this is an updated version of my summary revision video for aqa gcse combined science biology paper 1 which covers cells organization infection and bioenergetics this video gives you a brief overview of all the topics on the paper and it will also allow you to do a last-minute cram on the morning of the exam if you're taking the triple science exams there's a link in the description to an extended version of this video containing all of the additional content for gcc biology the first part of aqa gcc combined science biology paper 1 is all about cells we start with eukaryotic cells cells that contain a true nucleus like these animal and plant cells eukaryotic cells consist of a cell membrane wrapped around cytoplasm and they contain a nucleus a little pack of dna wrapped in a membrane eukaryotic cells also contain ribosomes where protein is made and these are drawn quite large here but in reality are too small and transparent to see with a light microscope they also contain mitochondria which are about the same size as a bacterium in addition plant cells and the cells of algae have a cell wall made from cellulose a polymer made of glucose these cells also contain green chloroplasts full of chlorophyll which absorb light for photosynthesis and a permanent vacuole full of cell sap you need to know the functions or jobs of each of these subcellular structures the nucleus contains the dna or genetic material and it's also responsible for controlling the actions of the cell the cytoplasm is the liquid gel where most of the chemical reactions in the cell take place the cell membrane is responsible for controlling what can go into and out of the cell the ribosomes are used to synthesize protein and mitochondria are the site for aerobic respiration which is used to release energy from glucose the cell wall that plants have is made of cellulose and it strengthens and gives the cell support the chloroplasts absorb light and aware photosynthesis takes place and then finally the permanent vacuole is a storage of cell sap and this is used to keep the cell rigid to support the plant prokaryotic cells such as bacteria are much smaller than eukaryotic cells a typical bacterium might be about two micrometers across whereas a skin cell would be more like 20 or 30 micrometers so around 10 times bigger prokaryotic cells don't have a nucleus instead their dna exists as a single circular chromosome some bacteria may also have small circles of dna called plasmids these often contain genes for things like antibiotic resistance or metabolism of a novel food source they're extra genes that it's useful for the bacteria to have but they're not life and death in addition to lacking a nucleus bacteria lack any membrane-bound subcellular structures like mitochondria or chloroplasts if you think about it a mitochondria is about the same size as a bacteria so there just isn't room inside the prokaryotic cell for a mitochondrion the ribosomes of prokaryotic cells are smaller than the ones in eukaryotic cells also the cell walls of the bacteria are not made of cellulose some bacteria may have a flagellum which acts like a tail and allows the bacteria to move around more easily cells can be specialized and this means they're adapted structurally to suit their function this could involve a change in shape or the presence of more or fewer subcellular structures a sperm cell has a tail to help it to move and it's also packed with mitochondria to release the energy that the sperm needs to get to the egg a nerve cell has a very branched shape and this allows one neuron to communicate with hundreds of others muscle cells are also packed with mitochondria for energy and ribosomes for synthesizing protein in animals this specialization of cells happens very early in development plant cells specialise much later and plants also retain unspecialized mere stem cells throughout their lifetime this is why you can clone a whole new plant from a very small piece of an older plant specialized plant cells include palisade cells in the leaf which have lots of extra chloroplasts for absorbing light energy root hair cells contain no chloroplasts because under the ground they can't photosynthesize because there's no light they also have an extended shape to increase their surface area and this allows them to maximize their absorbance of both water and minerals like nitrates and magnesium ions they're living cells and this means that they are able to use the energy that's released in respiration to actively transport these minerals into the cell the xylem and phloem are found in the vascular bundles of the leaf they're the plant equivalents of veins and arteries and they're also made of specialized cells the xylem are dead hollow tubes reinforced with a woody polymer called lignin there are no divisions in between the end of each cell the end of each cell has disintegrated and broken apart so it just forms a hollow tube the xylem transport water and mineral ions from the roots up to the leaves in a process called transpiration the phloem transports sugar from the leaves where it's made as part of photosynthesis all around the plant in an active process called translocation this requires a lot of energy but it's much faster than diffusion the phloem are made from two types of living cells the sieve tube elements which actually carry the phloem contents and an additional cell called a companion cell which supplies the sieve tubes with energy unlike the xylem phloem are not one continuous tube the end of each cell is perforated to allow materials to flow between the cells by going through those gaps but there are still ends to each cell speaking of xylem and transpiration the best conditions for transpiration are what you could think of as good laundry weather when it's hot dry light and there's lots of air movement this is because when it's warm the water molecules have more energy so they move faster and they evaporate more quickly when it's dry outside there's a steeper concentration gradient from the water in the plant leaf to the dry air the presence of wind or an air current will move water away from the plant so therefore it maintains that concentration gradient and the water will keep on moving out of the plant and when it's light the plant is able to do more photosynthesis and so it keeps the stomata wide open to get maximum amounts of carbon dioxide in but this also means that water can move out as part of transpiration now if we go back slightly to those specialized cells all specialized cells are originally derived from unspecialized or undifferentiated cells called stem cells as an adult human you have relatively few stem cells and the ones that you do have are quite limited in that they can only become a few different types of cell each in your bone marrow you have stem cells that can become red blood cells or white blood cells but they can't become a neuron and that means that while it's really useful that you can produce new blood cells throughout your lifetime those stem cells from your bone marrow couldn't be used to treat paralysis by growing a new nerve if you severed your spinal cord in contrast to that the stem cells found in embryos while they're developing are able to differentiate to become pretty much any kind of specialized cell these are more useful for treating conditions such as paralysis and also diabetes in therapeutic cloning an embryo is made that has the same genes as the patient this means that if those stem cells are then harvested and used to treat the patient they won't be rejected by the immune system because they're genetically the same as the patient this means that the patient won't have to take immunosuppressant