 I'm briefly going to review what I talked about on Friday, which was the structure of the cell, the function of some of the organelles, and in some detail, the mechanism by which proteins are sorted out. Those of you who missed lecture on Friday, be sure you watch the webcast because there's a lot of material in there. It's not in the book, so you've got to get it from me. Or the webcast. Very briefly, I talked about two pathways of how cells deal with targeting proteins. They end up in the right organelle. One was a co-translational import of proteins, and the organelles involved in this are the ruffiar, the Golgi secreted proteins, which are not an organelle. Lysosomes, the plasma membrane, and vacuoles. And what happens here is the proteins, as they are being synthesized, the ribosomes are, all protein synthesis starts and the cytosol, in this case, the ribosomes are taken to the ER membrane, to the ruffiar membrane, the proteins are inserted into the lumen, and then there's this complicated modification of these proteins, which involves glycosylation, the addition of sugars. And this basically gives each individual protein an address, and those proteins can then be delivered to where they're going, to their little mailboxes. I just wanted to mention something about the lysosome. The lysosome is an unusual organelle. It has an internal pH of about five, very acidic. There are about 40 different enzymes in lysosomes, and they're involved in breaking down macromolecules, but they break down these macromolecules hydrolytically. I didn't say that on Friday, so that's an important point. They add water across the various bonds, the glycosidic bonds or the peptide bonds, to break down these large molecules, and then the components, the smaller components are reused by the cell. The amino acids are reused to make new proteins. The other pathway that I talked about is maybe a little simpler. It involves post-translational import. In this case, the proteins are synthesized in the cytosol, and they remain in the cytosol. I'm sorry, that's not right. They are released into the cytosol, and then, so I'll put cytosol up here, because that is a destination, and then some of them are imported into various organelles. For example, the nucleus, mitochondria, chloroplasts, and peroxisomes. So, it's not a co-translational process, it's a post-translational process, and the nucleus import system is a little more complicated because there is a protein involved in binding the newly synthesized protein and taking it into the nucleus. In all of these cases, there is a signal sequence which directs the protein to the specific organelle, which is its destination. I mentioned at the end of the hour something about the chloroplasts and the mitochondria, and I'll come back to them in a minute in terms of what they do. These are what we call energy converting or energy transducing organelles, so they make ATP, and we will talk about how they make ATP in some detail over the last sort of two and a half weeks of the lectures. The other organelle that I wanna mention is the peroxisome. The peroxisome is an organelle that's involved in the oxidative degradation of fats and toxic compounds. It depends on what kind of cell where the peroxisome is. For example, liver cells detoxification is very important, so they are removing materials that can be or are dangerous to the cell. They do this in an oxidative process, not a hydrolytic process, so there's two different things going on between a lysosome and a peroxisome, and the two reactions involved in this detoxification process are a reaction of the compound, so this is the presumed toxic compound, with molecular oxygen to produce a less toxic product, and hydrogen peroxide, this is hydrogen peroxide. Now, hydrogen peroxide itself is a fairly bad compound to have around in cells. It's a very peroxide. It's a very strong oxidizing agent, and it's not anything you wanna have in a cell environment. So what peroxisomes have in them is a very high concentration of an enzyme called catalase, and the function of catalase is to react the hydrogen peroxide with another compound, our prime, various compounds from function here, to produce water as the product. So if you look at electron micrographs in the book or elsewhere, you will often see within the peroxisome a grid-like structure, it looks like a screen, and there's some organized structure in there, and that structure represents catalase. There's so much catalase in a peroxisome that it often forms a semi-crystalline array. So both lysosomes and peroxisomes are involved in breaking down things, but they do it by different methods. That was the point I wanted to make. Okay, coming back to what I was talking about at the end of the hour, which were chloroplasts and mitochondria. I'm not gonna talk about the internal structure of these organelles because we'll do that later. You should be aware they have internal membranes and are unique in this regard. But more unusual, they contain their own DNA, they contain their own