that happens step one the enzyme DNA helicase breaks hydrogen bonds between the complementary base pairs and that causes the double helix to unwind and the two strands to separate and both of those strands will act as a template free-floating nucleotides within the nucleus will then align opposite the complementary base pairs and the enzyme DNA polymerase will join adjacent nucleotides together so it's joining the nucleotides together to make phosphodiester bonds so now we have our newly synthesized DNA combined with the template Strand and that is our new molecule of DNA or the daughter DNA the evidence for this you need to know a little bit about also so it was Watson and Crick who discovered the structure of DNA so that double helix but they only managed to discover that because of Rosalind Franklin's research on x-ray diffraction meselson and style they conducted experiments which proved that DNA replication was semi-conservative now ATP is what we call a nucleotide derivative and that's because it's very similar in structure to DNA and RNA it still has a pentose sugar it still has a nitrogen containing base and a phosphate group but it actually has three phosphate groups it will always have ribose and it will always have adenine so adenosine triphosphate is what ATP stands for and its function is it's an immediate source of energy for biological processes so in other words it's used in metabolism which is all of the chemical reactions within a cell ATP is made during respiration by ADP and Pi which is inorganic phosphate joining together in a condensation reaction using the enzyme ATP synthase now ATP releases energy when it is hydrolyzed so when the bonds between the phosphates are broken and we then have ADP plus that inorganic phosphate that releases energy and the enzyme ATP hydrolase is what catalyzes that reaction now ATP can also do something called phosphorylation and that is when that inorganic phosphate is actually transferred and bonds to a different compound and in doing that the compound that it binds to or bonds to becomes more reactive and that happens to glucose at the start of respiration in glycolysis the next biological molecule is water and water has five Key properties that you need to know about now water is very very important because it makes up about 60 to 70 of your body and the reason it's so important links to these properties now most of the properties are due to the hydrogen bonds which form between water molecules and those will form between the oxygen of one water molecule and the hydrogen in another water is also described as being polar or dipolar because the oxygen has a slight negative charge and the hydrogen has a slight positive charge in the water molecule so the five properties number one water is a metabolite which means it's involved in chemical reactions so we've already seen in this video how it's involved in condensation and hydrolysis reactions but it's also involved in photosynthesis because of its dipolar nature it's a very very good solvent and this is important because if it can dissolve solutes and they can be easily transported around the body in the cytoplasm in a cell or in the plasma of blood or in the liquids and the phloem and the xylem implants chemical reactions also happen more readily in liquids it has a high heat capacity which means it takes a lot of energy to raise the temperature and in that way it can buffer temperatures so that means the fact that our body is mainly water it takes a lot of energy to increase our body temperature and that's good because we don't want enzymes to be denaturing it also has a large latent heat of vaporization and that provides a cooling effect because this property means a lot of energy is required to convert water from its liquid state into its gaseous state so if water does evaporate that means a lot of energy has been transferred in that process and it provides a cooling effect when we sweat or when plants are going through transpiration with water evaporation water also has strong cohesion and this is due to the hydrogen bonds between the different water molecules and cohesion means the water molecules are sticking together and in Plants this is particularly important because that means you get this continuous column of water moving up the xylem it is also what provides surface tension to water and that can actually provide a habitat for certain organisms inorganic ions is the last part of the specification for this topic and what you need to know is that ions can occur in solution so for example in the cytoplasm of cells in other bodily fluids as well like the blood sometimes in high concentration sometimes in low and there are a selection of inorganic ions that you need to know and what function they have now this actually links to other parts of the spec but the hydrogen ions you could look at the importance of them in terms of how they can alter the pH of a solution and that could have an impact on enzymes but also hemoglobin if you think about the bore effect that comes up in another topic or you can look at the importance of hydrogen ions in chemiosmosis in respiration and photosynthesis iron ions are a component of hemoglobin and are involved in the transport of oxygen sodium ions are involved in the co-transport of glucose and amino acids in absorption or you could look at the role of sodium ions in action potentials as well phosphate ions are found in DNA RNA and ATP the importance in DNA and RNA is the phosphate group is what where the phosphodiester bond forms in ATP the phosphate group can be added to other compounds to make them more reactive So within the eukaryotic cells the key organelles that you need to know the structure and function of are all the ones listed here and we're going to go through each of them one at a time the nucleus the key structures within that are the nuclear envelope nuclear pores nucleoplasm chromosomes and the nucleolus and the overall function of the nucleus is where DNA replication occurs and it's a site of transcription which is where mRNA is made and it contains the DNA which is a genetic code for the cell we also have within the nucleolus that is the site of RNA production and it's where ribosomes are made the endoplasmic reticulum you have the smooth and the rough and we can see here the top diagram we've got the rough endoplasmic reticulum and we can tell because we've got the ribosomes on the outside and the other side of this top diagram is the smooth endoplasmic reticulum and that has no ribosomes on the outside now the rer which is the rough endoplasmic reticulum that is where protein synthesis occurs because of those ribosomes on the outside the scr the smooth endoplasmic reticulum that is where you have the synthesis of lipids and carbohydrates and they also get stored there as well the Golgi apparatus is a folded membrane making this cystony shape and we have vesicles that will pinch off from those Systema once whatever it is that's going to be modified has been modified and packaged and we've got the whole list here of the different things that would happen so it could be that the carbohydrates are being added to proteins to make glycoproteins which might be embedded within the cell membrane you can also produce secretory enzymes carbohydrates transport and modify lipids we get lysosomes molecules are also labeled with their destination essentially meaning they're going to have a molecule added so they'll be able to bind to receptors on the target cell for where that molecule is needed lysosomes are bags of digestive enzymes and this is often involved in phagocytosis so the enzymes that are required to hydrolyze the bacteria or virus whatever it might be would be in a lysosome there'll be exocytosis which is where the products are going to be released to the outside after it's been destroyed the mitochondria is the site of aerobic respiration so you'll have lots of ATP production here it's a double membrane organelle with the outer membrane and the inner membrane is this folded part making up the Christie and the inner membrane is where oxidative phosphorylation happens which is one of the key stages in aerobic respiration they have their own Loop of DNA which is very similar to prokaryotic DNA and this is so they can code for the enzymes that they need in respiration ribosomes now these are found in prokaryotic and eukaryotic cells and that is because they are not membrane-bounds ribosomes are just made up of rrna and proteins you do have different size of zone so the ATS is the largest size ribosome and eukaryotic cells 70s is a smaller sized ribosome found in prokaryotic cells as well as in mitochondria and chloroplasts so they can make their own enzymes and it's the site of protein synthesis the vacuole which is only found in in the plant cells not in animal cells this structure is filled with fluid and surrounded by a single membrane called autonoplast and this is what helps to make the cell turgid and provide support you'll have temporary stores of sugars and amino acids within the fluid as well and the pigments within it may color the petals and that's what helps to attract pollinators chloroplasts again only found in plants and this is the site of photosynthesis you have again it's a double membrane organelle and we have the outer membrane the inner membrane and then on the inside we have even more membranes and that is these thylakoid membranes which stack up to create what we call a Grana if it's plural or just a granum if we're just looking at one of those stacks and the thylakoid membranes are embedded with proteins and pigments such as chlorophyll and therefore is the site of the light dependent reactions in photosynthesis the stroma which is the fluid part surrounding the thylakoid membranes you have lots of enzymes in this location which are needed for the light independent stages of photosynthesis cell walls are not in animal cells but they are in fungi and plant cells and they help to provide structural strength to the cells and prevent them bursting if lots of water moves in bosmosis and that's because they contain a particular molecule that provides strength and implants that cellulose and in fungi that would be chitin the plasma membrane is found in all cells and this is in the cell membrane so the cell surface membrane but also when we say it's a double bound or membrane-bound organelle it's also made up of this phospholipid bilayer with the different molecules embedded within it and the function of the plasma membrane is it controls what can enter and exit the cell prokaryotic cells for example bacteria you need to be aware of the key differences so it could come up as a comparison or a contrast question so first of all the cells are much much smaller they don't have any membrane-bound organelles they do have ribosomes though because ribosomes are not membrane-bound but they have the 70s size ribosomes they don't have a nucleus because that is a membrane-bound organelle so instead their DNA will just be as a single Loop which is free within the cytoplasm they do have a cell wall and that is made up of murin sometimes they might also contain plasmids which are additional Loops of DNA they might have a capsule around the cell which is to help prevent desiccation and to evade the immune system and sometimes they'll have flagella it might be one it might be many which is to help them to be able to move but those aren't always found those only sometimes found in prokaryotic organisms viruses are acellular and non-living the structure of virus particles consists of genetic material capsid and attachment proteins which we can see over here so we have the genetic material in the middle we have the capsid and then we have the attachment proteins or here they're labeled envelope proteins viruses replicate inside of host cells and that is why it's very difficult to destroy them because you would have to destroy the host cell to be able to destroy the virus now you need to know methods of studying cells and the reason this is so important is the way we've discovered these internal structures or organelles is because of these advances in the methods to study cells so advances in microscopes and then also cell fractionation and Ultra centrifugation to be able to isolate particular organelles to examine so we'll go through each of these methods of how to study cells first of all the microscopes there's a couple of key definitions you need to be aware of you also need to know the three main types of microscope there are Optical microscopes which are the light ones which should be using in school you then have electron microscopes but you can have either a transmission electron microscope or a scanning electron microscope the definition of magnification is how many times larger the images compared to the object resolution of a microscope is the minimum distance between two objects in which they can still be viewed as separate and the resolution of an optical microscope is determined by the wavelength of light and the resolution of the electron microscopes is determined by the wavelength of the beam of electrons and because that wavelength is much shorter that is why they have a higher or a more powerful that is why they have a much much higher resolution meaning that the distance between the two objects is smaller so let's compare the two general types of microscopes in more detail the optical microscope we've already said it's the light but it is a beam of Light which is it's a beam of light that is condensed to create the image in contrast an electron microscope it's a beam of electrons which is condensed to create the image and what is used to condense the light and the electrons is also different the beam of light is condensed using a lens whereas the beam of electrons is condensed using electromagnets the optical microscope has a poorer resolution due to the fact that light has a longer wavelength so that means that the electron microscapes because electrons have a shorter wavelength they have a much higher resolving power there's also a lower magnification for the optical higher for the electron microscapes you can have color images though and you can view living samples with the optical microscopes so those are two big advantages the electron microscopes can only produce black and white images and because electrons can be easily absorbed you have to have your sample in a vacuum and because it's in a vacuum you can't actually observe any living samples using an electron microscope now just go back to this black and white image point you can still get colored images if colors are added on programs like Photoshop afterwards because the optical microscopes have a lower resolution small organelles in a cell are not visible so you can't see details of the mitochondria chloroplasts ribosome you can just see some of the large organelles like we can see here you can see the nucleus you can see the DNA you can see the chromosomes the electron microscopes they have much higher resolving Powers as we said and the difference between the transmission and the electron is at the transmission you have extremely thin specimens and the electron gun produces a beam of electrons that transmits or passes through the specimen so some parts of the specimen will absorb the electrons and therefore appear dark some parts won't and the electrons would have passed through and they appear lighter and we get these 2D images where we have the darker and lighter depending on whether the electrons were absorbed or not and because there's a higher resolution that is why we can actually see the details internally of some of those small organelles a scanning electron microscope creates 3D images in contrast the specimen does not need to be thin and that's because the electrons are not going to be passing through the specimen instead the electrons are beamed onto the surface and the electrons are scattered in different ways depending on the Contours and that is how we get these 3D images which we can see here in these different blood cells in a sample from the blood you could be asked to calculate magnification of an image so the formula that you need to remember is I am and that is the image size equals actual size times magnification or you can rearrange it to work out the actual size or the magnification and the key thing that you need to be aware of when you're doing these calculations is the image size and the actual size have to be in the