drugs which would make them more susceptible to other infectious diseases on the other hand there are some ethical issues involved here the embryo can't consent to the procedure and so some people are not comfortable with the idea of bringing an embryo into the world only to destroy it once you've taken its stem cells also embryonic stem cell treatment can lead to viral transfer plants have meristems which contain stem cells that can become any kind of cell and this is why it's possible to really easily clone plants using cuttings this is really useful for helping to conserve endangered species or to produce large numbers of a particular plant such as a disease-resistant crop the next topic is microscopy the science of making things look bigger so that we can study them more easily you should know that resolution is the smallest measurement that you can make whereas magnification is how much bigger the image looks than the actual object when you take a zoomed in photo with a phone camera often you can zoom in at 10 or 20 times that's the magnification but the image is blurry because the resolution isn't high enough to tell the difference between objects that are that close together there are two types of microscopes light microscopes and electron microscopes light microscopes have existed since the 16th century and they give us a basic understanding of cells by focusing light through lenses but they can't show us things that are smaller than a certain size because their maximum magnification is around 1500 times and for the very best microscopes their resolution might be as good as 0.2 micrometers you won't see a ribosome using a light microscope because they're just too small and also because they're transparent you should know that the light microscopes you use in school are compound microscopes in other words they have two lenses an eyepiece lens and an objective lens and these work together to magnify the object you multiply the magnification of those two lenses together to get the overall magnification there are two types of electron microscope scanning electron microscopes and transmission electron microscopes and both of them have a much greater magnification and resolution than light microscopes because they use a beam of electrons rather than a ray of light so we can see things that are much smaller and we can also see them in greater detail the magnification of an electron microscope can be as good as 500 000 times and the resolution can be as good as a single nanometer in biology this means that we can view mitochondria and even the ultrastructure of subcellular structures so in other words what does the inside of the mitochondrion look like talking about microscopy is a good opportunity for the exam board to assess mathematical skills remember ten percent of the marks in your gcc biology exams are going to be for your math skills working out the magnification of an image or the actual size of an object from its image given its magnification is something that is likely to come up in biology paper one i'm generally not a fan of formula triangles because it's just another thing for you to remember but for me i am is quite memorable so this is pretty much the only one that i do use if you can remember this triangle just cover up one of the three letters and use the remainder to give you an equation so to calculate magnification i need to take the size of the image which is the picture on the exam paper that you've just measured with your ruler and divide it by the size of the actual object which you may have to work out using a scale bar or you may be told in the question remember the equation will only work if your units are the same for both measurements so if one is in millimeters and the other is in micrometers you'll need to convert until they match conversions in science always involve multiplying or dividing by a thousand the only exception is going to be if you have something that's in centimeters because centimeters aren't a proper scientific unit there are a thousand micrometers in a millimeter and a thousand nanometers in a micrometer the first required practical in biology paper one is about light microscopy you should be able to describe the method how you start with a stage as high as possible using the lowest power objective lens you focus firstly with the course focusing wheel and then with the fine focusing wheel you then switch to a more powerful objective lens if you need to and focus using just the fine focusing wheel you'll need to talk about how a stain is used to allow you to see the transparent structures of the cell and you'll also need to be able to troubleshoot if an image is out of focus you can use the focusing wheels to bring it into focus and make it sharper if the object is too small for you to see clearly then you need a higher power objective lens if you can't see an image at all it could be that the lamp isn't turned on or that the objective lens isn't fully in position the next topic we need to talk about is cell division and to describe cell division it's helpful to have a quick look at hierarchy within a eukaryotic cell there's a nucleus and within the nucleus of a human body cell there are 23 pairs of chromosomes they only look like this during cell division the rest of the time they're just long thin strings of chromatin each chromosome contains about a thousand different genes made of dna deoxyribonucleic acid that dna is going to be important in the genetics topic of paper two but it's also relevant here to mitosis one of the two types of cell division mitosis which is used by body cells for growth and repair usually comes up in paper one although sometimes it puts in an appearance in paper two don't get it confused with meiosis which is used to make gametes and which usually comes up in paper two if cell division is happening anywhere in the body except for the ovaries or the testicles it's mitosis also be very careful with your spelling there are a few pairs of words in biology where any ambiguity will cost you the mark mitosis and meiosis is one of those pairs so you need to make sure that you're spelling those words perfectly even though usually in science we don't lose marks or spelling mitosis occurs as part of the normal cell cycle following interphase where the cell grows and undertakes normal metabolism there's a dna replication phase where all of the dna is copied so that the cell has two copies twice as much dna as it did before and then the chromosomes are pulled to opposite ends of the cell and the cell divides once producing two diploid daughter cells that are identical to the original cell they were made from and identical to each other the next topic is about transported materials and there are three different processes you need to be able to describe the first one is diffusion which is a passive process so it doesn't require any energy diffusion is the overall movement of particles from an area of high concentration to an area of low concentration we say overall movement because the particles continue to move in both directions but once they're at equilibrium they're going back and forth at the same rate so overall there's no more change if you want to when describing diffusion you can use the phrase down the concentration gradient but it must be down not across and not along because those are wrong and they won't get you the mark diffusion occurs in both gases and in solutions and it can occur either with or without a membrane as an example here you can see these oxygen molecules diffusing from outside the cell where there are lots of them to inside the cell where there are a few diffusion is important in both animals and plants and