all right so chapter 7 is going to focus on the cell membrane cell membranes are are fluid mosaics that are made up of lipids and proteins the plasma membrane is the gatekeeper for your cell it's what allows materials in and what allows things to leave it has selective permeability as a result phospholipids are the primary lipid that's found in your plasma membrane they have amphipathic properties they have both hydrophobic and hydrophilic regions and the fluid mosaic model which is the model that is thought to make up our cell membranes is described as a fluid structure that has a mosaic of various proteins that are embedded throughout it so there's your cell membrane and you can see how the hydrophobic tails don't necessarily just line up as a straight line and then the hydrophilic heads are going to be found on the exterior portions facing the cytosol within the cell as well as the extracellular fluid outside of the cell membrane so scientists were attempting to describe what the structure of the cell membrane was davidson and danielle came up with this idea of a sandwich model in which the phospholipid bilayer basically was the peanut butter in your sandwich and then there were globular proteins outside of it so the proteins were the bread in this sandwich there were some issues with this model especially with the membrane proteins because they have both hydrophilic and hydrophobic regions and so if they were just facing the exterior they would only be facing a hydrophilic environment finger and nicholson came up with the fluid mosaic model back in 1972 where only the hydrophilic regions get exposed to water and freeze fracture studies were able to verify those they basically split a membrane among along the middle of the phospholipid bilayer and so there you can see what the inside of the extracellular layer looks like that would be that top portion and you can see what the inside of the cytoplasmic layer looks like so you can kind of see how the proteins are embedded throughout not just on the top or on the bottom membranes need to stay fluid the phospholipids actually can move they can drift laterally and they do quite frequently but they can also flip-flop across the membrane that does not happen nearly as frequently whereas the lateral movement happens um 10 to the seventh times per second so quite quite often um flip-flopping occurs maybe once a month and so that's just showing you the structure you saw a little bit about this back in chapter six where you can see the phospholipid bilayer you can see some cholesterol molecules embedded in the bilayer and then the different mosaic of proteins you've got your peripheral proteins your integrins or your integral proteins and you've got different tags on the surface the glycolipids that could be attached to just the phospholipid bilayer or you could have one attached to one of the surface proteins a glycoprotein um you can also just have carbohydrates on the surface as well and you can see how the extracellular matrix kind of helps tie everything in together coordinating the interactions between the cell membrane and what surrounds it as well as on the inside of the cell membrane you can see the cytoskeleton microfilaments that kind of help to can maintain the cell structure so why membranes are able to be fluid when temperatures cool the membranes will try to switch to a solid state the temperature will depend on what types of lipids are present if there are more unsaturated fatty acids present just like we learned about when we're studying macromolecules they tend to stay more fluid than those that are saturated because there's going to be less of the ability to stack the membranes need to stay fluid to be able to work you want to be a fluid similar to what you would see with salad oil and cholesterol will make up part of our membranes because it does have an impact on their fluidity when they are warmer cholesterol keeps the phospholipids from moving around too much and at cooler temperatures it's able to help them stay fluid by preventing that type packing we would see in the solid state so unsaturated hydrocarbon tails are going to help to increase fluidity um saturated hydrocarbon tails are going to make it a more viscous environment and then if you have cholesterol molecules and these are within animal some cell membranes since plants are going to have the cell walls surrounding it those are also going to help to prevent packing as well so your membrane does have lots of different proteins that are embedded and the proteins are what are going to determine what that membrane is going to allow to enter and exit you can have peripheral proteins that are bound just to the surface of your membrane and then you can have integral proteins that will go through that hydrophobic um the the lipid portion of your membrane when they span the membrane completely they're called transmembrane proteins and when you happen to have one of those the hydrophobic regions are going to be made up primarily of nonpolar amino acids that get coiled into alpha helices and those are going to be what is seen in the interior of the integral protein and so there's just kind of an example your more hydrophilic portions of your protein will be facing the extracellular side as well as the cytoplasmic side functions of membrane proteins i'm actually going to talk about each of these on the next slide transport enzymatic activity signal transductions cell cell recognition intracellular joining and attaching to your extracellular matrix inside a skeleton so transport is one thing we're going to be spending a lot of time on in this specific chapter we'll talk a little bit more about enzymes in chapter 8. signal transduction will be coming our way how we can use those proteins to act as rep receptors for signaling molecules i believe that's going to be in chapter 11. having cells be able to recognize one another intracellular joining we talked a little bit about that in our last chapter with the different ways that