same units so you need to be familiar with how you convert from millimeters to micrometers or micrometers back into millimeters those are the two most common conversions in a magnification question but if you want to know any of the other conversions here they are you might also need to be able to measure the size of a specimen using the eyepiece graticule and inside of the optical microscope there is a scale on a glass disc which is called the eyepiece graticule so it'd be just here within that eyepiece and this can be used to measure the size of objects you're reviewing under the microscope however each time you change the objective lens and therefore the magnification you have to recalibrate the eyepiece to work out what the distance between each of these different divisions is actually worth now you need to know how to do that but I'm not going to go through that in the summary video but I'll link the video up here of how to calibrate the eyepiece graticule cell fractionation is the final way that you can study cells and this is used to isolate the different organelles so they can be studied further first thing that happens is the cells are broken open to release the contents and the organelles the cells need to be prepared in a cold isotonic and buffered solution and you need to know why it has to be those three conditions so it has to be cold to reduce enzyme activity and that's because when you break open the cell you'll be releasing enzymes which wouldn't typically be in contact with the organelles and they might damage them it has to be isotonic so that you don't have excess water moving in or out of the organelles by osmosis because we don't want the organelles to burst or shrivel because then we won't be able to study them and it has to be buffered because if it became too acidic or alkaline again it could damage the organelles now cell fractionation is a two-step process homogenization and Ultra centrifugation so first of all her modernization is when you break open the cells and that can be done in a blender as long as you have as we said the solution that you're blending up your sample in as long as the solution that you're blending your sample in is cold isotonic and buffered then we'd need to filter to remove the large debris and then the filtrate we can then use ultra centrifugation to isolate the different organelles so we'd put our sample into a centrifuge and we'd spin at different speeds the organelles are going to separate according to their densities and that is why we have to spin our sample at different speeds and this is what differential centrifugation is so when we spin our sample the centrifugal forces cause pellets to form at the bottom with the most dense organelles so we can see here in this first image we've got all of the organelles equally distributed and if we first spin at low speed the most dense organelle will form a pellet at the bottom we would then remove that pellet and spin the rest of the filtrate again at a slightly faster speed we'd then get the next most dense organelle forming the pellets and we repeat this process at increasingly faster speeds so each time that liquid which is the supernatant is removed and the pellet is what the organelles will be in and we can examine those now just to let you know the order of the density in that first centrifugation the nuclei are the most dense so those would be isolated in the first spin the next most dense are the chloroplasts and the mitochondria then the lysosomes endoplasmic reticulum and finally the ribosomes so that would be the order in which you'd collect your different organelles we then move on to cell division and eukaryotic cells enter the cell cycle and divide by mitosis or meiosis but in topic one you only learn about mitosis in comparison prokaryotic cells replicate by binary fission and viruses do not undergo cell division at all because they are non-living they do still replicate though but viruses replicate inside of a host cell they will invade that host cell by injecting in their nucleic acid so their genetic material and then it will be the host cell that uses that genetic material to replicate the virus particle cell cycle that eukaryotic cells will be going through includes these key stages interphase is the longest stage of the cell cycle and it includes g1s G2 G1 is when the cell is going to be increasing in size and the organelles will double S phase is when DNA replication happens G2 you'll have further growth but also it says preparation for mitosis in G2 you'll have this error check stage so if there are any errors in the DNA replication the cell would be destroyed at that stage nuclear division is either mitosis or meiosis but in topic 2 we just focus on mitosis the final stage of the cell cycle is cytokinesis and this is when the cytoplasm divides to create the two new cells if it's mitosis mitosis is split into four key stages which are our pmat prophase metaphase anaphase and telophase the key facts about mitosis are is only one round of division genetically identical cells are created the cells are diploid which means there are two copies of every chromosome and this Sage is used for growth and repair so a specific example of that which we'll see later in this video is the clonal expansion of B cells so that is growth in the sense of we're creating lots of new cells so in prophase the chromosomes will condense and at this point they become visible and in the animal cells the pairs of centrioles will move to opposite poles which means opposite sides of the cells the scent trails are going to be creating these spindle fibers which are released from both poles to create the spindle apparatus and these will attach to the centromere and the chromatids on the chromosomes in the later stages ants have a spindle apparatus but they don't have the centrioles in metaphase we can see that the chromosomes will then line up in single file along the Equator and the spindle fibers are released from those centrioles at the poles and they'll attach to the centromere and also the chromatids in anaphase the spindle fibers will start to retract and pull back towards the centrioles and in doing that they'll pull on the centromere and chromatids and this causes the centromere to divide in two and the individual chromatids are pulled to the opposite poles of the cell and therefore it separates the chromatids and once those chromatids are separated we now actually call them chromosomes now this stage requires energy in the form of ATP and that's provided by respiration from the mitochondria until phase the chromosomes are now at each pole of the cell and become longer and thinner the spindle fibers will then disintegrate and the nucleus starts to reform the final stage in the cell cycle is when the cytoplasm splits into to create the two new genetically identical cells which the mitotic index can be calculated by counting first of all how many cells are visible in the field of view and then counting the number of cells that are also visible but in a stage of mitosis you can then do a percentage calculation so you'd be doing the number of cells that you can see in a stage of mitosis divided by the total number of cells present and times that by 100 prokaryotic cells don't go through mitosis instead they go through binary fission the first step of that would be the replication of the circular DNA and of the plasmids and then the second stage would be division of the cytoplasm to produce two door cells each with a single copy of the circular DNA and a variable number of plasmids viruses are non-living so they don't undergo the cell division instead they inject their nucleic acid into the host and the host cell replicates the virus particles so now onto plasma membranes all cells and organelle membranes have the same structure it's this fluid mosaic model which is named because of the fact that it does have some slight movement and also it's composed of a range of different molecules such as phospholipids proteins glycoprotein teens and glycolipids all of these molecules are arranged within the phospholipid bilayer and they create the partially permeable membrane to create the bilayer the phospholipids Align in this manner because of the fact that the phospholipid has a hydrophilic head due to that negative charge on the phosphate group and that causes it to be attracted to water whereas the hydrophobic Tails those are repelled by water but can interact with lipids so the phospholipids end up spinning rounds so that the tails are opposite each other the heads are on the outside where they can interact with water collect present in some membranes too and this will restrict the lateral movement of other molecules in the membrane this is useful as it makes a membrane less fluid at high temperatures and prevents water and dissolved ions from leaking out of the cell the other components of the membrane are mainly the proteins and these are embedded across the cell surface membrane either as peripheral meaning they're just on the outside sometimes called extrinsic or it could be integral sometimes called intrinsic meaning it spans from one side of the bilayer to the other the peripheral proteins provide mechanical support or they can be connected to proteins or lipids to make glycoproteins and glycolipids the peripheral proteins provide mechanical supports or they can connect to carbohydrates to make glycoproteins and also carbohydrates can bind to the lipids directly to make glycolipids the function of these is cell recognition or as receptors the integral proteins are protein carriers or protein channels involved in the transport of molecules across the membrane protein channels are tubes and these fill with water which will enable water-soluble ions to diffuse across the membrane whereas the carrier proteins will enable molecules to bind with them and therefore larger molecules like glucose and amino acids can cause those carrier proteins to change shape and therefore transport the molecule to the other side of the membrane so when we say that the membrane is partially permeable what we mean by that is only lipid soluble substances and very small molecules can pass across the membrane by simple diffusion other molecules such as water-soluble or polar substances and large molecules can't simply diffuse across the membrane therefore they have to be transported across by other means which could be facilitated diffusion active transport or if it's water osmosis so simple diffusion first of all this is the net movement of molecules from an area of higher concentration to an area of lower concentration and this will continue until an equilibrium is reached and this process does not require any ATP the molecules do still have energy to be able to move but this is just due to the kinetic energy that they possess and that is why they're able to move constantly as a fluid for molecules to diffuse across the membrane they have to be lipid soluble and small if they're not lipid soluble or small then it will be facilitated diffusion instead and this is still a passive process because the molecules are moving from an area of higher concentration to lower but because they can't simply diffuse through the phospholipids instead they have to diffuse through proteins now this could be through protein channels where we have those tubes filled with water and therefore water soluble ions can dissolve and then diffuse through this is still selective though as the channel proteins only open in the presence of certain ions when they bind to the protein carrier proteins will bind with a molecule such as glucose which causes a change in the shape of the protein this change enables the molecule to be released to the other side of the membrane osmosis is the movement of water and that will be from an area of higher water potentials to an area of a lower water potential across a partially permeable membrane the water potential is the pressure created by water molecules and it's measured in kilopascals and represented by this symbol here pure water or distilled water has a water potential of zero so you cannot get one any higher than zero it's always going to be zero or negative and that means when solutes are dissolved in water the water potential will be negative and the more negative the water potential the more solutes there must be dissolved in it now three key terms that you need to know linked to osmosis are isotonic hypertonic and hypertonic an isotonic solution is when the water potential is the same in the solution and the cell hypotonic is when the water potential of the solution is more positive so closer to zero or closer to Pure Water compared to the cell hypertonic is when the water potential of the solution is more negative than the cell so it'd be more solutes dissolved in it and in an animal cell we can see the impact that would have in an isotonic solution because the water potential is the same there's not going to be any net gain or loss of water in a hypertonic solution there'll be more water moving out of the cells by osmosis and that can cause animal cells to shrivel or crenate if animal cells such as red blood cells are put into a hypotonic solution that would mean water would be moving into the cell by osmosis and if enough water moves in it can cause the cell to burst transport is the movement of substances from a lower concentration to a higher concentration and this requires metabolic energy and a carrier protein so the process by which this happens is first of all it's always through carrier proteins only it is not channel proteins for active transport the molecule will bind to the carrier protein and it can only bind to proteins which have a complementary receptor shape so it is still selective the ATP binds to the carrier protein as well from the inside of the cell and it is hydrolyzed into ADP and pi this causes the carrier protein to change shape and therefore it releases the molecule to the other side of the membrane the phosphate ion is then released and the protein can return to its original shape and the process can then happen again as long as there is plenty of ATP presence co-transport is a type of Acts of transport and you learn about it across the course but one of the key examples you learn about is the co-transport of glucose or it could be amino acid with sodium ions in the ilium and to absorb glucose from the Lumen of the guts there has to be a higher concentration of glucose in the Lumen compared to the epithelial cell but there's usually more glucose in the epithelial cells and that is why active transport and co-transports are required so the first thing that has to happen is the sodium ions which are shown in blue hair are actively transported out of the epithelial cell into the capillaries and that will result in a lower concentration of sodium ions within the epithelial cells and that creates this concentration gradient from the ilium to the epithelial cell and therefore the sodium ions can diffuse down their concentration gradient into the epithelial cells the proteins the sodium ions diffuse through though is a co-transporter protein so either glucose or amino acids will also attach and are transported into the epithelial cell with the sodium ions except the glucose is going in against its concentration gradients the glucose once in the epithelial cell can then move by facilitated diffusion from the epithelial cell into the blood now you also need to be aware that cells may be adapted for Rapid transport across they're internal or external membranes so it could be that cells have an increased surface area so things like microvilli for example or there could be an increase in the number of protein channels and carrier molecules within the membrane as another way to increase the rate of Transport the last section of topic 2 is all about immunity And We Begin by looking at identifying self and non-self cells so your body's immune system has cells to be able to identify the presence of pathogens and potential harmful foreign substances compared to your own body cells and that's so it can destroy the foreign items and not your own cells and those cells are called lymphocytes how can lymphocytes then actually distinguish between a pathogen and your own body cells well each type of cell has specific molecules on its surface that identify it and these molecules are usually proteins so they'll have this unique 3D tertiary structure and this is how they can identify whether