you should be able to name examples of these for instance the waste product urea diffuses from cells into the blood plasma to be removed oxygen diffuses from the lungs into the bloodstream while carbon dioxide diffuses in the opposite direction just to be clear we're not talking about inhaling and exhaling which are massive pressure changes brought about by the physical movement of your diaphragm and your ribs and the muscles around there we're talking about the movement of the gases from the alveoli inside your lungs across one cell to get into the bloodstream within the plant-spongy mesophyll carbon dioxide also diffuses through the leaf certain tissues such as the lungs small intestines and gills in fish are adapted to improve the speed with which diffusion can occur tissues like the alveoli in the lungs or the villi in the small intestine have a folded structure and this increases their surface area so there are more places for the molecules to be absorbed and diffusion happens faster the villi even have microvilli each cell has these little extra projections to increase the surface area even more only having a thin membrane means there's less distance for the absorbed substances to travel and having a good blood supply like a strong capillary network or good ventilation in the lungs will help to maintain that concentration gradient so that the substance keeps moving this also happens in plants as we've seen already root hair cells have a very large surface area too there's a bit of crossover here with the chemistry gcse if you cut an object into smaller pieces or if you give it folds this gives it a bigger surface area to volume ratio which will speed up transport or speed up chemical reactions if you look at my cube here on the left it has a total volume of 64 centimeters cubed four times four times four and it has a surface area of 96 centimeters squared four times four is 16 and then there are six faces and 16 times 6 is 96 centimeters squared if i cut it in half in all three dimensions to make eight smaller cubes then the volume is still the same because i have the same amount of material i've just broken it up but the surface area of those eight smaller cubes put together is now 192 centimeters squared in other words by cutting it in half on all three dimensions i've doubled the surface area to volume ratio the second transport mechanism you need to know about is osmosis and the crucial thing about osmosis is that it is always 100 of the time the movement of water like diffusion it's a passive process which doesn't require any energy in osmosis water will move across a partially permeable membrane which is a membrane that will allow water to go through it but it won't allow the solutes that are dissolved in the water to move to define osmosis you can take your diffusion definition and add in the words of water after high concentration and low concentration but some people find that confusing so i tend to stick to talking about dilute solutions and concentrated solutions osmosis is the diffusion or movement of water from a dilute solution to a concentrated solution through a partially permeable membrane you have to bear in mind that the water is moving to balance out the solutes so when the process is finished there won't be the same amount of water everywhere there'll be more water molecules wherever there was more solute to begin with in the required practical for osmosis you use a cork borer or a knife to make pieces of a vegetable such as potato or turnip and then you'll have investigated putting these into solutions of different concentrations of a solute like sugar or salt as with all of the required practical activities you need to know what the variables are the independent variable is the concentration of solution the dependent variable is the change in mass or the change in the length of the cylinder the control variables would include the type of vegetable material so if you're using potato for one test you use potato for all the tests the mass or the length of the cylinder or chip to start with so you want all of your initial chips to have the same mass and then the length of time that the cylinders were left in the different solutions for if the solution contains more water than the tissue fluid in the cells then the water will move into the plant cells in order to dilute them and then the cylinders will get bigger if the solution is more concentrated than the cells and contains less water then water will be sucked out of the plant material and the cells will shrivel and shrink and this will make your potato cylinder get shorter which you can measure with a ruler or also lighter which you can measure by weighing it using a balance if the potato doesn't change mass or doesn't change size that tells you that the solution it's in has the same concentration as the fluid in its cells it's important that if you're weighing your cylinders rather than measuring their length that you dry them thoroughly first because otherwise you'll just end up including the mass of water and that would make your values inaccurate there's also another opportunity here for some maths working out the percentage change in mass of the pieces of potato firstly you work out the absolute change in other words how many grams has the mass changed by so you take the end mass and you subtract the starting mass and then once you have that you divide by the starting mass and multiply by 100 to get a percentage the third transport process is active transport and you can think of this as being kind of the opposite of diffusion because it's the overall movement of particles from a low concentration to a high concentration this requires the use of both carrier proteins and energy from respiration you therefore expect to see that tissues that do a lot of active transport will have lots of mitochondria to respire aerobically and provide them with this energy we see active transport anywhere that tissues are trying to absorb more than 50 of a particular nutrient for instance in plant roots where root hair cells absorb mineral ions like magnesium and nitrates and also in the villi of the small intestines where sugar is absorbed into the bloodstream in both these scenarios it allows the organism to absorb over 50 of the available nutrients which is all they would be able to get via diffusion alone the second topic in biology paper 1 of gcc combined science is organization although we've discussed this a little bit as part of unit 1 we also need to talk about things that are bigger than cells so you should know that cells are the basic building blocks of all living organisms and that a tissue is a group of cells that have a similar structure and function and that work together where you have multiple different tissues that perform a specific function together we call that an organ like the heart or the lungs or the pancreas organs can be organized into organ systems like the digestive system and the circulatory system and these work together to form organisms you're probably quite familiar with this idea when it comes to animals but plants can be a little bit less familiar if we take a whole plant we can break that down into organs like leaves stems roots and flowers and then if we take a leaf as an example we can break that down further into tissues the first part of the leaf is the waxy cuticle on top this isn't actually a living tissue it's a layer of wax that prevents water loss from the plant beneath the waxy cuticle is a transparent layer called the epidermis or the epidermal layer it's transparent to allow maximum light to get through to the palisade mesophyll layer mesoville literally means middle of the leaf the palisade