RNA, and they contain their own ribosomes, which means these organelles can synthesize within them some of the proteins that will be functioning within that organelle. And I mentioned something like 20% of the proteins in the organelle are synthesized in the organelle, 80% are synthesized on free ribosomes out in the cytosol and transported into the organelle. This is unusual, right? Because there's no other organelle that does this. There's no other organelle that synthesizes proteins inside. There's some interesting features about this. The DNA in both the chloroplast and the mitochondria is circular, one circular DNA molecule. The ribosomes are, quote, small. By that I mean, if you remember, the numbers I put up about ribosome structure, the eukaryotic ribosomes are much larger than the so-called bacterial or prokaryotic ribosomes. So these ribosomes in these organelles are of the small variety, and more interestingly, they are inhibited, protein synthesis is inhibited in the organelle by antibiotics, the same antibiotics that will inhibit prokaryotic protein synthesis. Well, with all of this information, various people have proposed mechanisms or theories or models as to how chloroplast and mitochondria arose. Where did they come from? Because of these unusual features. And I think the theory that is most widely accepted is called the endosymbiotic theory. And what this theory maintains is that there was a eukaryotic cell out in the world somewhere which engulfed a prokaryotic organism, either something that was doing photosynthesis, which became a chloroplast, or something that was doing respiration, which became a mitochondrion. So this smaller cell was engulfed, but it was not totally eliminated. Many of the components within that small cell were retained in the larger eukaryotic cell. There must have been some value to the larger cell as well as to the smaller cell to not get rid of everything that was inside the small cell. What's also interesting is that some of the information within the genome of the prokaryotic organism must have been transferred to the nucleus. Because the nucleus encodes most of the proteins that are in these two different organelles. So it's very hard to prove this, but it's also very difficult to come up with an alternative explanation for the unusual features of the chloroplast and the mitochondria in that they have this ability to synthesize their own proteins. The last thing I wanna point out to you, I just mentioned to you, is I'm not gonna talk about any of the other components that are in the cell, the ones in particular that I'm not gonna talk about are components associated with what's called the cytoskeleton. And this is the only time I will tell you to read that section of the chapter in Campbell. There are three different cytoskeleton elements, and I want you, there's an enormous amount of detail in these last few pages of that chapter. They go overboard, I think, in terms of structure and organization. And the only thing I want you will be responsible for is to understand what the elements are, what are the components that are found in each one, and something about what their functions are in the cell. And when you see the amount of material on the cytoskeleton, you will thank me for only making you responsible for a small portion of this material. Okay. I'm gonna continue to talk now specifically about biological membranes. We're sort of getting outside the cell, the structure of the cell, and we're talking now about membranes. And again, if you look at the reader, you can see in figure, oh, incidentally, table 55 is a summary. It's a summary of all these cell components, most of them I think I've talked about, and their components and their function, and that's a good table for you to be familiar with. That's the sort of take home lesson of this last couple of lectures. Figure 56 shows a cell, a eukaryotic cell, and the number of different membranes that are found in that cell, and some gives you some indication of the function of some of these membranes. We will be talking about the structure of biological membranes, and then we will be talking about some of the functions, particularly in this portion of the lectures, the transport function, and then later, as I said, we'll talk about the membranes in chloroplastin, mitochondria, they have specific functions. You can see the first obvious function of the biological membrane is it's a barrier. It separates an inside and an outside, so when you put a membrane around a cell, there's an internal environment, and there's an external environment. However, this has to be a permeable barrier. In other words, you have to get things in and out of the cell, in and out of membranes, so it's not as simple as it initially seems. There are other specific functions of the membranes that I won't talk about, such as signal detection, things like that, but they're listed and shown in figure 56. The membranes, biological membranes are selectively permeable. They allow some materials to come into the