it is an antigen from a pathogen or whether it is one of your self antigens if a non-self cell is detected a response will be triggered to destroy that cell and these different surface molecules enable them to identify pathogens which we've mentioned so bacteria fungi viruses but it could also be cells from other organisms of the same species and that can be harmful for individuals who have had organ transplants it could also be abnormal body cells for example cancer cells and finally toxins because some pathogens release toxins into the blood such as the bacteria cholera now an antigen is a molecule that generates an immune response and this is because it triggers the lymphocytes they are usually proteins and they are located on the surface of the cells now antigen variability is this concept that pathogens DNA can mutate frequently if a mutation occurs in the gene which codes for the antigen then that means the shape of the antigen on the outside of the virus the fungus the bacteria will change and if that happens any previous immunity that you may have had to the pathogen is going to be lost so it will no longer be effective as all of the memory cells in the Bloods that you have will have a memory of the old antigen shape and not the new one and this is known as antigen variability the influenza virus mutates and changes its antigens very quickly and this is why a new flu vaccine is created every year to account for that change so looking at your immune response then if a pathogen gets past the chemical and physical barriers so for example your skin is a physical barrier and stomach acid is a chemical barrier then your white blood cells are the next line of defense your white blood cells have a specific and also non-specific response the phagocytes are a non-specific response meaning that they will destroy any foreign item that they come across lymphocytes have a specific response though and this is where the idea of detecting using antigens will come into it so let's look at phagocytosis first of all phagocyte is a macrophage cell which is a type of white blood cell and they are found in the blood but also tissues phagocytosis as we said is a non-specific response so any non-self cell that is detected will trigger the same response every time to cause the destruction of that object so the stages of Phagocytosis are first of all the phagocytes are in the blood and the tissues and any chemicals or debris released by The pathogens or abnormal cells attract the phagocytes and that will cause those white blood cells to move towards the cell there are many receptor binding points on the surface of the phagocyte which we can see here they will attach to the chemicals or the antigen on the pathogen via The receptors the phagocyte then changes shape to move around the pathogen and engulf it and once engulfed the pathogen is contained within a vesicle which we call a phagesome a lysosome within the phagocyte will then fuse with the phager zone and it will release its contents and the contents is a lytic enzyme called lysozyme analytic enzymes are able to hydrolyze so the result is that enzyme will hydrolyze and it breaks down the pathogen any soluble products are absorbed and used by the phagocyte and anything else is released as waste and debris so that is our non-specific response but now if we look at the specific responses which are by the lymphocytes first lymphocyte we're going to look at are the T lymphocytes or the T cells so all lymphocytes are made in the bone marrow but the T cells mature in the thymus which is why they're called t cells and this is classed as the cell mediated response so the first thing to be aware of is the cell mediated response is specific to T cells responding to antigens on the surface of cells so it involves antigen presenting cells or APC an angstrom presenting cell is any cell that presents a non-self antigen on their surface infected body cells will present the viral antigens on their surface if they are infected a macrophage which has engulfed and destroyed a pathogen will also present the antigens on their surface cells of a transplanted organ will have different shaped antigens on their surface compared to your own self cells and cancer cells will have abnormal shaped self antigens as well so all of these are presenting antigens on the cells so they are Androgen presenting cells and can therefore trigger an immune response so the cell mediated response then once a pathogen has been engulfed and destroyed by a phagocytes the antigens are positioned on the cell surface and that is why we'd Now call that phagocyte an antigen presenting cell helper T cells which are a type of T lymphocytes have receptors on their surface that can attach and bind to the antigen on Androgen presenting cells and once those two cells are bound it activates the help of T Cell to start to divide by mitosis and that then means we get a very large number of cloned cells so essentially this stage is to make sure you end up with lots of helper T cells which have the correct shape to bind on to that particular antigen so you'll have lots of helper T cells within your blood that have different shape receptors to be able to bind onto different shaped antigens but if you have one Collide that will then activate that particular T cell with that particular shape receptor to divide by mitosis to make large cloned copies those cloned help with T cells then differentiate into particular types of T cells some remain as helper T cells which then go on to activate the B lymphocytes some can stimulate macrophages to perform more phagocytosis some become memory cells for that particular shaped antigen and some become cytotoxic killer cells or cytotoxic T cells and what those cytotoxic T cells do is destroy the abnormal or infected cells that have the antigen on their surface and they do this by releasing a protein called perforin which embeds in the cell surface membrane to make a hole or a pore so that any substance can now enter or leave the cell now this causes death because you can have lots of water move in and therefore the cell will burst or you could have lots of water moving out and therefore the cell will actually shrivel up and die this is most common environment infections because viruses infect body cells of the host body cells therefore have to be sacrificed to prevent further viral replication and this is actually why you get a really sore throat when you have a cold because those cytotoxic T cells are destroying the infected body cells in your throat to try and prevent that virus dividing and spreading further in your body next time we move on to the B lymphocytes or the B cells as we already said all lymphocytes are made in the bone marrow but B cells mature in the bone marrow as well which is why they're called B cells this is now called the humoral response and this is the one that involves antibodies antibodies are soluble and transport in bodily fluids and humor is an old term for body fluids and that's why this is called the humoral response there are approximately 10 million different B cells which have antibodies on their surface complementary to 10 million different antigens and antigens in the blood will collide with their complementary antibody on a B cell and the B cell takes in the antigen by endocytosis and then presents it on its cell surface membrane when that B cell collides with a helper T cell that can then activate the B cell to go through clonal expansion and differentiation or clonal selection so this is the step where there's a link between the cell mediated response with the help of T cells binding via the receptor to the antigens on the B cells to activate them once activated the B cells undergo mitosis to make large numbers of the cells which have that particular antigen on their outside and these can then differentiate into plasma cells or memory B cells the plasma cells will go on to make antibodies complementary in shape to that particular antigen and the memory cells can divide rapidly into plasma cells if you are reinfected with the same pathogen and therefore antigen later on and that means that you can make large numbers of the correct shaped antibody so rapidly that you should be able to destroy the pathogen before it causes any damage and symptoms so the memory B cells can live for decades in your body whereas the plasma cells are only short-lived memory B cells though cannot make antibodies but they can divide by mitosis and then differentiate into plasma cells so this is what results in the large numbers of antibodies being produced so rapidly that the pathogen is destroyed before any symptoms occur and that's what it means when we say you are immune to a particular disease it's because you have these memory B cells so you can destroy the pathogen before it causes any symptoms and that would be an example of active immunity primary and secondary response is referring to the number and the speed at which antibodies are produced when you are first exposed to an antigen compared to what your second exposure so the first time you're exposed to a new pathogen and therefore a new antigen it takes a little bit longer for the antibodies to be produced because you have to go through that initial step of those 10 million different B cells colliding until you have the correct shape antigen antibody binding so that can take a couple of days and that's why you have a slower response in producing the antibodies and also you don't actually make as many but because memory B cells are made if you are exposed for a second time you will be able to create those antibodies very very rapidly and in much larger numbers and this is what is meant by the Primary Response and the secondary response once so if we look a bit more at antibodies antibodies are an example of quaternary structure proteins because they're made up of four polypeptide chains the part shown in purple is described as the variable region and that is where you would have the part of your antibody that binds onto particular shaped antigens the part in red is constant so would be the same for different antibodies we can then see we have one longer chain which is heavier so we call it the heavy chain and we have a shorter chain which is called the light chain and this bit here where it is variable as we said that is the antigen binding sites now those antibodies are actually flexible and that enables an antibody to bind to multiple antigens so we can see we've got binding to two antigens here and then this antibodies binding to two as well and as a result they all end up clumping together and we call this agglutination the advantage of glutination is if you've got a big clump of antibodies and antigens which are going to be attached to the pathogen it makes it much easier for the phagocytes to locate and therefore destroy the pathogens more rapidly so we've talked about active immunity passive immunity though is when the antibodies are introduced to the body so you have made the antibodies yourself you are just gaining the antibodies so that means that the pathogen didn't cause the creation differentiation of plasma cells or any memory cells so you won't have any long-term immunity examples of passive immunity could be antibodies passed to a fetus through the placenta or through breast milk from the mother to their baby acts of immunity as we said is when the immunity is created by your own immune system following the exposure to pathogen but this can be either natural or artificial and natural is following an infection where you've actually had that pathogen within your body artificial is following the introduction of a weakened version of the pathogen or antigens via a vaccine so if we go through then how vaccines work small amounts of weakened or dead pathogen or it could just be the antigens are introduced either to the mouth or injected into the body and exposure to those antigens activates the B cells to go through clonal expansion and differentiation cells the canal selection part where we have mitosis occurring to make large numbers of those B cells those then differentiate into plasma cells and memory B cells as we said earlier the plasma cells will make antibodies but the key bit of importance for a vaccine is the fact that b memory cells will also be produced and those can stay in the blood for years so that means that if you are infected with the actual pathogen those memory B cells if they collide with the antigen will divide rapidly into plasma cells when you are reinfected and therefore you create large numbers of antibodies so rapidly that you shouldn't get the symptoms of the disease or if you do they'll be very very mild and you should be able to overcome the disease more quickly now not everyone is able to take vaccines for various reasons but the concept of herd immunity is if enough of the population are vaccinated the pathogen cannot spread easily amongst the population anymore so this provides protection for those who aren't vaccinated for example if you already have another illness which means it'd be too dangerous for you to have the vaccine if you have a lowered immunity if you're too young and sometimes if you're pregnant you're not able to take vaccines as well now the structure of HIV is made up of four key components we have the core and that is the genetic material which is RNA and the enzyme reverse transcriptase and this is needed for the viral replication when it's in the host you also have a capsid which is an outer protein which we can see just here you then have the envelope which is an extra outer layer made out of the membrane taken from the host cell membrane and then we have protein attachments on the exterior of the envelope to enable the virus to attach to the host's helper T cells so the way that HIV replicates in helper T cells is that first of all it's transported in the blood until it attaches onto a CD4 protein which is one of the attachment proteins on the outside of helper T cells which we can see just here the HIV protein capsule then fuses with the helper T cell membrane enabling that RNA and enzyme to be injected or enter into the host's cell the HIV enzyme reverse transcriptase copies the viral RNA that's been injected into a DNA copy and moves to the helper T cell nucleus and this is why it's called a retrovirus here the MRNA is transcribed and the helper T cell starts to create viral proteins to make the New Path articles someone subscribers HIV positive when they are infected with HIV AIDS is when they're replicating viruses in the helper T cells interfere with the normal functioning of the immune system so someone who is HIV positive doesn't necessarily have AIDS with the help of T cells being destroyed by the virus the host is unable to produce an adequate immune response to other pathogens and is left vulnerable to infections and cancer it is the destruction of the immune system that leads to death rather than the HIV directly so the very final thing is this concept of monoclonal antibodies mono meaning one and clonal meaning identical antibodies are proteins which we looked at and they have these binding sites complementary and shaped to a particular antigen and this has been manipulated to create monoclonal antibodies and that can be for Medical Treatments medical diagnosis it's used in pregnancy tests it's also used in tests such as drug testing and testing for other viral diseases so targeted medication we're going to go through that example first this is an example of direct monoclonal antibody therapy so what we mean by that is some cancer can be treated using monoclonal antibodies which are designed with a binding site complementary and shape to the antigens on the outside of the cancer cells the antibodies are given to the cancer patients and they will then attach onto the cancer cells only while the antibodies are bound to the cancer cells this prevents chemicals binding to the cancer cell which will enable uncontrolled cell division therefore The Binding of these monoclonal antibodies prevent the cancer cells from growing and as they are designed to only attach to the cancer cells they're not going to harm any other normal cells indirect monoclonal antibody therapy can also be used to treat some cancers the monoclonal antibody is created exactly the same except there will be a drug attached to them indirect monoclonal antibody therapy starts with the same process of creating a monoclonal antibody which has a binding site