mesophyll is specialized by having extra chloroplasts to absorb light energy because it's where the majority of photosynthesis happens below the palisade mesophyll is the spongy mesophyll and this has gas spaces to allow oxygen and carbon dioxide and water vapor to diffuse through the leaf embedded inside that spongy mesoville are the vascular bundles which are made up of the xylem and phloem and these are responsible for the movement of water and sugar respectively in transpiration and translocation at the bottom of the leaf there's another epidermal layer and within this there are stomata those pores that allow gas exchange to happen there are some stomata in the top epidermal layer as well but there are far far fewer of them each stoma or stomal pore is surrounded on both sides by a pair of guard cells guard cells are the pairs of cells that are responsible for keeping a particular stoma open or closed based on how well hydrated the plant is when there's a lot of water available the plant uses energy and active transport to pump ions into the guard cells and this causes water to then move in by osmosis so the cells become full of water and turgid and this will leave the stoma to be open when there's less water available the water moves out of the guard cells and they become flaccid or shrunken and shriveled and the stone or pore closes up and this prevents further transpiration from happening and stops the plant from getting any more dehydrated you should also be able to use the human digestive system as a case study for an organ system so you need to be able to label the mouth the esophagus the stomach the small and large intestines and also the liver and the pancreas which are responsible for producing other chemicals that are important in the digestion of food the purpose of digestion is to turn large insoluble food molecules which can't dissolve into smaller soluble food molecules that can be absorbed through the small intestine it's vital for digestion that your cells are able to make enzymes these are biological catalysts molecules that can speed up the rate of chemical reactions like respiration photosynthesis and digestion without being used up or changed themselves they're made out of protein so any cell that needs to make enzymes will contain lots of ribosomes each enzyme is specific it just works on one particular molecule and this means that the enzyme has a particular shape and its active site fits exactly the shape of the substrate that it interacts with it's possible for the enzyme to be denatured and stop working if its active site is permanently deformed by extremes of temperature or ph the human body makes thousands of different enzymes but when talking about the digestive system you need to be able to discuss three groups of them each of them has a name that ends with the suffix a's which indicates that it's an enzyme amylase is an example of a carbohydrates which is a wider group of enzymes that all break down carbohydrates amylase in particular breaks down a carbohydrate called starch it breaks this down to make sugars now in addition to being made in the pancreas and the small intestine amylase is also made in the mouth in the salivary glands the reason that you need two different sources of amylase and two different versions of amylase is because the mouth has a neutral ph so your salivary amylase has an optimum ph of about seven but then you swallow it and it goes into your stomach where there's a very very low ph and so that initial amylase from your salivary glands is denatured and so then you need some more amylase to work in your small intestine so the small intestine and pancreas both also make amylase and this has a much higher optimum ph than the amylase that's produced in your mouth the second enzyme that you need to know about are proteases which break down proteins into amino acids as well as being made in the pancreas and the small intestine they are also made in the stomach and the stomach proteases have a very low optimum ph because the stomach is full of hydrochloric acid lipases are enzymes that can break down lipids also known as fats and oils and lipids are broken down into two monomers fatty acids and glycerol again lipase is made by the pancreas and the small intestine and it works in the small intestine in addition to these three groups of enzymes you also need to know about two further chemicals bile is produced by the liver and stored in the gall bladder it emulsifies fats which means it breaks apart large globules into much smaller droplets and because this increases the surface area it enables lipases to access more of the fat at once and speeds up digestion bile is also alkaline so it neutralizes the stomach acid to give the optimum ph for the enzymes that are working in the small intestine the stomach also makes hydrochloric acid which is responsible for giving the protease enzymes their optimum ph and also for killing pathogenic bacteria for the food test required practical you need to be able to describe how to test for the presence of carbohydrates like starch and sugars lipids and proteins to test for starch we use an orange brown reagent called iodine solution which turns blue black in the presence of starch to test for glucose we boil the sample with benedict solution this starts out bright blue because benedict's solution contains a lot of copper sulfate but when reducing sugars like glucose are present as it's boiled it turns green then yellow then orange and then eventually red depending on how much sugar is present it's really important that you talk about boiling it not merely warming it because it does need a very high temperature in order to work to test for proteins we use a different blue solution called buret reagent and again be very careful of this spelling as it will cost you marks if you spell it wrong buret reagent turns a lilac purple color if there's protein present now there isn't one named test for lipids in your specification so any good test will do but you could describe using tracing paper to test for solid foods and looking for a greasy stain or you could talk about adding ethanol and water and shaking to produce a white emulsion if lipids are present or you could talk about using sudan 3 which will turn red linked to the food test required practical is a further required practical where you investigate the optimum conditions for a particular enzyme to work remember the optimum conditions are those where the enzyme is able to fastest catalyze the particular reaction that it works on so in this investigation we test the speed with which amylase breaks down starch to make sugars the faster this happens the closer it is to the optimum conditions as we saw in the previous slide we can test for the presence of starch using iodine solution which if starch is present will be blue black but when there's no starch left will be orange brown we use a continuous sampling method which means that every 30 seconds we remove a sample and test it and we see if there's any starch left at that point initially you have several tubes each containing starch and a ph buffer and these have been left in a water bath until they've reached a particular temperature such as 30 degrees you add the same volume of amylase solution to each one of these tubes and then every 30 seconds you remove a sample and add it to the iodine in the spotting tile to see whether or not there's still starch present in that sample has the enzyme done its job yet and has it broken that starch down going to end up with a results table that looks something like this so the ticks indicate that there's still starch present whereas the crosses indicate that all the starch