cell and leave and some to not come in, and I've already given you some indication about the structure of a biological membrane by saying when we talked about phospholipids that phospholipids are a major component in a biological membrane. They form this so-called phospholipid bilayer, and we talk about phospholipid bilayers and membranes in the same sentence. And I've just put these numbers up. Again, the point here is simple. The numbers are only indicative. These are not numbers you're ever going to be asked to repeat anywhere in this section of the class. The numbers are the amount of protein, lipid, and carbohydrate in various types of membranes, different membranes. And you can see that a plasma membrane, the ratio is near one. It's almost as much protein as lipid. But as soon as you go to some other membranes, that ratio varies dramatically. The ER membrane, that ratio is two. I'm sorry, the nuclear envelope and the ER membrane. There's much more protein in those membranes than there is lipid. The inner membrane of the chloroplast and the inner membrane of the mitochondria have a lot more protein than lipid, particularly the mitochondrial membrane. That ratio is now 3.5, 3.5 times more protein than lipid in the mitochondrial inner membrane. Well, we still say the mitochondrial membrane is a phospholipid bilayer membrane, but you might as well say it's a protein membrane with a bit of phospholipid in it, because that's clearly much more accurate. So the amount of lipid and the amount of protein in a membrane can vary, and they can vary depending on the specific function of the membrane. When we look at membranes and membrane structure, we discover that there are two types of association of proteins with membranes, and I've drawn this typical figure here, you know, the little tadpoles with things in it, and the association is, firstly, there are proteins that are actually embedded in the membrane. That's what this glob is, this is a protein, and this is a protein, and that's a protein. And some of those proteins have regions that stick out of the membrane on one side or the other, but the major part of the membrane, the major part of the protein, is within the membrane. This protein I've put in here, and I don't have much sticking out, so all of this protein is essentially embedded in the membrane. These proteins are either called intrinsic or integral, they're integral to the membrane. I use these terms interchangeably, I can't be consistent. Sometimes I'll just say intrinsic, and another time I'll say integral, so they mean the same thing. The other kinds of proteins that are associated with the membranes are basically on the surface of the membrane. They're not in the membrane, they are interacting with the phospholipid head groups, which are polar or charged, through ionic interactions, and these proteins are known as extrinsic or peripheral. Extrinsic proteins are relatively easily removed from membranes. If you take membranes and you wash them with salt concentrations, high salt concentrations, or even if you vary the pH, these proteins may be released from the membrane. The integral proteins are tightly bound to the membrane, and the interaction keeping those proteins in the membrane is a hydrophobic interaction between portions of the protein and the phospholipid side chains. These proteins are not easily removed from the membrane, and in fact, generally, to remove them from the membrane, you have to destroy the membrane, and we'll talk about how you do that. The fact that these proteins are so tightly bound to membranes has made it very difficult to study them. So our understanding of membrane protein structure, particularly of the integral proteins, lags far behind our understanding of the structure of the so-called water-soluble globular proteins that one finds in the cell. If you look at figure 57, that is essentially an amplification of what I've drawn here. It shows the various ways you can have integral proteins within a membrane. Some of them cross the membrane one time, such as B, single-pass protein, and some of them cross the membrane many times, such as C or D, and then there are regions that stick out on some proteins and regions that don't stick out. So there are a lot of different ways membrane proteins are in the membrane. The most widely accepted model for the structure of a membrane, a biological membrane, is something, I'm going to erase something, okay? And I think this point that I made is pretty simple and you don't have to look at these numbers. So membrane structure, a model put forth by two biochemists down in San Diego in the 1970s, Singer and Nicholson. The model is known as the fluid mosaic model. And if you look at figure 58, it's hard to show this because this is a static picture and this model is a dynamic model. The main feature of the model is that yes, you have two kinds of interactions of proteins. The hydrophobic interaction of the integral proteins and the hydrophilic interaction of the peripheral proteins. But an important feature of