complementary in shape to the antigens on the outside of cancer cells but this time a drug is also attached to the antibody the Cancer drugs are therefore delivered directly to the cancer cells and can destroy those cells only and this reduces the harmful side effects that traditional chemotherapy and radiotherapy can produce and this is sometimes called a bullet drug because it's been directed to just one particular site medical diagnosis is another use and as we said this could be in pregnancy tests viral tests so influenza could be in bacterial tests as well cancer tests most recently we've seen in kobit 19 lateral flow tests as well and these use the Alyssa or the Elisa test so the Elisa test which stands for enzyme-linked immunosorbent assay or Elisa Elisa uses two antibodies so first of all the first mobile antibody which is complementary to the antigen being tested for and has a colored dye attached to it we can see just here part C we have a second antibody which is complementary in shape to the Androgen as well but this one is immobile so it's attached to this fixed position there is then a third antibody which is immobilized at Point D and it is complementary in shape to the first antibody so what this means is if you are pregnant if we think about this in terms of a pregnancy test you contain the hormone HCG within your urine that will then bind to the antibody and any antibodies with the HCG hormone bound to it will then attach to the immobilized antigen at Point C and therefore you get a colored blue line at this point because we have that blue bead held in that fixed position you should always have a second blue line whether you're pregnant or not at position D though because this second immobilized antibody is just complementary to the shape of the first one so it's basically just the control to check that you do definitely have that first antibody in your sample and it is able to move along the test kit now another test we can look at is this one here for the Elisa test where you would add the test sample from a patient to the base of a beaker wash to remove any Unbound test sample then we add an antibody complementary in shape to the engine you are testing the presence of in the test sample again we then wash to remove any Unbound antibody add a second antibody that is complementary in shape to the first antibody and binds to the first second antibody we can see here it has an enzyme attached to it again we have to wrench just to make sure if there's any Unbound antibody those are taken off and then the substrate for the enzyme which is colorless is added and this substrate produces a colored product when that enzyme is present so the presence of color indicates the presence of the antigen in the test sample and the intensity of the color indicates the quantity presence now there are some ethical issues surrounding monoclonal antibodies and that is because within the creation of monoclonal antibodies you do require animals such as mice to produce the antibodies and tumor cells and that can lead to ethical debates as to whether the use of animals is Justified to enable the better treatment of cancer in humans and to detect diseases because this will cause discomfort harm and death to the animals that are being used so we begin with surface area to volume ratio and this is where we have this particular formula of whatever the surface area of an organism is you divide it by its volume and the relationship between how large an organism is or it could be the different shapes it has are linked to the surface area to volume ratio so this concept has a big role within this topic where we look at how different cells or even organs organ systems have adaptations to maximize gas exchange and other types of transports across surfaces so here we just have an example where we can see this pattern of the larger an organism is now in this case we've just got cubes to demonstrate but the larger an organism is the surface area divided by the volume or the surface area to volume ratio will decrease so what that means in terms of biology is small organisms like single-celled organisms like amoeba have very large surface areas compared to their volume and that means that they don't have to have any additional special adaptations to meet their needs in terms of Transport across their surface they can just diffuse gases across their surface and because they've got such a large surface area compared to their volume they'll be able to get enough gases in and out to meet their metabolic needs however large organisms because the surface area is so much smaller compared to the volume that means that if we were just to get gases into the organism through simple diffusion the cells right in the center of the organism are so far away that the gases wouldn't be able to reach there in sufficient time for their metabolic needs now on top of that larger organisms typically have higher metabolic needs as well and what that means is the chemical reactions within a cell are happening faster so that is another reason why large organisms have to have adaptations for mass transport or just for transport across cells and that's what most of this topic is looking at the adaptations that organisms have for their exchange surfaces to make exchange more efficient so some of the key adaptations are the Villi and the microvilli So within the small intestines those two particular types of tissue and then cells to maximize the absorption of digested food the alveoli and bronchial so they're involved in gas exchange sphericals and tracheals involved in gas exchange but that's for terrestrial insects whereas alveoli and bronchioles are for mammals Gill filaments and lamelli is the gas exchange adaptation surface for fish and then we'll be looking at plants as well they exchange gases through this thermometer on the leaves but we'll be looking at some of those adaptations as well now in addition to that when we get the idea of the mass transport as well we'll see a link to gas exchange whereby most of these gas exchange surfaces have a capillary Network nearby as well with the exception of plants because they don't have blood so some of the key terms linked to the gas exchange and ventilation topic that you need to be familiar with are breathing so this is the movement of air in and out of the lungs this is different to respiration because respiration is a chemical reaction that results in the release of energy in the form of ATP ventilation is the scientific term for breathing so we'll be using the term ventilate quite a lot in this video and gas exchange is the diffusion of typically oxygen and carbon dioxide in and out of cells now it's not just the alveoli that is just for mammals so those are the key terms just to be familiar with and these the key structures that you need to know for the human gas exchange system so the alveoli bronchioles bronchi trachea and lungs which are all labeled here so if you need to pause just to note that down then pause that just here so we have a look at how humans ventilate it involves the muscle which is the diaphragm it also involves antagonistic muscles which are surrounding the ribs and this is where we have the external intercostal muscles but we also have internal intercostal muscles now antagonistic means that as one muscle is Contracting the other relaxes and we'll see that in our intercostal muscles how the internal and external will always be doing the opposite and that is what controls the movement of the rib cage in and out and to help with ventilation so a bit more about these internal and external intercostal muscles antagonistic muscles always occur as Pairs and the external intercostal muscle the outside layer of the ribs and they will contract and cause the rib cage to move out and air to flow in or inspiration the internal intercostal muscles you can kind of see as it moves around 3D they'd be on the inside layer around the ribs and when these ones contract that will pull the rib cage closer inwards and therefore the air will flow out or you'll be expiring but just to have a summary then of what is happening to all of the muscles that are controlling the movement of the ribcage and therefore whether you're inhaling or exhaling or inspiring or expiring so when you are breathing in that is when your external intercostal muscles will be contracting and that pulls the rib cage up and outwards that means that the internal intercostal muscles are doing the opposite they're relaxing the diaphragm muscle contracts and when that contracts it causes it to move downwards and it becomes more flat now as a result that means that the volume of the thorax will be larger so the volume increases and by that we don't mean the volume of gases we mean the volume is in the cavity space so the available volume the actual space available and because the lung volume the thorax space is larger that causes the pressure to drop and because you now have a lower pressure within the lungs compared to outside air is going to flow in so that is what happens when you breathe in now when you exhale it's exactly the offsets so this time the external intercostal muscles relax the internal will contract and that pulls the rib cage down and back in the diaphragm relaxes which causes it to dome upwards and as a result the actual volume in the thorax that space is smaller and because we have a smaller space or a smaller volume that causes the pressure to increase and that is why the air is forced out of the lungs now the pulmonary ventilation calculation is one that you do need to know and it's the total volume of air that's moved into the lungs during one minute and the units would be decimeters cubed for volume and per minutes so to work this out you would be doing the tidal volume times the ventilation rate so tidal volume is the volume total volume of air and the ventilation rate is how many times you breathe per minute so then if we go on to the gas exchange and look at the alveolar epithelium once the gases are in the alveoli so as a result of ventilation this is where gas exchange occurs and it's between the epithelium and the blood alveolia tiny air sacs and you have around 300 million in each lung and that's actually why lung tissue floats compared to let's say muscular heart tissue which would sink um so all of those alveoli air sacs make it flakes it's safe full of air now the alveoli epithelium cells are also very very thin and that is to minimize the diffusion distance so we can see here there's just one layer of cells and they're a thin layer each alveolus is surrounded by a network of capillaries and this helps to maintain the concentration gradients because as soon as that oxygen diffuses into the blood it is then carried away in the capillaries and replaced by deoxygenated blood terrestrial insects they do have a large surface area to volume because they're small however the issue they have to overcome is the fact that they have a waterproof exoskeleton and that's an advantage to prevent water loss but because it's made up of this hard fibrous material it also means gases can't exchange across it now insects don't have lungs but they do have a tracheal system which is where ventilation and gas exchange occurs so their tracheal system then we have a look at how this differs to the human lungs first of all it includes a trachea tracheals and sphericals the sphericals we can see here they're the tiny holes they're round valve-like openings that run along the abdomen and this is where oxygen and carbon dioxide are going to be entering and leaving so that is instead of like the nose or the mouth for humans and those sphericals are going to be attached to trachea so that's the next structure and the trachea are tubes they have rings within them to strengthen the tubes to stop them from collapsing and that keeps the tubes permanently open and then the trachea Branch into smaller tubes which are deeper in the abdomen of the insect and those are called the tracheals and those extend throughout all of the tissue in the insects to deliver oxygen to all of the respiron cells so here we have the sphericals connected to the trachea and then the tracheal systems branching all the way through into the center where the abdomen is and to all of the respiring tissues now there's three methods of moving gases into that tracheal system the first method is gases can just move in by diffusion and that is because when the cells within the insect are aspiring they're using up oxygen producing carbon dioxide and that creates a concentration gradient from the tracheals compared to the atmosphere so some of the gases will simply diffuse in and out the second method of gas exchange is mass transport and this is where it's slightly similar to ventilation in humans in that the abdominal muscles can contract and relax and that is a way to ventilate to move gases on mass in and out of the tracheal system the final method is when the insects are in Flight the muscle cells will actually start to respire anaerobically and when they do that they produce lactate or lactic acid and this lowers the water potential of the cells and as a result it draws water into the cells from the tracheals that decreases the volume in the tracheals and as a result more air from the atmosphere is drawn in so overall the adaptations that terrestrial insects have for gas exchange are the fact that they have a large number of These Fine tracheals provide in a large surface area the walls of the tracheals are very thin and there's a short diffusion distance between the sphericals and the tracheals so that will speed up the rate of diffusion and lastly they use oxygen and they produce carbon dioxide and that is how the Steep concentration gradient is maintained now the next challenge that the terrestrial insects have to overcome is limiting the amount of water that is lost at these gas exchange surfaces because a gas exchange surface is also ideal for evaporation and they do not want to be losing excess water so here are the ways that water loss is prevented number one is the surface area where gas exchange actually happens is very small so it's not the entire insect we're just looking at the spherical itself and then the tracheal system that is a very small area compared to the whole volume of the insect and therefore it's going to limit evaporation the rest of the insect has this waterproof exoskeleton and the sphericals can actually open and close a bit like stomata on a plant to reduce water loss also so next is gas exchange in fish now fish are waterproof and they have a small surface area to volume ratio so those are the two reasons why fish have to have a gas exchange surface with adaptations and for fish it's gills now fish obtained their oxygen from the water but there is 30 times less oxygen dissolved in water than there is available in the atmosphere in the air so they have to have an additional adaptation to help maintain the diffusion and concentration gradient across their gills so just as a reminder for all of these gas exchange surfaces they have to have these three features a large surface area compared to the volume a short diffusion distance and they have to have a mechanism to maintain the concentration gradients now those features can be used to work out the rate of diffusion using fixed law and this is just here so diffusion is proportional to the surface area times the difference in concentration divided by the length of diffusion pathway so you could get questions linked to this testing that math skill but let's have a look then at the fish skills to see how they have these three features maximized in their adaptations so we're going to have a look at the fish skill Anatomy so there are four layers of gills on both sides of the head and the gills are made up of stacks of Gill filament each Gill filament is covered in Gill lamelli and these are positioned at right angles to the filament so this here is the melee and this part is the filaments now because there are so many that creates the large surface area so that's the first adaptation ticked off because there's so many guilameli and Gill filaments it creates that large surface area now when a fish opens its mouth while it is swimming water rushes in and then actually comes out the side of their head which means that the water rushes over the gills so the water enters here and then it goes over the gills on both sides of the head where they have these four layers of