has been digested and the reaction is over so as you can see here at ph 7 the enzyme is working very very quickly and all of the starch is broken down within the first 30 seconds whereas it ph 9 and ph 5 it's working a little bit more slowly and at ph 2 and ph 12 it seems like the enzyme has been completely denatured because it's unable to break down the starch even after six minutes as part of this required practical you might be asked about the control variables and one of the things we need to control is the temperature but remember a control variable is something that you are actively taking charge of and doing something to force it to be exactly the same in every single repeat so it's not enough to say that you would do the experiment at room temperature we need to actually control the temperature by keeping the tubes of starch and amylase in a water bath or by warming them using an electric heater you need to be able to identify and label a number of organs of the human body for the lungs you should label the trachea which is the proper name for the windpipe and this splits into two bronchi one in each lung which in turn split into bronchioles these finally split into thousands of tiny alveoli little cloud-like structures that give the lungs a bubbly texture and they increase the surface area for gas exchange each alveolus is served by lots of capillaries to increase the rate of diffusion underneath the lungs there's a thick sheath of muscle called the diaphragm and when this moves downwards it increases the volume of the thoracic cavity leading the lungs to inflate as part of the circulatory system you should recognize three types of blood vessels the arteries carry blood away from the heart and the high pressure so they have thick layers of both the muscle and the elastic tissue the veins carry blood towards the heart and it's under lower pressure so they have a larger lumen which is the hole in the middle and thinner walls both the elastic tissue and the muscle they also have valves to prevent backflow of blood the capillaries are the tiny little blood vessels that allow blood to get to every cell in your body and they have walls that are just one cell thick human blood is made up of four key components red blood cells carry oxygen in the form of oxyhemoglobin their concave shape maximizes their surface area and they have no nucleus to allow more room for hemoglobin white blood cells such as phagocytes and lymphocytes fight infection either by engulfing pathogens producing antibodies or antitoxins or as acting as memory cells platelets are fragments of dead cells and they're responsible for making the blood clot plasma is the liquid part of the blood it carries dissolved carbon dioxide urea glucose and amino acids as well as carrying the cells and also some other large insoluble molecules such as hormones the human heart is part of a double circulatory system which means that the right side and the left side function independently to pump the blood to different places you must remember that the heart is labeled from the point of view of the person whose heart it is so that means that the left hand side of the diagram is called the right hand side of the heart and vice versa blood enters the right atrium from the vena cava the most important vein which carries blood from the body between the right atrium and the right ventricle there's a valve to prevent backflow from the right ventricle the blood moves to the lungs to become oxygenated through the pulmonary artery the blood then returns to the heart into the left atrium and again there's a valve separating the left atrium from the left ventricle to prevent black flow from the left ventricle the blood leaves the heart through the aorta which is the biggest artery in the body and it goes to the rest of the body you'll notice from the diagram that the left hand side of the heart is much more muscular than the right hand side because it's pumping the blood a greater distance located in the right atrium there's a small group of cells that control the natural resting heart and this is called the pacemaker if you suffer from arrhythmia it's possible to have an artificial pacemaker fitted which is a small electrical device that's used to control irregularities in the heart rate health is defined as the state of physical and mental well-being and ill health can be caused by a number of factors including communicable and non-communicable diseases as well as other factors like diet stress and lifestyle these factors may interact which means influence each other so for instance if there's a problem with your immune system you're much more likely to suffer from infectious diseases but also certain viruses like hpv can be responsible for triggering certain cancers it's possible to have an immune response to pathogens that cause allergies like rashes and asthma and also if you have poor physical health then this may lead to mental illness where the precise cause of a disease isn't known often the first step for scientists is to look for risk factors these are things that may or may not actually be causing the disease but we do find them occurring in the same people who have the disease so they could be aspects of a person's lifestyle or substances that are found in a person's body or their environment sometimes we'll go on to find that these are the things causing the disease and sometimes not so for some examples we know that cardiovascular disease is more likely to occur in people who have a poor diet who smoke or who don't do enough exercise we know that obesity is a real risk factor for people having type 2 diabetes we know that if you abuse alcohol you're more likely to have liver and brain issues and we know of course that if you smoke you're much more likely to have lung disease or lung cancer and it can have a really negative impact on unborn babies if their mothers are smoking while pregnant finally carcinogens like certain chemicals and ionizing radiation can lead to cancer coronary heart disease or chd is an example of a non-communicable disease in other words it can't be passed from person to person because it's not caused by a pathogen it happens when a fatty deposit called plaque yes the same word that we use for the stuff on your teeth causes the arteries to narrow this reduces the blood flow to the cardiac muscle in the heart and therefore the oxygen supply it can be treated with stents with drugs called statins or with a heart transplant and you may be asked in a question to evaluate which of these would be the best option remember anywhere that you meet the word evaluate in gcc science you're being asked to compare the evidence and write a conclusion to reach level three of an extended response question you need to say this one is the best and these are the reasons why you could consider factors like the dangers associated with surgery the need to take immunosuppressant drugs following a transplant which would then mean you're more susceptible to infection difficulties in obtaining transplant organs and the fact that a stent or a transplant is sort of an immediate response so it will help much more quickly but it won't help long term unless you change your lifestyle in contrast taking statins takes longer to have an impact but it does have a long-term effect however you do need to remember to be able to take the drugs often for these evaluate questions there isn't one right or wrong answer but it's about justifying your conclusion with the evidence cancer is another example of a non-communicable disease it occurs when normal body cells undergo a series of mutations that lead to their uncontrolled growth and cell division whereas a