cells can connect to each other and then the physical attaching to your cytoskeleton and extracellular matrix so carbohydrates how they're able to help cells recognize one another these cells will bind to surface molecules that often have those carbohydrates on your plasma membrane's extracellular surface as i talked about they could be um covalently bonded to lipids or to proteins aka forming glycolipids or glycoproteins the carbohydrates that are present will vary not just among individuals but also within different species and within different cell types so it does help cells to recognize what actually the cell is function is in that particular organism and so here is one example of how you can use these recognition sequences to be able to prevent entry for things as well as unfortunately sometimes for things to enter we're running into that a lot right now with covid i'm going to talk about that a little bit more on um in class so hiv is able to infect cells that have the ccr5 on its surface which is found in most individuals so you've got the receptor for cd4 and then the co-receptor ccr5 if there is only cd4 present but there is no ccr5 present then individuals that are infected with hiv will not take that up it will not be able to replicate itself because it's not able to enter and take over the cells machinery so we talked a little bit about sightedness with the golgi apparatus in chapter six synthesis and sadness of membranes membranes do have specific or distinct insides and outside faces that you want those carbohydrates to join to the extracellular side of the membrane having them on the interior would not be very beneficial they're not going to recognize things there and so the asymmetrical distribution of proteins lipids and carbohydrates in the plasma membrane is determined when the er and the golgi work in concert to build that particular membrane remember that you have your smooth er which makes lipids and then you have your rough er which is going to make your proteins and both are going to be transporting what they're making via vesicles to the golgi apparatus which is going to be able to recognize where things need to go and to prepare them to be able to either leave the cell completely or to join on in terms of the membrane composition import the membrane compartment of the cell or wherever else those items are needed within the cell itself so there is a logic to that process of both proteins and lipids being synthesized within the cell so the membrane structure does result in selective permeability cells need to be able to allow things to enter and allow things to leave the plasma membranes are able to help regulate that there are some molecules that are able to diffuse passively through the membrane and that would be your hydrophobic substances that are pretty small if it's polar it's not nearly as likely to cross without assistance you can have some water molecules move through the phospholipid membrane but it definitely helps when there are other ways for water to get in and one of those ways is through a transport protein transport proteins would allow your more polar molecules to move through the membrane you can have channel proteins and you can have carrier proteins channel proteins basically are going to allow a specific molecular ion to move straight through and there are channel proteins called aquaporins that will allow large amounts of water molecules to move into a cell if it needs it there are other transport proteins as i said carrier proteins that actually help they assist moving that protein or moving whatever it is that polar molecule through from the extracellular fluid into the cell they cause result in changes in shape which allow things to enter and exit there are specific transport proteins for whatever substance it is that needs to move in and out of the cell okay so your channel proteins they just open the gate and everything moves in whereas your carrier proteins they will change shape to allow certain things to move in but both of these occur passively they do not require energy for these proteins to be able to allow molecules to enter the cell so speaking of passive transport passive transport is when a substance is able to come across without requiring any energy to make that occur diffusion is one of those ways that we can use passive transport where molecules basically are able to spread out into their available space you can have diffusion occur in one direction you can have it occur in multiple directions and when you are at a state of equilibrium you can have molecules moving in both directions but you will have the rate at which they are moving in be equal to the rate at which they are moving out so there you see dye in the first example um the dye is moving in and you're having a few move back across and then when you reach equilibrium roughly the same amount is on moving towards the water side while the same amount is moving in towards the side where the molecules initially were present if you happen to have two solutes you would see the same process occur where you would have the diffusion occur in opposite directions until you have roughly the same amount of ions or the same rate at which the ions are moving occurring on both sides so what allows diffusion to take place passively is that they substances that are diffusing are moving down their concentration gradient they're moving from an area of high concentration to an area of less concentration so since no energy is needed for this to occur as they move from an area of higher concentration to an area of less concentration diffusion is considered to be a passive process we can see this with water and sugar here in this particular example via osmosis you see that you have greater numbers of sugar molecules on the right side of this youtube while you have fewer on the left side and so as that membrane allows water to move in both directions more water is going to move towards the side with the greater concentration of solute trying