the model is that the membrane is fluid. That is the proteins and the lipids can move. The membrane is not static. There's movement, particularly in this direction and that direction and this direction and that direction. You have to visualize this in three dimensions. What it really is like is icebergs floating around in the ocean, right? The icebergs are sitting in the ocean. Some of them are above the surface, some of them are below the surface, but they can move around in the fluid environment of the ocean. The proteins can move around in the fluid environment of the phospholipid bilayer. Fluidity would be expected to be lower in membranes that have high concentrations of proteins, for example, the mitochondrial membrane. If you lower the temperature of a membrane, you start to basically freeze out this movement. If you look at lipids, you can see in figure 59 and 60, this relates back to what we were talking about before, the phospholipids. It shows two kinds of phospholipids. It shows phospholipids that has a high concentration of unsaturated fatty acids, side chains. That's shown, the best model is figure 60, the space-filling model. The lower portion of that figure are phospholipids that are, you can see there are unsaturated fatty acids. The upper one has saturated fatty acids. The packing in the upper one is tighter than the one with unsaturated fatty acids, which means a membrane that has a high concentration of saturated fatty acid is going to be less fluid than a membrane that has a lower concentration of saturated and a higher concentration of unsaturated fatty acids. And it's got to do with the packing of the lipids. And I like the figure 60 because I think you can see right there the difference in the actual structure of the molecules. In the top, the packing is tight. At the bottom, the packing is much looser. I have to deal with cholesterol, which is a bit confusing. Cholesterol is a component which is found in biological membranes in animal cells. Plants don't have cholesterol. If you probably don't remember the structure of cholesterol from an earlier figure, it's shown in figure 62. It's shown inserted into a membrane. The problem with cholesterol is it's difficult to predict whether it will increase or decrease the fluidity of a membrane. It's a temperature dependent to fit. If you look at figure 62, you might say, well, cholesterol is stuck in between. It's like in here and it's in here and it might disrupt the fluidity and make these membranes more fluid than if they were not cholesterol was not there. It's not that simple. I mentioned cholesterol because it is something that everybody has heard about and a lot of discussion of cholesterol and what it does. You should at least be aware. It's found in membranes and let's just say it affects fluidity in a rather complicated manner. How's that? That's called hedging. Another important feature of these membranes which is shown, let's go back to our picture. Yeah, okay, let's go back to the picture of the fluid mosaic model, figure 58. I think even from figure 58 or from, yeah, that's fine. You can see that the membrane is asymmetric. Both sides of the membrane are not the same. They're not the same for a number of reasons. One is the protein that sticks out on one side of the membrane, may not stick out on the other or it's going to be different from that. So that produces an asymmetry to the membrane and also the lipids. Have been found to be different on one face of the membrane to the other face of the membrane. So this nice way of drawing all these little tadpoles with all the tadpoles being the same, that's a gross oversimplification of membrane structure. Membranes are asymmetric and this is important for some of the processes that we will talk about later where there is the side of the membrane is important in the function of that particular membrane. Okay, I want to talk in more detail about how one characterizes membrane proteins and what kind of information we have gotten over the years concerning the structure of membrane proteins. If you look at figure 62, which is in some ways like 58, what is shown is you've got a membrane and I'll draw a bit of phospholipid for you and then there's a protein that goes like this and it goes like this. There's an alpha helix. What has been figured out over the years is that the region of the protein that crosses the lipid bilayer is an alpha helix, forms an alpha helix and you can see in figure 63 that protein, there's one alpha helix on the one on the left, there are three alpha helices on the one on the right. Actual structure of some membrane proteins, not structure, but models of proteins. Again, these are real proteins shown in figure 64. Again, a protein that has a single alpha helix crossing the membrane and the protein next to it, bacteria radopsin, we'll talk about that a little bit later, has many, many hydrophobic alpha helices crossing the membrane. How do we get this information? Well, I mentioned it's not easy to work with these proteins. What one has