normal cell will divide a set number of times before stopping dividing and eventually dying in contrast a cancer cell will continue to divide and eventually begin growing its own blood vessels and just getting bigger and bigger and taking over and when this begins to happen the tumor can be classified as either benign which means that it's not considered cancerous because those cells are contained by membrane and stay in one part of the body or malignant which means that the tumor is cancerous and it may begin invading neighboring tissues and forming secondary tumors in other organs communicable diseases are those that it's possible for an infected person to pass on to somebody else because the disease is caused by a pathogen a disease-causing microorganism which could be a bacterium a virus a fungus or a protist bacteria make us ill by making poisons called toxins and they can be killed by antibiotics viruses on the other hand reproduce by hijacking our cells and damaging them and that's what makes you feel ill viruses can't be killed by antibiotics partly because they're not alive but also because they're protected by being inside your cells it's possible to treat the symptoms of a viral infection using a painkiller but these are treating the symptoms the way that you feel they won't actually cure the infection or get the virus out of your system any faster there are three examples of communicable diseases caused by viruses that you need to know about for the exam measles causes fever and a red skin rash and it's spread by inhaling droplets from the coughs and sneezes of people who have the disease it can be prevented by vaccination such as the mmr vaccine which also protects against mumps and rubella human immunodeficiency virus or hiv firstly causes a flu-like illness but then it goes on to attack the white blood cells and cause an immune deficiency syndrome called aids hiv can be spread through the exchange of bodily fluids such as blood if people share needles or semen through sexual contact hiv infection can be managed by antiretroviral drugs but if the disease has progressed far enough for aids to begin then that process cannot be reversed tobacco mosaic virus or tmv is a plant infecting virus that causes a mosaic pattern of discoloration where parts of the leaves of infected plants don't contain sufficient chlorophyll so they're yellow rather than green this can happen to lots of different plants it's not just tobacco but that's the one that it's originally been isolated from and that's hence the name this yellow pattern or chlorosis affects growth because the bits of the leaf that don't contain chlorophyll are unable to absorb light for photosynthesis tmv can be spread through the contact of the infected leaves or by contaminated tools and it can be managed by removing the infected leaves cleaning tools and also rotating crops salmonella and gonorrhea are two examples of bacterial pathogen salmonella is one microorganism responsible for causing food poisoning the bacteria make toxins and these toxins lead to fever abdominal cramps vomiting and diarrhea it can spread on contaminated food that hasn't been cooked properly or has been prepared in unhygienic conditions in the uk a particular source of salmonella is chicken or poultry so we vaccinate our poultry to try to prevent the spread of salmonella this helps because vaccinated chickens will have fewer bacteria living on them and therefore people eating that chicken will ingest fewer pathogens it's also important to keep chicken in the fridge because at that colder temperature the salmonella bacteria are going to take a lot longer to reproduce and therefore there'll be fewer bacteria gonorrhea is a sexually transmitted disease that causes a thick yellow or green discharge from the penis or vagina as well as pain when you urinate it's spread through sexual contact and can be prevented by using barrier methods like condoms or it can be treated with antibiotics however antibiotic resistant strains are beginning to arise and this is making the disease much harder to treat rose black spot is a plant disease caused by a fungus rather like tmv it doesn't just infect roses it affects lots of different plants it causes purple or black spots to develop on the leaves which then turn yellow and drop off completely it can be spread as spores through water or wind and it's treated by fungicides which are chemicals that kill funguses or by removing and burning the affected leaves so both removal and burning can be used to treat rose black spot and tmv but fungicides can only be used for rose black spot because it's a fungus malaria is caused by a protist and it causes recurrent episodes of fever it's spread by mosquitoes which we call a vector a vector is something that can pick up a pathogen and move it to somewhere else the protein spends part of its life in a human and part of its life in a mosquito so it has two different hosts prior to laying eggs the female mosquito needs to have a blood meal so she bites a human and if that human already has the malarial protist living in its blood then she sort of sucks up the protist and it's in her and then the next time that she goes to have a blood meal and she bites a new human she inoculates them with protest to control the spread of the disease we need to stop mosquitoes from biting humans so either we drain the standing water that they like to lay their eggs in sort of bogs and swamps or we use mosquito nets to avoid the mosquitoes getting to humans to bite them while they're sleeping to protect us against pathogens the skin acts as a barrier preventing those pathogens from entering the bloodstream any time that you cut yourself platelets allow that to clot really quickly to maintain that barrier in the nose and the shakia there are goblet cells which produce thick mucus which traps any pathogens that manage to enter those ways and then small hair-like structures called cilia waft that pathogeny mucus so that it goes into your esophagus and you can swallow it so that the pathogens end up in your stomach where they're killed by the stomach acid if a pathogen does make it past those first lines of defense it encounters the immune system there are several different types of white blood cells badger sites engulf pathogens and they digest them in a process called phagocytosis lymphocytes can produce antitoxins and antibodies antitoxins are responsible for immobilizing the poisonous toxins that bacteria can make antibodies are responsible for immobilizing the pathogens themselves the antibodies are specific to the antigens the little protein bumps on the outside of the pathogen the antibody will immobilize that particular pathogen by grabbing onto its antigens and then squashing lots of pathogens together in one space so that one phagocyte can come and engulf several pathogens in one go vaccination is a procedure that protects us from certain diseases by building artificial immunity in advance of you ever meeting that disease for real so it harnesses the ability of the white blood cells to make these antibodies and just ensures that the first time that you meet a pathogen you're able to make a lot more of them and make them a lot more quickly in its crudest form vaccination involves injecting a dead pathogen which we call inoculation but now we tend to inject an inactive version or just purified antigen proteins so we make the pure protein from the outside of the virus or the bacterium without actually needing the pathogen itself you're not expected to be able to discuss the new rna vaccines like the ones we're using to protect against covid in response to those antigens the lymphocytes in your bloodstream are able