gills with the Gill filaments and the Gill lamelli so the next thing is looking at the additional adaptations we've talked about the large surface area to volume ratio but the next thing is the fact that there is a short diffusion distance and that is because of the network of capillaries in every Gill lamelli so we're zooming in here on the Gill lamelli and there is our network of capillaries within each one and that provides a short Fusion distance how the concentration gradient is maintained is by something called the counter current flow mechanism so the count current flow mechanism is where water flows over the gills in the opposite direction to which the blood is flowing through the capillaries the reason this is an advantage is it makes sure that the equilibrium of the concentration of oxygen is never reached and because we don't have equilibrium being reached that means that oxygen is able to continue diffusing from the water into the capillaries in the Gila melee across the entire Gill lamelli and that's in bold and underlined because that is the key marking point now that's demonstrated in these two diagrams here where we look at concurrent flow in comparison to counter current flow so in concurrent not co-currence and you'd have the water and the blood flowing in the same direction so about halfway across the lamelli you'd be reaching equilibrium and therefore you wouldn't have any more diffusion of gases whereas here we have the water and the blood flowing in opposite directions which means you should never actually have equilibrium and there will always be a higher concentration of oxygen in the water compared to the blood and that is why we maintain the concentration or the diffusion gradient across the entire Gill lamelli so lastly is looking at gas exchange in leaves so you could be asked to look at some of the structures in the leaf so we have the tissue layer Palisade mesophyll which is where photosynthesis mainly happens and then we have the spongy mesophyll where there's lots of air spaces and we have the stomata which is where the gas is actually diffused in and out of so oxygen will actually diffuse out of the stomata if it's not being used in respiration and carbon dioxide diffuses in because it's needed for photosynthesis to reduce water loss by evaporation stomats are closed at night when it's dark and they'll open in the daytime when it's bright and this is linked to idiophage synthesis because it's a light dependent reaction now there are compromises though and in particular linked to xerophytic plants which are plants that are adapted to survive in environments with very limited water so they have adaptations to minimize water loss because there's such little water available already and over here looking at this Marin grass under the microscope it demonstrates some of those adaptations really well so the leaves aren't actually flat they roll up and we have stomata which are really deep and sunken in and we have lots of tiny hairs sticking out now all of those adaptations are to reduce water loss because any water that is evaporating out is going to get trapped on these hairs and the fact it's curled in it gets trapped in this space so it makes this area very very humid and if it's very humid that will reduce further evaporation now in addition to that they also have a thicker cuticle to reduce water loss and we can't see it on this particular image but they do have a longer root Network to be able to reach further distances to try and find any available water so we now move on to digestion and absorption and during digestion large biological molecules are hydrolyzed into smaller soluble molecules which can be absorbed across the cell membranes now there's three biological molecules that you need to know the digestion of and that's carbohydrates lipids and proteins so with the carbohydrates there's actually more than one enzyme that's needed to hydrolyze them from the polysaccharides to the monosaccharides we have amylases and membrane-bounds disaccharidases so digestion begins in the mouth it will then continue in the duodenum and then it's completed in the ilium and the first thing is the amylase that is produced by the salivary glands is going to be doing the digestion in the mouth but you also have amylase produced in the pancreas and that is going to be releasing the amylase into the duodenum and this hydrolyzes the polysaccharides into the disaccharide maltose and that is by breaking the glycosidic bonds between the molecules in the polysaccharide then we have suitcase and it could be lactase as well which are membrane-bound enzymes that hydrolyze sucrose and lactose into monosaccharides you could also have maltes which would be hydrolyzing maltose into glucose for proteins these are large biological molecules they're polymers and they are hydrolyzed by three enzymes we have the endopeptidases which hydrolyze the peptide bonds between amino acids in the middle of a polymer chain we have the exopeptidases which are going to do hydrolysis and break the peptide bonds between amino acids at the end of the chains and then the membrane-bound dipeptidasia will be breaking the peptide bonds between two amino acids so it hydrolyzes those dipeptides now where this is actually happening is firstly in the stomach so that's where it begins it continues in the duodenum and then it's fully digested within the ilium lipids are digested by lipase or lipase and also through the action of bile salts now lipase is produced in the pancreas and it can hydrolyze the estebonds in triglycerides to form monoglycerides and fatty acids what the bile salts do after they've been produced by the liver is emulsify lipids to form tiny droplets called missiles and these increase the surface area available for the lipase to bind onto and therefore act on so they're going to be making this digestion more efficient so we can see here the physical and the chemical digestion physical is the emulsification and that missile formation chemical digestion is the action of the lipase so the physical this is where the lipids are coated in bile souls to create an emulsion many of those small droplets of lipids provide a larger surface area to make hydrolysis faster and as we said the chemical is just the action of lipase now a muscle is a vesicle formed of fatty acids glycerol monoglycerides and bile salts but we're going to have a look at how that aids in the absorption of lipids so lipids are digested into monoglycerides and fatty acids by the action of lipase and the bile salts and that is how we create these tiny structures which are from the cells when the missiles encounter the ilium epithelial cells due to that non-polar nature of the fatty acids and monoglycerides they can simply diffuse across the cell surface membrane to enter the cells of the epithelium once in the cell these will then be modified back into triglycerides inside of the endoplasmic reticulum and the Golgi body and then they can form vesicles and be released from the cell into the lactial and be transported around the body so absorption in mammals is taking place in the ilium and this is where it links to the Villi and the microvilli and adaptations that these structures have so the ilium wall is covered in Villi and they have thin walls surrounded by a network of capillaries and they also are covered in epithelial cells that have even smaller microvilli on them and that is creating that large surface area the fact that the walls are so thin is providing a short diffusion distance and that network of capillaries is maintaining the concentration gradients now this actually links to as well topic two when you learned about co-transport and that is because the monosaccharides like glucose and amino acids can only be absorbed by active transport in the form of co-transport and that is because there's usually a higher concentration of either amino acids or glucose already in the epithelial cell so in order to be able to move even more from the ilium into the epithelial cell we require this co-transport but that theory is covered in topic two now hemoglobin is involved in the mass transport of oxygen around the body and it's an example of a quaternary structure protein because it's made up of four polypeptide chains you also have a range of different types of hemoglobins one that we'll be looking at is myoglobin which is found in the muscle tissue in vertebrates and also in fetuses the oxyhemoglobin dissociation curve is a way to look at how hemoglobin behaves in different conditions oxygen is loaded in regions with a high partial pressure of oxygen so what that means is when hemoglobin is in areas with a lot of oxygen available such as the alveoli it will be able to pick up lots of oxygen in regions with a low partial pressure of oxygen for example respiring tissues hemoglobin unloads the oxygen and that is how we get the shape of this oxyhemoglobin dissociation curve we can see the different partial pressures how saturated hemoglobin is with oxygen and the more saturated it is that means it must have loaded up with more oxygen the less saturated it is it must have unloaded oxygen and that's why it's not holding very much anymore so we can see here we've got our respiring tissues compared to the alveoli now this graph also demonstrates Cooperative binding and this Cooperative nature of oxygen binding to hemoglobin is due to the hemoglobin change in shape when the first oxygen binds it then makes it easier for the subsequent oxygens to bind now you can also see dissociation curves that demonstrate the bore effects and the bore effects is when high carbon dioxide concentration or partial pressure causes the oxyhemoglobin curve to shift to the right the affinity for oxygen decreases and that is because the acidic carbon dioxide changes the shape of the hemoglobin slightly and therefore it means the hemoglobin behaves differently and it's more likely to unload the hemoglobin even at the same partial pressures so we can see here we've got three curves at different PHS and the lower the ph the more carbon dioxide there would be presents so we can see here at pH 7.6 compared to let's say 7.2 the curve has shifted to the right and even at the same partial pressure so we'll pick 20. we can see that the saturation is only about 25 but for this one it's just under 60 percent and that is because of this bore effect so low partial pressure of carbon dioxide would typically be in the alveoli because you are exhaling that carbon dioxide a high partial pressure of carbon dioxide would be at respiring tissues because carbon dioxide is produced in respiration now that would be an advantage because it means that hemoglobin will behave differently it will unload oxygen more readily and therefore it's unloading the oxygen at the respiring tissues now different animals have hemoglobin adapted to their particular needs and environments also and that is one thing that you could get application questions on so a fetus and this is a human fetus they will have myoglobin or fetal hemoglobin and the fetal hemoglobin has an even higher affinity for oxygen even at the same partial pressures compared to adult hemoglobin and that's an advantage because it means that as the blood is circulating through the umbilical cord the fetus is hemoglobin is able to load the oxygen off of the mother's adult hemoglobin llamas are found at high altitudes where we have very very low partial pressures of oxygen so for the Llama we can see that they also have hemoglobin that has a higher affinity for oxygen even at lower partial pressures so we can see that would mean that if there isn't very much oxygen available which there wouldn't be a high altitude then the hemoglobin is still able to load up animals like doves for example their needs are that they need more oxygen to match their fossa metabolism because they're flying so much they need oxygen for the muscle contraction so the hemoglobin of a dove the curve actually is shifted to the right compared to human hemoglobin and that means that the hemoglobin has a lower affinity for oxygen and therefore it will more readily unload the oxygen which is needed for respiration earthworms a different example so they will be underground a lot where there's very low partial pressures of oxygen so they have hemoglobin that has very high affinities even at low partial pressures so that their hemoglobin can load up with whatever oxygen is available in mammals the circulatory system is described as closed and double closed meaning that the blood remains within the blood vessels the entire time and double referring to the fact that the blood passes through the heart twice in each circuit so there is one circuit that delivers blood from the heart to the lungs and the other circuit delivers the blood from the heart to the rest of the body mammals require to have a double circulatory system to manage the pressure of blood flow the blood flows through the lungs at a lower pressure and this is to prevent damage to the capillaries in the alveoli but it also means that the blood will flow at a slower speed so there is more time for gas exchange the oxygenated blood from the lungs then goes back to the heart and it's pumped out at a higher pressure to the rest of the body and this is important to make sure that the blood is able to reach all of those respiring cells in the body now the key blood vessels that you need to know about are first of all the coronary arteries and these are the arteries that cover the heart itself to supply the heart muscle or cardiac muscle with oxygenated blood you also need to know the four blood vessels that are delivering blood into and out of the hearts the vena cava aorta pulmonary artery and the pulmonary vein you need to know the blood vessels that deliver blood to the lungs and carry away so the pulmonary artery and the pulmonary vein so any time that you see pulmonary that is referring to the lungs and you can see here the pulmonary arteries carrying blood away from the heart to the lungs the lungs will then oxygenate the blood and the pulmonary vein is delivering that blood back into the heart the kidneys we have the renal artery and the renal vein so where you see the word renal that is referring to a blood vessel attached to the kidneys those major blood vessels are connected within this double circulatory system via arteries arterials capillaries and veins the cardiac muscle has a range of special features now the walls of the heart have a very very thick muscular layer so that it can contract with high Force to deliver high pressure blood to all of the body cells now the unique properties that the cardiac muscle has is first of all it's myogenic and that means it can contract and relax without nervous or hormonal stimulation it also never fatigues so as long as it has a constant supply of oxygen and glucose it will be able to respire aerobically the coronary arteries we can see here they are what Supply the cardiac muscle with this oxygen and glucose so that it never fatigues they Branch off from the aorta which we can see here and if one of those coronary arteries was to become blocked that would then mean that the heart muscle or the cardiac muscle wouldn't be receiving the oxygen or glucose therefore the cardiac muscle wouldn't be able to respire and it would stop Contracting and that would cause a myocardial infarction or in other words a heart attack so you need to know some of the key structures of the hearts first of all there are four chambers we have two atria at the top and we have the two ventricles at the bottom the Atria have thinner muscular walls and that's because they don't need to contract with as much force because they're only delivering the blood from the Atria into the ventricles they also have elastic walls that they can stretch when the blood is entering the ventricles have much thicker muscular walls and that is so they can contract with more force and pump the blood out at higher pressure because they are carrying the blood further distances either to the lungs or to the rest of the body now the right ventricle is pumping the blood to the lungs and that is at a lower pressure as we said to prevent damage to the capillaries and go at slow speed so comparatively the right ventricle wall has a thinner muscular layer the left ventricle has a much thicker muscular layer in its wall because it has to contract with more Force the pump of the blood at high pressure around the body