to reach a point where they are at equilibrium where you have roughly the same concentration of sugar in the water molecules on both sides of that selectively permeable membrane so we're going to talk about how cells are able to maintain balance with water and their surroundings we're going to look at it both for animal cells those without cell walls and plant cells those with cell walls tonicity is whether a surrounding solution is able to make a cell take in water or to have a cell give up water for it to gain or lose it when animal cells are placed in an isotonic solution the solute concentration that is surrounding the cell is the same as the solute concentration inside of the cell and when that takes place you would have no net change in movement you are at equilibrium when a cell is placed in a hypertonic solution there is a greater amount of solute outside of the cell than there is inside of the cell and so water is going to diffuse across and as it diffuses across it's going to cause those animal cells to shrivel in a hypotonic environment you have a solute concentration inside of the cell that is sorry you have a solute concentration outside of the cell that is less than that inside of the cell and so water moves into the cells to try to balance that out and when that occurs your cells are going to get larger and larger until they will eventually lyse and burst it's a little bit different with plant cells because they have a cell wall and so there's a process a topic we're going to get into more later on in the spring called turgor pressure that is going to help them um be able to balance out the water changes due to the solute concentration surrounding their cells a little differently than animal cells in a hypotonic environment the plant cells are going to take in water and they're going to continue to take in more water than they might ex you might expect that they would need but the cell wall basically pushes back against it and will only allow them to go to a certain extent they can't truly lyse like we saw with the animal cells so when they are in a hypotonic solution and they are taking in water to try to reach that equilibrium balance they will conform target cells or turgid cells they'll be very firm that's what you want to see in your plant cells that they are able to stand up if they are in an isotonic environment they are not going to take in any water and they're not going to give up any water and when that occurs the cell membrane is able to be a little bit more separated from your cell wall and you would see a more flaccid cell taking place that your plants would be more limp and you would tend to see wilting occurring and that's usually your your go-to sign that you need to make sure you water your plants in a hypertonic environment because there is more of a solute concentration outside of the cell you're going to see water leave and eventually that cell membrane is going to pull so much apart from the cell wall that is going to cause it to go through plasmolysis or plasmolysis where the cell basically is ripped up and so that plant cell will die so there's the animal versus the plant cell so an animal cell isotonic solution is the go-to for normal while in a plant cell because of the cell wall being able to exert the turgor pressure and not allowing it to be able to be broken open a hypotonic solution is the better bet if you're in a hypotonic solution with an animal cell um as your cell takes in water um it will eventually burst in lice well if you're in an isotonic solution with a plant cell you are going to have room for more water and that's what's going to allow those plant cells to become flaccid and in a hypertonic solution in both cases you're going to be losing lots and lots of water and an animal cell is going to become shriveled and in a plant cell it's going to separate the cell membrane from the cell wall and causing it to go through plasmolysis so we're going to talk a little bit more we talked about how water can move in and out through diffusion and through there are definitely proteins that can act passively as well and those are going to be through facilitated diffusion you can have channel proteins and carrier proteins act like this they help to speed up the process of passively moving molecules across the plasma membrane so i talked about the aquaporins earlier so that's a specific protein that is able to move water through the from the extracellular fluid into the cytoplasm of your cell and you can also have carrier proteins which will pick up ions or polar molecules change shape and allow them entry into the cell the ion channels are going to open or close in response to a stimulus so they're again acting as a gate and these those carrier proteins go through that slight change in shape that allow things to move from outside of the cell membrane to the interior so there you can see a channel protein and there you can see the change in shape with a carrier protein active transport is not able to be done without the use of atp so we saw with diffusion how that was a passive transport we saw with facilitated diffusion through the carrier or the channel proteins that that was passive because no energy was required to allow that transport process to occur some transport proteins are able to transport against their concentration gradient and when they do that they require atp and when that is involved we have active transport occurring and there are reasons why cells would want to transport ions against the gradient there are legitimate reasons why they would want to go against the grain so the energy again is typically in the form of atp there are some exceptions but atp is your go-to energy currency in your cells so specific proteins are able to participate in active transport and the main one that you're going to see this year is the sodium potassium pump so your sodium concentrations tend to be higher in the extracellular fluid and your potassium tends to be lower and that's the opposite in