to produce specific antibodies based on the antigens they meet remember specific means they have a complementary shape rather like the lock and key theory of enzymes these antibodies will immobilize the pathogen to allow vagiocytosis or biocytosis to happen special lymphocytes called memory cells stay in the blood and they remember how to make that particular antibody if you become re-infected these memory cells produce very large quantities of specific antibodies much much faster than the very first time they encountered that antigen when you were inoculated or vaccinated the pathogen is killed before you even really start feeling ill you're now considered to be immune to that particular strain of the pathogen any question asking you to discuss the advantages of vaccination is expecting you to know that if you've been vaccinated then when you meet the pathogen for the first time sort of in the wild you'll produce far more antibodies you'll produce them much more rapidly and also the antibodies will persist in your bloodstream for a longer time you should be able to differentiate between antibiotics and painkillers antibiotics can only be used to treat bacterial infections they don't work on viruses because these are protected by the cells that they're in painkillers can be used to relieve the symptoms of infections such as if you've got a fever when you have a cold but they don't kill the pathogen you should know that while most new drugs are synthesized by chemists in the pharmaceutical industry many different drugs have natural starting points digitalis is a cardiac medicine given to strengthen heart contractions and it was originally isolated from fox gloves aspirin was originally isolated from willow bark which people used to chew as a painkiller in the days before they had access to medicines and penicillin was the first antibiotic to be isolated and it came from a fungus called penicillium and this was discovered by alexander fleming drug development is often a very lengthy process and it's entirely normal for it to take 10 years or even longer for a new drug to come to market partly this is because of the number of different stages that are part of this process but also that the whole thing can be slowed down by difficulties receiving funding for the drug development and also recruiting volunteers for the later stages we begin with pre-clinical testing this is testing that doesn't involve any humans and instead uses cells tissues and eventually animals as part of this we're looking for two things firstly toxicity so a lot of the drugs that could kill pathogens or kill cancerous cells also have the potential to be damaging to normal human cells so here we're looking to see is that damage going to occur by just looking at individual cells or tissues secondly we're looking for stability so if we make that new medicine and we leave it on the shelf for a month is it still going to be usable at the end of that time or is it going to break down because it's just too unstable then we move on to clinical testing all of the tests that involve human volunteers within this process we're going to look for side effects so is the drug actually going to make somebody feel unwell efficacy does it work better than a placebo or better than the current available drugs and also what the dosage is so what would an appropriate amount be to give to somebody who was receiving that drug initially we give a very low dose to healthy volunteers the reason for using healthy volunteers is threefold firstly because those volunteers are healthy it's far less likely that they end up being made unwell if the drug is not appropriate so it's far less dangerous secondly if they do experience any side effects we know that it's because they're being caused by the drug it's not that person is just feeling unwell because they're already very sick and finally that healthy volunteer is probably not going to be taking any other drugs which could interact with the new drug after we've tested those healthy volunteers to work out whether it's causing side effects we can then move on to volunteers who actually have the illness or disease that we're trying to treat and at this stage we can start looking at the efficacy of the drug and does it make them feel better than they did before or cure their disease and also what the dosage is so how much are they receiving throughout all of these clinical tests it's really important that the person receiving the drug doesn't know whether they're having the actual new drug or a placebo a placebo is a version of the medicine that doesn't contain any of the active ingredients so it could be a sugar pill or a saline injection so some of the people on the trial are going to be actually not being treated but they're still going to have some kind of active interventions they're still going to be receiving the same number of injections or taking the same number of tablets it's also really important that the doctors treating them don't know who is receiving the actual medicine and who is receiving the placebo and this process where neither the patients or the doctors know who's receiving what is called a double-blind trial once the scientists have accumulated a large amount of data this is subject to peer review where other scientists and other doctors will look at it to check that there isn't any bias and to make sure that the data is valid even after the drug has been licensed there'll be continued monitoring in case new information comes to light or we find out things that mean that it's no longer appropriate for that drug to be on the market the final topic in aqa combined science biology paper 1 is bioenergetics and we begin with photosynthesis photosynthesis is an endothermic chemical reaction used to store energy in glucose as you know from chemistry endothermic reactions absorb energy from their surroundings and this may lead to a temperature drop but it doesn't have to be a transfer of energy by heating in photosynthesis the energy transfer is a type of radiation light the light is absorbed by the chlorophyll in the chloroplasts within plant and algae cells you need to know the word equation and the symbol equation for this reaction and it's really important that you don't mix them up or chop and change if the question asks you for words you have to use words if it asks for symbols you have to use symbols it's also really important with those symbols that you make sure that the numbers in the molecular formulae are subscripts they're small and they go below the line if you write a squared sign instead of a subscript two you won't get the mark the glucose-maiden photosynthesis can be used for respiration to release energy for the plant to use it can be combined with nitrate ions to make amino acids which can then be used to make proteins it can be used to synthesize an insoluble polymer called starch for storage it can be made into cellulose for cell walls or it can be used to make lipids which are also used for storage of energy the rate of a process is its speed and we can calculate the rate of photosynthesis by using an aquatic plant submerged in water and looking at the number or volume of bubbles of oxygen gas that are produced in a certain amount of time it's vital when you're discussing rate that you mentioned that you're going to time it as without this you can't calculate a rate the limiting factor of a reaction is the thing that's slowing it down it could be a reactant there isn't enough of or a condition that isn't optimum for photosynthesis the rate could be limited by light intensity carbon dioxide concentration temperature or the amount of chlorophyll to absorb the light when you're