highlighted here we have the four key blood vessels we've got the aorta and the pulmonary arteries the vena cava and the pulmonary veins but for some of them there's actually multiple so we can see we have the superior vena cava and the inferior vena carbon we also have the pulmonary artery which is carrying the blood away from the heart to the lungs but we have one coming out of the left and one out the right same with the pulmonary veins which is carrying blood into the left atrium from the lungs we have one on the left side one on the right side and the reason for both of those is we have a right lung and a left lung but aorta is carrying blood from the left ventricle to the rest of the body now some of the ways just to try and remember what each of these are doing are first of all if you see veins veins are carrying blood into the heart and vena cava means vein and Carver means body so that is carrying the deoxygenated blood from the body into the right atrium pulmonary resid means lungs and again it's a vein so it's carrying oxygenated blood from the lungs into the left atrium arteries think a awaits carrying blood away from the heart so pulmonary arteries carrying deoxygenated blood from the right ventricle to the lungs and the aorta is the major artery which is carrying oxygenated blood from the left ventricle to the rest of the body now the vowels that you need to know about are the semilunar valves which are in the aorta and the pulmonary artery as well as the atrioventricular valves which are between the Atria and The ventricle sometimes you'll see those called the bicuspid and the tricuspid valves and we can see these labeled here on the diagram of the hearts now valves are to prevent the backflow of blood and they do this by only opening when the pressure is higher behind the valve compared to in front so if the pressure is higher in front it causes the valve to shut and that is how it stops the blood from flowing backwards the septum is running through the middle of the heart to separate the blood on the deoxygenated and the oxygenated side and that's to help to maintain a high concentration of oxygen in the oxygenated blood to make sure that these diffusion gradients are maintained so that diffusion can occur at respiring cells looking at the blood vessels are connecting all of the major blood vessels together we have the arteries arterials capillaries and veins arteries as we said think a away they carry blood away from the heart towards the arterials the arterioles are smaller than arteries and connect to the capillaries the capillaries connect the arterials to the veins and the veins will then carry blood back into the heart so we can see here Our arteries connected to our arterials we then have this capillary bed or network of capillaries connecting to the venules and then the vein so if the structures of the arteries and vein are quite different because of the fact that one is carrying blood away from the heart and one's carrying it into so the pressure of the blood they're carrying will vary so arteries have a much thicker muscular layer compared to the veins and that is so that constriction and dilation can occur to control the volume of blood flowing through them they also have a thicker elastic layer than the veins and that's to help maintain the blood pressure so the walls can actually stretch and recoil in response to the heartbeats so overall the thickness of the wall is thicker because of those two thick layers but also it stops the blood vessels from bursting at that high pressure so we can see here how thick those walls are they also don't have any valves so in comparison veins have a much thinner muscular layer thinner elastic layer and thinner walls in general and that is because the blood is at lower pressure so they're not at risk of bursting however because the blood is at lower pressure they do require valves to help prevent the backflow of the blood to make sure that the blood is going to be pumped back into the heart the capillaries are very very narrow in diameter and this is to make sure that the blood speed is going to slow right down as it's passing through the capillaries and that is to allow time for gas exchange and tissue fluid formation in the capillaries so we've already talked about the arteries and the veins but if we now add in the arterials and capillaries the arterials are thicker than in the arteries the muscular layer and that is to help restrict the blood flow to the capillaries the elastic layer is thinner than the arteries and that is because the pressure is now slightly lower and overall the wall is thinner compared to the arteries because the pressure is slightly lower but they still don't have valves capillaries do not have any muscular or elastic tissue layer they are only one cell thick and that is to make sure there's a really short diffusion distance because the function of the capillaries is that is where exchange of materials between the blood and the cells occur the cardiac cycle is split into three stages now people pronounce this differently so I'm going to say both diastol or distally atrial systole or atrial systole and ventricular systole or systole and we're going to go through what happens at each stage so in diastole the Atria and ventricle muscles are relaxed and this is when blood will enter the Atria through the vena cava and the pulmonary vein and the blood flowing into the Atria then causes an increase in the pressure because we've now got a larger volume of liquid there then we get atrial systole occurring and that is when the Atria muscles contract and that increases the pressure even more in the Atria and because we now have this high pressure that causes the atrioventricular valves to open and blood moves from the Atria into the ventricles and at this stage The ventricle muscles are relaxed we have ventricular diastole the last stage then we have is ventricular systole which is when The ventricle muscles contract and that happens after a short delay and it will increase the pressure beyond that of the Atria which causes these atrial ventricular valves to shut but when they've contracted high enough to increase the pressure above that of the Atria and pulmonary artery the semilunar valves will open and that causes blood to be pumped out of those two blood vessels you could be asked to calculate the cardiac outputs and that is the volume of blood which leaves one ventricle in one minute and it can be calculated by doing the heart rate times the stroke volume and the heart rate is Beats of the heart in one minute strength volume is the volume of blood that leaves the heart each beats and that's in decimeters cubed tissue fluid is a fluid that contains water glucose amino acids fatty acids ions and oxygen and it's the liquid that is forced out of the capillaries to bathe the cells to make sure the cells are getting all of the minerals nutrients that they require from the blood now it's formed due to the fact that capillaries have very very small gaps between each of the cells that make up the capillary walls and as the blood enters the capillaries from the arterials the smaller diameter results in a very very high hydrostatic pressure of the blood and because we have this High hydrostatic pressure and Tiny gaps between the cells that make up the capillary walls water and small molecules are forced out so glucose amino acids fatty acids ions oxygen are all forced out of the capillary to the surrounding cells and that is called ultra filtration so the molecules that can be forced out we just went through all of those but anything that is too big cannot get out of those tiny gaps between the capillary cells and that would be things like the red blood cells platelets and large proteins so those will remain in the blood and the fact that they remain in the blood is actually what enables the liquid from the tissue fluid to be reabsorbed again so towards the venue end of the capillary which is over here the hydrostatic pressure has decreased and that's because so much liquid has been forced out the pressure within the capillary has now dropped by the time you got to The Venue end of the capillary however we don't have as much liquid but we have lots of those large proteins that have remained behind which has lowered the water potential of the blood in the capillary compared to the water potential in the liquid surrounding the cells which is the tissue fluid and as a result that liquid is going to or the water is going to re-enter by osmosis and it will carry with it waste that has dissolved in that tissue fluid that was released from these cells so things like carbon dioxide and urea so that gets absorbed into the capillaries barsmosis and then it'll be transported around the body to be removed as waste now not all of the liquid will be reabsorbed by osmosis because eventually equilibrium will be reached and the water potential inside of the capillary and of the tissue fluid will be the same so any of that water that doesn't get reabsorbed by osmosis will enter the lymphatic system instead and then when we have it transported around the lymphatic system it will eventually get to one of the lymph vessels near the heart where that liquid is drained into the blood again and that is how we don't run out of liquid in the blood so the last bit of topic three is mass transport in plants and we start with the mass transport of water looking at first of all transpiration so transpiration is the loss of water vapor from the stomata so water evaporates out of stomata on the leaves and there are four key factors that affect the rate of transpiration first of all light intensity the more light there is present the more stomata will open and therefore there's a larger surface area for evaporation if it's hotter that means that the water molecules will have more kinetic energy and therefore they'll be moving faster and evaporate at a faster rate with humidity the more water vapor that is in the air that will actually make the water potential more positive outside of the leaf and it reduces the water potential gradient so what that means is the more humid it is the less transpiration there will be occurring with wind because the wind is carrying away air that contains water vapor the wind will actually be maintaining the water potential gradients so the more windy it is the faster the rate of transpiration so how water actually moves from The Roots all the way up the xylem to transpire out of the stomata links to this idea of the cohesion tension Theory so water has to move up the plant from The Roots Against Gravity and that can be a very very large distance if it's a huge tree and this is possible because of the cohesion the capillarity or adhesion and root pressure so if we look at cohesion first this links to the theory of water is dipolar meaning we've got two slight charges two different charges a slight negative on the oxygen and a slight positive on the hydrogen atoms and that means hydrogen bonds can form between the hydrogen and oxygen of different water molecules and it creates this cohesion or sticking together of the water molecules now that's an advantage because as the water moves up the xylem that means it's all bonded together with these hydrogen bonds and it moves up as a continuous column of water and it's much easier to pull up a column than it is individual water molecules the next idea was this idea of adhesion whereby the water molecules will actually also stick to the walls of the xylem and if you have a really narrow xylem that will increase this capillarity effect and therefore the liquid will be moving up more just from this sticking this adhesion effect so the narrower the xylem the easier it's going to be to transport the liquid up the xylem Against Gravity root pressure is the final concept and this is as the water moves into the roots of Osmosis it increases the volume of liquid inside the roots and therefore the pressure inside the root increases as well and we call that root pressure and that creates this positive pressure which means it's a pushing force because we have lots of water in the roots it pushes all of the water above it upwards so it helps push the water up the xylem so those three concepts all work together for the cohesion tension Theory and that's how water moves up against gravity so just have a look at this at a hole the first thing that happens is the water evaporates out of the stomata and as that water vapor is leaving that is then leaving behind this lower pressure because liquid has been lost and that creates this negative pressure or in other words a pulling force and that pulls the water column up the xylem and because of cohesion we have that water column whereby all the water molecules are stuck together the water molecules are also adhering or sticking to the walls of the xylem to help pull the column upwards and as the column of water is pulling up it also creates tension in the xylem which actually pulls the xylem inwards making it narrower which increases that impact of the capillarity or the adhesion and it helps to move the water up even more now the second type of mass transport in plants is looking at how the organic molecules like glucose produce in photosynthesis are transported around and this now happens in the phloem now the phloem tissue contains two key cells we have the sieve tube elements and the companion cells and the sieve cheap Elements which are here are living cells but they don't contain any nuclei and they're very few organelles and that is so it's pretty much hollowed to make it easy for the solutions to transport through the tube the companion cells are on the outside and they provide the ATP required for active transport of the organic substances so they have all the organelles that the sieve tube elements don't so they can provide the resources needed now the transport of these organic molecules within solution is often explained using this source to sync model so we have here the xylem next to the phloem and the source in this model would be a photosynthesizing cell the sink which is where the sugars are going to be delivered is a respiring cell so at the source we have photosynthesizing cells and the sucrose or glucose that have been made in photosynthesis is going to lower the water potential of those cells and therefore any surrounding water in the plant or from the xylem is going to be entering those cells by osmosis at the other end where we have our respiring sink cells because they'll be using up those sugars in respiration there will be a more positive water potential inside of the cell compared to outside and therefore water is going to leave the respiron cells by osmosis to other cells in the plant or even the xylem so the effect that has is there'll be an increase in the hydrostatic pressure in the source cell but a decrease in the zinc cell and because of those pressure changes the liquids that are within that Source cell will be forced by that high hydrostatic pressure through the xylem all the way to the zinc cell now that is part of the story but also you need to know the translocation steps so how those sugars within the leaf cell actually make it into the phloem for that pressure then to actually move the liquid along so photosynthesis is occurring in a chloroplasts in the leaves and we're calling those The Source cells that we said creates a high concentration of sucrose now as well as that affecting the water potential that also means that the sucrose can diffuse down its concentration gradients into the companion cell by facilitated diffusion we then get active transport of protons or hydrogen ions from the companion cell into the space within the cell walls and that uses energy because it's active transports now that creates a concentration gradient and therefore the protons move down their gradient via carrier proteins into the sieve tube elements and the co-transport of sucrose with those hydrogen ions occurs via a protein co-transporter and that is how the sucrose goes from being in the companion cell into the phloem even though there is a high concentration often of sugar or sucrose in the phloem already so the next step then is we're looking at the movement of that sucrose within the phloem SIV tube element so the increase of sucrose in the Civ tube element is going to lower that water potential and therefore water enters those sieve cheap elements from design and vessels which are tightly compacted next to the