your cytoplasm so if we go to picture one in the top left hand corner you can see how that protein is open to the cytoplasm and sodium ions join on to the protein at specific binding sites and then atp comes along and phosphorylates that protein and when it phosphorylates that protein it causes the protein to go through a confirmation shape and as it changes shape and it opens up to the extracellular portion of your cell membrane those sodium ions are able to exit when they exit as a result of that protein changing shape potassium ions are able to fill in onto some of the new binding sites and when they fill into those new binding sites they actually de-phosphorylate that protein and cause that phospho phosphate group to leave when that phosphate group leaves the protein changes its conformation again and now is open back to the cytoplasm side of your cell membrane and those potassium ions can now move into the cell so this allows sodium ions to move to the extracellular fluid where they're already high in concentration and this also allows potassium ions to move into the cell where they're at a much higher concentration so we've got two ions basically going against their concentration gradients through the sodium potassium pump so we talked about how passive transport is going to be consisting of diffusion and specific channel and carrier proteins while active transport again you're going against your gradient so atp or some sort of energy molecule is going to be needed to force the concentration gradient to work in the opposite direction so how ion pumps help to maintain your membrane potential the membrane potential is basically the voltage difference across your membrane due to the positive and negative ions that are moving back and forth between the extracellular portion of your membrane and the cytoplasm portion of your membrane the extra electrochemical gradient is able to diffuse or is able to work to diffuse ions across your membrane either one via chemically through the concentration gradient for your ion or because of how those ions are able to have an impact on the membrane potential of your cell the electrical force an electrogenic pump is a transport port protein that is able to generate voltage across your membrane and then that energy can be used to do additional work within the cell and the sodium potassium pump is the key pump that we see in most animal cells that acts as an electrogenic pump so why would we want to move things across the gradient against their concentration gradient i said that you can use these to kind of store energy and co-transport is one way that this occurs so here we have a proton pump which is allowing hydrogen ions to leave the cytoplasm to move to the extracellular fluid and when they leave those hydrogen ions are able to help to change the shape of the co-transporter sucrose channel protein and when they join on along with sucrose they help to move sucrose molecules into your cell which we know is kind of important since that's going to help to provide a source of energy for your cells so bulk transport across the membrane because what we've been talking about are pretty small amounts of things moving in and out with the exception of the aquaporins um there are definitely going to be times when you're going to want larger amounts of materials to both exit and enter the cell and they can obviously still do it through the lipid bilayer or through your transport proteins but if you have larger molecules we typically tend to see it in the form of vesicles and those vesicles the bulk transport forms are going to require energy so examples of larger molecules would be macromolecules your polysaccharides and your proteins so exocytosis is when you have a transport molecule that moves towards the membrane it will join on with the membrane it fuses to it and then it is able to release what is inside its contents and this is what many of your secretory cells will do to export their products back out of the cell across the cell membrane so they can go to their desired location within that particular organism in endocytosis the cell takes in macromolecules by taking by making vesicles that then will join with the plasma membrane actually i take that back it's when the cell takes in macromolecules by forming vesicles from its plasma membrane so it is the reverse um but a little bit different because with the exocytosis the membrane is already there as part of the vesicle and endocytosis the vesicle gets formed from the actual cell membrane you can have phagocytosis occur pinocytosis and receptor mediated endocytosis phagocytosis is when a cell takes in something um engulfs a vacuole some sort of food material that can then infuse with the lysosome to digest it and allow the cell to use its contents hydrocytosis is when a cell takes in molecules by taking in some of that extracellular fluid that gets sculpt in and then finally we have receptor-mediated endocytosis and that's when specific ligands will bind to receptors on the cell membrane surface that will trigger vesicles for being formed and those ligands can only bind to specific receptor sites so there's something that those ligands are aware of that is present around the cell membrane that that cell might want to take in and so there you see phagocytosis we're seeing an example of this with a pseudopod how it's taking in some sort of particle and forming a food vacuole which then the cell can put to work to use we can see that with pinocytosis where you see part of the plasma membrane pinch off in both examples there to form a vesicle that just has ions and fluid in it and then in the receptor-mediated endocytosis you can see how the ligands have joined up to the receptors and as they join up they may take some of them into the cell as well but they also will take in other particles that are present around and those it will form a coated pit that kind of helps the cell to recognize that this is not just a random exercise endocytosis event taking place