trying to identify the limiting factor for photosynthesis by looking at a graph it's important that you know which part of the graph you're looking at if we take this light intensity graph if i split it in two here you can see that in the first part of the graph as i give the plants more light the rate of photosynthesis increases this tells me that light is the limiting factor however once we get to a certain level the graph plateaus or flattens off and at this point it doesn't matter how much more light i give the plant it's not going to photosynthesize any faster this tells me that something else must be limiting the rate of photosynthesis maybe the temperature isn't optimum or maybe there isn't enough carbon dioxide for the plant to photosynthesize any faster the graph of carbon dioxide looks very similar to the light graph up to a point as you increase the level of carbon dioxide the rate will increase but once we've reached a certain level of carbon dioxide there's no point giving any more to the plant the graph of temperature looks a bit different as we increase the temperature this will increase the rate of a chemical reaction however because this is a chemical reaction catalyzed by an enzyme that enzyme will eventually be denatured by the high temperatures leading the rate of photosynthesis to full in the final required practical of this paper we look at the effect of light intensity on the rate of photosynthesis you should describe your setup an aquatic plant submerged in water to allow you to count bubbles or collect the gas some system of collecting gas such as a gas syringe or an upturned measuring cylinder and a light that is set up a specific distance away from the plant you need to describe how you're going to change the light intensity so the normal way to do this is to have a light and have a ruler and have a set distance from the plant and then once you've completed the experiment move the lamp to a different location you could also use a light that you could turn up or down you need to describe that to measure the rate you're going to take a set time such as 10 seconds or a minute and then use that aquatic plant under water so that you can count the bubbles that are being given off or measure the volume of gas produced within that time frame this will allow you to calculate a rate in bubbles per minute or centimeters cubed per second you should be able to name the variables the light intensity is the independent variable the rate of photosynthesis is the dependent variable the control variables will include the temperature the carbon dioxide concentration and the plant used to keep the temperature same we could use a heat screen which stops the heat from the light bulb influencing the plant or you could use an led bulb because they tend to stay quite cold but make sure you're explicitly saying that you're doing it so that it doesn't affect the temperature you could also use a water bath that remains a precise temperature say it's exactly 25 degrees and it's being monitored with a thermostat you can use chemicals like sodium bicarbonate to make sure that the water is absolutely saturated with carbon dioxide and there's no way that that could become a limiting factor if you're taking the higher tier exams then you may be asked in the context of this practical to discuss inverse square law this basically means that however much you increase the distance by from a plant to a light you're going to decrease the amount of light that each square centimeter receives by that number squared so if you move the light double the distance away then each centimeter squared of the plant will receive one quarter of the amount of light also if you're taking higher tier you may be asked how limiting factors are important in the economics of the conditions in greenhouses basically to maximize photosynthesis the light should be permanently on the temperature should be optimum as warm as it can be without having the enzymes being denatured and carbon dioxide needs to be popped in so that it isn't limiting however all of these things cost money so for instance we'll pump in carbon dioxide until it's no longer limiting but then after that point there's no point wasting your money pumping in any more don't forget that all living things including plants carry out respiration and they have to do it all the time otherwise they would die you may be asked about the gas exchange that plants do during the night time or the point at which photosynthesis and respiration equal each other out so you could have a graph like this one that shows that photosynthesis is happening lots during the day when it's light but of course at night it stops respiration is continuing at a constant level and this means that overnight we expect that plants will be releasing carbon dioxide whereas during the day they're releasing oxygen respiration is an exothermic chemical reaction that occurs in all living cells all the time it's used to release or transfer the energy that's stored in the chemical energy store of glucose and organisms can then use this energy to build larger molecules like starch glycogen and proteins for movement and for keeping warm you should be able to discuss the difference between aerobic respiration which uses oxygen and anaerobic restoration which doesn't aerobic respiration breaks down glucose to produce carbon dioxide and water whereas anaerobic respiration in animals breaks it down into lactic acid and in plants and yeast breaks down glucose into ethanol and carbon dioxide anaerobic respiration and yeast is really important in the food industry it's called fermentation and it's responsible for making the ethanol that goes into beer and wine and also for making bread rise because the carbon dioxide bubbles push the dough up aerobic respiration is an example of complete oxidation so in other words as much oxygen as possible bonds with all the different atoms that are in that glucose and therefore it releases about 19 times more energy than anaerobic respiration and it doesn't lead to an oxygen debt whereas anaerobic respiration is incomplete oxidation releases far less energy and leads to a large oxygen depth which later needs to be got rid of when we exercise we need more energy so we need to respire more therefore the heart pumps faster to supply more oxygen and glucose to the muscle cells and breathing rate and breath volume also increase to supply more oxygen and remove carbon dioxide without the glucose and oxygen the cells can't continue to respire aerobically which is the best way to supply them with the most amount of energy if this doesn't happen then anaerobic respiration will take place instead and this leads to muscle fatigue when the muscles fail to contract efficiently glycogen is broken down to form glucose during exercise to continue supplying glucose to the cells if you're taking higher tier you need to know the oxygen deck can be defined as the amount of extra oxygen needed to react with the accumulated lactic acid so that it can be removed the blood flowing through the muscles transports lactic acid back to the liver where it can be converted back into glucose following exercise acquiring that extra oxygen to break down the lactic acid is why you continue to breathe heavily and your heart rate remains elevated following exercise thank you very much for watching this summary revision video for biology paper one and i hope you found this useful in your revision for gcc combined science if you did find it useful then don't forget to like and subscribe for more science videos and let me know in the comments if you found it helpful or if there are particular topics that you'd like more detail for