phloem the increase in water volume in the sieve tube element at this position will increase the hydrostatic pressure and this is where we have the idea of source to sink it causes the liquid to be forced from The Source area down to the zinc cells lastly the sucrose is then used in respiration at the sink or it might be stored where to start if it's not currently required but that means more sucrose is actually transported into the zinc cells and that is going to cause water potential to decrease and as a result um we'll have water moving biosmosis from the sieve cheap elements into the zinc cell some water will also be returning from the Civic element into the xylem the removal of that water is decreasing the volume in the sieve tube element and therefore the hydrostatic pressure decreases so the movement of soluble organic substances is due to the difference in the hydrostatic pressure between the source and the sink end of the sieve tube element now you could be asked about two particular investigations that prove translocation the first one is called traces and this is where we have tracing involving radioactively labeling carbon plants are provided with only radioactively labeled carbon dioxide and over time they'll be absorbing that in through the stomata using it in photosynthesis and the organic substances like the sugars created will all contain that radioactively labeled carbon thin slices from the stems are then cut and placed on x-ray film that will turn black when exposed to radioactive material when the stems are placed on the X-ray film the section of the stem containing the sugars turn black and this highlights where the phloem are and it can show the sugars are transported in the phloem and it also means you can track the root that is taken ringing experiments is when a ring of bark and phloem are peeled and removed off the trunk like we can see here the result of removing the phloem is that the trunk swells above the removed section and Analysis of the liquid in this swelling shows it contains sugar so this shows that when the phloem is removed the sugars cannot be transported and it therefore proves the flow of transports sugars so we start topic 4 by comparing the DNA in eukaryotic cells and prokaryotic cells and comparisons mean similarities and differences so our two key similarities are the fact that the DNA is made up of DNA nucleotides for both meaning it contains deoxyribose a phosphate group and a nitrogenous base the nucleotides are also joined together by phosphodiester bonds to make the polymer chain the three key differences are that eukaryotic DNA is longer eukaryotic DNA is linear so it occurs as straight lines in the chromosomes whereas prokaryotic DNA you get circular Loops of DNA the DNA in eukaryotes is associated with histones but for prokaryotic DNA it is not associated with proteins those histones so those are your key differences in the DNA and this is just showing you here the organization of the DNA in a eukaryotic cell so we have our chromosomes within the nucleus that is tightly coiled to make the chromosome if we unwind that you can then see the nucleosomes which is actually the DNA wrapped around the histone proteins and that is how we tightly coil to fit it all in the nucleus now you also need to be able to compare the DNA that is found in the mitochondria and chloroplasts of cells to the DNA that you find in prokaryotic cells because they actually have quite a few similarities so we can see here our mitochondria and you have just a loop of DNA and the same within your chloroplast you just have this circular Loop of DNA and they have their own DNA so that they can transcribe translate their enzymes that they need for photosynthesis and the chloroplasts and respiration in the mitochondria so the similarities are they are both short sequences of DNA they are both circular and neither of them are wrapped around histone proteins so that then takes us on to looking at what a gene is and a gene is a sequence of DNA and it codes for the amino acid sequence for a particular polypeptide and also a functional RNA so it codes for an mRNA molecule and we can see here one of the extra key terms as well and that is Locus and Locus is the exact position that one particular Gene is found on a chromosome so Locus is location so that's the way to remember it Locus location of the gene okay so the genetic code you need to know a few features of the genetic code one thing first of all is knowing that a sequence of three bases on DNA is called a triplets and those three bases will code for a particular amino acid so the three features are it is a degenerate code it's Universal and it's non-overlapping so if we ever think about this idea of degenerates there are 20 amino acids that we said exist and we have four possible DNA bases and the way they actually worked out that it was three DNA bases that codes for one amino acids was actually mathematically they looked at all the possible code options you could get if you only had one base coding for one amino acid and that would only give you four possible codes GCT or a that's not enough to code for 20 different amino acids so then if you think about could it be two bases if you had only two bases that would give you 16 possible different codes and that's still insufficient so they realized it must be three bases that code for one amino acid because that actually gives you 64 possible different options for what those Triplets of bases could be and that is more than enough to code for the 20 amino acids and that is why the genetic code is degenerate and what we literally mean by this definition is there are more than one triple of bases that codes for the same amino acid that would be your key definition and this table here is just showing you an example of that so you can have a look for example at let's look at glycine here gly g g g g g a GGC ggu all four of those triplets are bases code for Glycine and this is an advantage of the genetic code because if there is a mutation and one of the bases in a triplet is changed you might still have the new triplet coding for the same amino acid and therefore it has no effect on the overall polypeptide chain Universal means that the same triplet of bases codes for the same amino acid in all organisms non-overlapping is the fact that each base is only involved in one triplet so if we just draw boxes around this to show you what I mean this base a is only in this one triplets and this C is only in the triplet this G is only in this triplets we don't have this G also making up a second tripler basis so every codon or tripler bases is read as a discrete unit this is an advantage as if a point mutation occurs it will only affect one codon and therefore one amino acid so it will minimize any potential harm now in your DNA you have sections of Base sequences which are introns and you have sequences of DNA that are exons introns are sequences of DNA bases that do not code for polypeptides and you actually have a lot of introns making up your nuclear DNA the exons are sequences of DNA bases that do code for the amino acids so the exons are the codeine regions and when we say codon a codon is three bases on mRNA that codes for a specific amino acid a start codon is three bases that you find at the start of every Gene and that is what initiates translation to occur a stop codon is the final three bases that you have at the end of every Gene and those three bases will cause the ribosome to detach during translation and therefore it stops the translation of the polypeptide chain a genome is what we call an organism's complete set of genes in a Cell so that is your definition of a genome whereas the proteome is the full range of proteins that a cell is able to produce the genome should never change unless there are mutations whereas the proteome of a cell can constantly change depending on which proteins are needed in a specialized cell because you'll have some genes have been switched off or on and that's what makes it specialized the Genome of an organism will really differ between different species so for example a bacteria contain on average 600 000 DNA based pairs within their genome whereas humans we have three billion DNA base pairs so this then starts to take us onto RNA before we get into protein synthesis so messenger RNA is what mRNA stands for and we can see it here on the picture it is short compared to DNA because it's only a copy of one gene whereas the DNA is the entire genome it's single stranded and it's found in both the cytoplasm and the nucleus so it's made during transcription and that happens in the nucleus but then once it's been modified it leaves the nucleus enters the cytoplasm to attach to a ribosome it's three bases on mRNA that are codons so three bases which can code for a particular amino acid TRNA is Transfer RNA and this is found in the cytoplasm it has an amino acid binding site which we can see up here at the top and each TRNA molecule will have a particular or specific amino acid attached to that binding site the TRNA molecule also has three bases on it at the bottom here and we call those three bases the anticodon and they will be complementary to a particular codon on mRNA and when those align they're held in place so that amino acids can start to bond together during translation so TRNA is involved in translation the second stage of protein synthesis a ribosome will be holding it in place to enable the joining of amino acids it has this Cloverleaf shape that's what we call this shape and we can see here these lines are representing hydrogen bonds so it's still single stranded it's just folded to create this shape and that shape is held in place by hydrogen bonds so that then takes us onto protein synthesis and it's split into two steps transcription which is where one gene at a time from DNA is copied into mRNA then we have translation where the MRNA will join with a ribosome and corresponding tRNA molecules will then bring specific amino acids so first of all we have transcription so a complementary mRNA copy of one gene on the DNA is created in the nucleus mRNA is much shorter we've already said and that is because it's only copying one particular Gene and therefore is able to leave the nucleus because it is smaller your key steps are all here so if you did have a long answer question describe the process of transcription these would be your key marking points with the key marks put in bold so first of all the DNA Helix unwinds to expose bases and you have one strand acting as a template and that's our second Mark like with DNA replication that is caused by DNA helicase breaking the hydrogen bonds in the nucleus you then have free floating mRNA nucleotides and they will align opposite their complementary DNA base pairs on the template strand the enzyme RNA polymerase will then join together those RNA nucleotides to create the MRNA polymer chain once it's copied it then has to be modified and then it leaves the nucleus via a nuclear pore the modification that happens is this here splicing in eukaryotes after transcription we actually call the molecule pre-mrna and that is because it still contains the introns which are those non-coding sequences of bases and that's because the DNA the gene that was copied there will be introns within it which are the non-coding sequences so the RNA that is copied will still contain those introns so the introns need to be removed and we call that splicing they're spliced out so cut out they're spliced out by a protein called a splicosome and now we have finished mRNA that is ready to leave the nucleus that stage doesn't happen in prokaryotes because they don't have introns translation is the next stage in the creation of the polypeptide chain and it involves both mRNA and TRNA if you are asked to describe that whole process again these are your six key marking points and in bold are the key terms you would have to include once modified mRNA is left the nucleus it will then bind to or bind with a ribosome in the cytoplasm the ribosome will attach at the start codon of the MRNA molecule the tRNA molecules with complementary anticodons to the start codon will then align opposite and they're held in place by the ribosome which we can see here in the picture the ribosome holds together two tRNA molecules at a time the two amino acids that have been delivered by the TRNA molecule are joined by a peptide bond and that reaction does require energy in the form of ATP and an enzyme but you don't need to know the name of it once that happens the TRNA molecule will be released and the ribosome moves along one codon so the next TRNA molecule can then align its anticodons to its codons so this continues until the ribosome reaches the stop codon at the end of the MRNA molecule and when it does that causes the ribosome to detach and therefore translation ends now the modifications because we now just have a polypeptide chain the modifications will occur in the Golgi body for folding to create that secondary tertiary or quarternary structure we then move on to how variation is introduced and G mutations is one way a change in the base sequence of DNA is what a gene mutation is and they randomly occur during DNA replication So within the in interface part of the cell cycle these random mutations are more likely to occur if you're exposed to mutagenic agents which can interfere with the DNA replication that includes high energy radiation like UV light ionizing radiation like gamma rays and x-rays and also some chemicals which we call carcinogens for example mustard gas and cigarette smoke a g mutation can result in either a base being deleted or swapped so substituted for a different base so here are our examples we have our original DNA sequence this is shown a substitution instead of cytosine that has been swapped or substituted for adenine this one is shown a deletion because that base C has now been deleted and that's actually caused what we call a frame shift everything Downstream of the mutation has shifted to the left a base mutation they might have no impact at all because the new codon may still code for the same amino acid and that's because the genetic code is degenerate chromosome mutations can also occur and chromosome mutations are changes in the number of chromosomes and this spontaneously occurs during meiosis in a process called non-disjunction say non-disjunction is when the chromosomes or it could be the chromatids do not equally split during anaphase of either meiosis one or meiosis II so that's what we can see here non-disjunction occurring because the chromatids didn't separate and instead all of them are being pulled to the same pole of the cell now this can occur in two forms either a change in the whole set of chromosomes which we call polyploidy or changes in the number of individual chromosomes which is aneuploidy so we'll go through polyploidy first which we said is a change in the whole sets of chromosomes so you could end up instead of being diploid having two copies of every chromosome which we have in humans you could have three copies or four copies of every chromosome which would be called triploid or tetraploid now in humans that would be fatal you don't see triploid or tetraploid humans but it's actually quite common in plants so how this would occur then each homologous pair is doubled in replication and that happens in interphase in this example we have non-disjunction in meiosis one for some reason the spindle fibers haven't attached to the chromosome on this side and they have attached to all of the chromosomes on the other side so when the spindle fibers retract it's going to pull all of the chromosomes to one side of the cell and therefore they're all going to be in this cell and there'll be no chromosomes in the next one in meiosis II that would mean that these two gametes will contain no chromosomes at all so those gametes will not function these gametes though meiosis II is happening normally and we do have complete separation of all of the chroma tids but we now have two copies of every chromosome in the gametes so instead of having a haploid gametes we have a diploid gametes and if a diploid gametes fuses with a haploid gamete that is how we then end up with three copies so we get two chromosomes from this gamete instead of one and we get just one chromosome from the haploid gametes so that is polyploidy changes in the whole sets of chromosomes that could also happen if you have non-disjunction in meiosis II in this example we can see the chromosomes in meiosis one did separate equally and then we had normal meiosis 2 in this example so we have two haploid gametes but for this cell in meiosis II there was non-disjunction so the spindle fibers didn't form on this side so the chromatids aren't separated equally and instead they're all pulled to this cell so again we end up with a two n gamete to diploid gametes and this gamete has no chromosomes in anuploidy this is different this is when you have changes in the number of individual chromosomes so sometimes individual homologous pairs of chromosomes fail to separate during meiosis it's still called non-disjunction but instead of it being affecting every single chromosome or chromatid it's just one and this is how Down syndrome occurs you have non-disjunction on chromosome 21 so you end up with three copies of that chromosome instead of two so let's see how that might occurred we can see in this one we have non-disjunction occurring at meiosis one because these spindle fibers for just that one chromosome or that one homologous pair of chromosomes is attaching and it pulls them both to this cell and this cell does not get a copy of that red chromosome if meiosis II occurs normally so no non-disjunction all of the chromatids are separated equally however because of the non-disjunction of myosis one this gamete is haploid it has one copy of all the chromosomes except for the red so we describe that as haploid plus one extra chromosome so n plus one these two are still haploid but they're missing a chromosome so we describe it as n for haploid minus one now if an M plus 1 so a haploid with an additional chromosome is to fuse with a haploid chromosome that is how you can get Trisomy which means three copies so try trisomies three three copies of one particular chromosome and that is how Down syndrome occurs three copies of chromosome 21. now you could also have non-disjunction occurring in meiosis II so we can see their normal cell division occurred in the first round of meiosis but now we have non-disjunction in meiosis II because the chromatids are not separated equally for the red chromosome they're all pulled to this one gametes so that would be n plus one and this one is missing the red chromosome so it's n minus one now another way that variation can be introduced is in meiosis and meiosis creates gametes and it creates four genetically different haploid gametes by two nuclear divisions so meiosis is how variation can be introduced as well and that's through two mechanisms independent segregation of homologous chromosomes and crossing over between homologous chromosomes and both of these occur within the first round of division in meiosis independent segregation is when the homologous pairs of chromosomes line up opposite each other at the equator to form bivalence this is random which side of the Equator the paternal and maternal chromosomes from each monogous pair a line so we can see on this side by chance two purples Two Reds but equally it could have been a purple and a red red and a purple in meiosis those homologous pairs of chromosomes are separated in meiosis one so one of each homologous pair ends up in the daughter cells eventually this creates a large number of possible combinations of chromosomes in the daughter cells produced and it can actually calculate this by doing 2 to the power of n 2 because it is homologous pairs so your pairs are chromosomes and N you would substitute in for how many homologous pairs of chromosomes that species has say for humans that would be 2 to the power of 23 we have 23 pairs of chromosomes which means we can make over 8 million different possible gametes just from independent segregation now crossing over also occurs sometimes it's actually quite rare but it can occur and again it occurs when the homologous pairs of chromosomes line up at the equator and form by valence which is what we call it when you have both of them next to each other and you have chromatids from each of those chromosomes cross over and they can get twisted around each other that puts tension on the chroma tids causing part of the chromatid to break and swap and in doing that we create new combinations of alleles which is represented by the letters here so originally this chromosome only had the dominant allele but now we've got a dominant and recessive someone that's pulled apart we've got new combinations of alleles on that chromosome now comparing meiosis and mitosis meiosis is two nuclear divisions whereas mitosis is only one that's why meiosis results in haploid cells whereas mitosis is diploid cells meiosis introduces genetic variation through crossing over an independent segregation mitosis creates genetically identical cells now you could be asked to identify meiosis in an unfamiliar life cycle and what you need to do here is look for where you have cells that were diploid or 2N dividing to then create cells that are haploid because that's what happens in meiosis you go from 2N to n It won't always be gametes because not all organisms have life cycles like humans where it is the creation of the gametes that is meiosis so for example we can see here we have the zygote which is 2N and then it makes something called Zeus Force which are n so that would be the meiosis stage so that's what you're looking for 2N moving to n so again that there would be meiosis genetic diversity this is the number of different alleles of genes in a population and this is what enables natural selection natural selection can only occur if there is genetic diversity natural selection is the process that leads the evolution and our definition of evolution is the change in allyl frequency over many generations natural selection is really important to the survival of the whole species because it results in the species becoming better adapted to their environment and these adaptations might be anatomical physiological or behavioral so the key marking points that you'd need to describe for this process are first of all you would have new alleles for a gene being created by random mutations if those new alleles increase the chances of the individual surviving in that particular environment then they're more likely to survive and therefore more likely to reproduce and pass on that new advantageous allele to the Next Generation over many generations of that occurring that new allele will now become more common in the gene pool or in other words we've increased the allele frequency now the types of selection that occur are directional selection and stabilizing directional selection is when the advantageous allele is coding for an extreme trait so this links to your antibiotic resistance example and if we think about the traits being low resistance medium high resistance the extreme traits would be either very low resistance or very high resistance and in the case of antibiotic resistance when there was a change in the environment which is Introduction of antibiotics the bacteria which had the alleles for high resistance antibiotics were more likely to survive and pass on that allele for antibiotic resistance and that's why we saw this shift and antibiotic resistance allele became far more common in these species stabilizing selection is when whatever is the middling trait Remains The Selective advantage and that would be the case if there's no change in the environment and this is exemplified by human birth weights so being a middling birth weight increases your likelihood of surviving because if you're very very small then you might be very premature and might have under underdeveloped lungs difficulties regulating your temperature are more vulnerable to infection if you're very very heavy then it's going to be a more complicated birth and that could result in difficulties so that would mean that the modal trait Remains The Selective advantage and we can see that the range of traits or alleles decreases over many many generations the range and the standard deviation decrease next then it's thinking about species and taxonomy so the species is when two organisms are able to produce fertile offspring and a species must reproduce and pass on advantageous alleles in order for the whole species to be able to survive and this is where courtship Behavior comes in this behavior is essential for successful mating meaning mating and creating fertile offspring in order for species to survive so courtship behavior is a sequence of actions which is unique to every species so it is genetically coded for this behavior and it is how animals are able to identify members of their own species to make sure they are reproducing with members of their own species to make sure that they can create fertile offspring the behavior the sequence of actions is normally carried out by the male and then the female picks of whether they are worthy of mating with now this sequence could include dance moves creation of sounds release of pheromones display of feathers fighting whatever it is is always a unique sequence to that species the female then observes the ritual and decides if they look like they have a good enough set of alleles based on their Fitness their performance to mate with so the reason this is important is we said to ensure successful reproduction and the way it does that is first of all because the behavior is unique to every species it allows them to recognize members of their own species and also the opposite sex that is because to make fertile offspring you'd need sperm and egg to fuse it also synchronizes mating behavior and what this means is it makes sure that the male and the female are mating when they are sexually mature so the female is releasing eggs and the male is able to produce and release sperm it can actually also help the survival of The Offspring once it's born in some animals and that's because this Behavior this courtship ritual can help form a really strong bond which we call a pair bond between the parents so they're more likely to stay together and if they are together for some animals like penguins it increases their likelihood of survival because you need one penguin to look after the chick one penguin to go and find food it also enables that strong and healthy mates are selected for and therefore the advantageous alleles are being passed on to the next Generations to ensure the survival of the entire species so this could be used as a way for us to identify how closely related different species are as well so for example we've been shown three Ducks here and the sequence of their courtship Behavior and we can see that ducks 1 and 2 must be more closely related because their sequence of behaviors in the courtship ritual are more similar than duck one and three or Ducks two and three and because this behavior is genetically coded for because the sequence of behaviors is more similar they're DNA based sequence is likely to be more similar to phylogenetic classification is another way to look at how closely related different species are and also how recent their shared common ancestors were phylogenetic classification it's arranging groups according to evolutionary origins and relationships so humans and chimpanzees we can see are most closely related to each other because they branch in the tree most recently compared to the others so that means they evolved from a shared common ancestor more recently than they did compared to any of the other species so that means they have had less time to accumulate different mutations in the human and chimpanzee populations compared to let's say the human and the horse population because humans and horses their recent common ancestor is back here and there's no time scale on this but this is normally going back like this would probably be at the very start here this could be looking at maybe 13 20 million years ago and so on so it's millions of years of accumulating mutations in these separate species now you can also classify and group using a hierarchy and a hierarchy is when you have smaller groups arranged within larger groups and there's no overlap between those groups so this is one particular hierarchy you need to know off by heart domain kingdom phylum class order family genus and species we can see here that within one genus you can have multiple different species so that is our smaller groups within larger groups but there's no overlap in those species they're still distinct groups the binomial system is a universal way of identifying organisms and it's using two names that's what binomial means the first name is the genus and the second name is the species so for humans our genus is homo and our species is sapiens so our binomial name is homo sapiens and we can see an example here of common names which would be unique to every language versus the binomial system which is universally used and that gives you more information on how closely related they are because calling them both Robins is misleading but they are closely related but actually we can see they're not the same species and they're not even the same genus so the final thing is looking at biodiversity and there's different ways that you can classify and measure biodiversity it could be looking at the range of different habitats that exist it could be the variety of genes amongst all the individuals of a population of one species or it could be looking at the number of different species and individuals within each species in a community species diversity as well if we go into that further that takes into account species richness and species richness just means the number of different species in a community biodiversity can be used to describe a range of habitats you could be looking at the biodiversity of a very small local habitat like a forest or it can be talking about the entire Earth having a low biodiversity isn't actually a cause for concern for example in the Arctic or deserts you would expect that to be low but if you have a decrease in biodiversity that is of course for concern because that could be caused by human activity causing destruction and farming techniques is an example of that farming techniques can reduce biodiversity but it is needed to provide food for humans so we have to come up with some kind of compromise between the two you could get application questions where you have to suggest different farming techniques that could reduce the biodiversity so I've got some of those listed here and if you want more detail on that then go to my actual biodiversity video to hear all of the examples and how it reduces the biodiversity and what the compromise would be the way that we measure biodiversity is using the index of diversity and that describes the relationship between the number of species in a community which is the species richness and the number of individuals within each species so this is the formula to calculate it and they would give you that in the exam but they won't give you this that capital N is the total number of organisms of all species lowercase n is the population size of one species one is the lowest value you can get and the larger the value the greater that index of diversity the greater the biodiversity so here's an example if you want to have a go pause the video now and write it down and then have a look at the answer if not let's go through so the first bit the formula we're going to look at is the lowercase n times n minus 1. so that would be 6 as our lower case the population size of species a and N minus 1 that'd be five six times five would be 30. so we then do that for all of the others and we need the sum of that so the sum of that column is 180. capital M we said was the total number of all individuals and in this example it's 25. so 25 times 24 divided by 180 is 3.3 genetic diversity within or between species can also be measured by comparing different factors so you can compare observable characteristics but that can actually be quite inaccurate because members of different species that aren't even closely related might look similar because they live in similar environments so more accurate ways to compare how closely related species are is through comparing the DNA based sequence the the MRNA based sequence or the amino acid sequence for proteins and the more similar those sequences the more closely related they must be so that is it for topic four I hope you found it helpful and if you 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