to do to study the hydrophobic proteins of a membrane is to remove them from the membrane. To do this, one requires compounds which are known as detergents. That sounds familiar because these are actually the kinds of detergents that one uses to wash your clothes or wash your dishes. These are compounds, such as shown in figure, I'm jumping around in figures, figure 69, there are the structure of some detergents that are used. These are chemical compounds, non-physiological chemical compounds, which when added to membranes dissociate the membrane and the proteins, hydrophobic proteins, stay in solution because the detergent acts as a sort of pseudo lipid. That's what's shown in figure 68. The detergent molecules are essentially coating the protein and allowing that protein to remain in solution. You can see a compound like sodium cholate. It looks very much like a steroid. Octoglucoside looks sort of like a sugar, but it's got a long hydrocarbon side chain. And sodium dodecyl sulfate looks like a fatty acid. The compound that is used most often is this compound, sodium dodecyl sulfate. It is very effective at dissolving away membranes. A couple of problems arise when you take these proteins out of the membranes. One is they may not have any activity. When you remove a protein from a membrane with sodium dodecyl sulfate, it is usually inactive. It doesn't do anything. Some detergents are, quote, milder. That is, you can remove a component from the membrane and that component will retain its biological activity. But that is tricky to do. What can one do with these proteins? You've got now, let's say, a red blood cell membrane, and you want to understand what proteins are in the red blood cell, the membrane of the red blood cell, and what they do. So you, firstly, can remove them from the membrane with a compound like sodium dodecyl sulfate. And you can carry out something, and I didn't bring my little gel today, something called sodium dodecyl sulfate polyacrylamide gel electrophoresis. That is all spelled out in Figure 70. I'm not going to write out polyacrylamide gel electrophoresis. SDS page, it's an electrophoretic technique where you make a gel, essentially, out of acrylamide. And it's got a crust, basically, it's like a pores of different sizes. And compounds will migrate in through these pores depending on their molecular weight. So what Figure 70 shows is you take a solution of membrane fragments, which have been obtained by, say, treating with sodium dodecyl sulfate. You load this onto a gel, and you put this in an electric field, and the proteins migrate down. They migrate down depending on their molecular mass. After a certain period of time, you stop the thing from running, you take the gel out, and you stain it with a compound, which gives a blue band wherever there is a protein, cumissie blue, it's called. And if you look at the right of Figure 70, the lower figure, you can see there are various, these are not blue bands. If you see an actual gel, you will have blue bands on your gel, and there are seven or eight or 10 different bands. Each one represents an individual protein. They've been separated because they have different molecular weights. Then you can actually, if you're clever and careful, you can cut these little bands out, and you can determine some amino acid sequence of those bands. And hopefully, it's enough so that you will be able to identify the specific protein that it is. Well, why is that useful? Well, when you start to determine the amino acid sequence of these proteins, there are various techniques which allow you to predict the structure of the protein. There's a technique, and these are all computer-based. So you take your amino acid sequence, and you feed it into a computer, and you carry out an analysis which yields something called a hydropathy profile. And a hydropathy profile essentially looks at the entire sequence, and it is based on the fact that the very hydrophobic amino acids are going to be in one region, the very hydrophilic amino acids are going to be in another region. And each amino acid is different, even though maybe hydrophobic some are much more hydrophobic than others. And you can get a program that will basically plot out the relative hydrophobicity or hydrophilicity of an individual protein. That's what is shown in figure 66. That's a hydropathic profile for a particular protein. There are some regions which are very hydrophobic. Those are shaded. There's four regions in this protein, actually, that are very hydrophobic. And then the rest of the protein, there's not much more of it, are fairly hydrophilic. But there are four regions. I think you can see those shaded regions. I'm talking about this figure. So this is hydrophobic. Anything in that, and this is hydrophilic. And you've got various points representing individual amino acids. And in this particular protein, you've got a region like that. There is a little bit of nothing, essentially, small region. Then another region above the line, then a larger connecting region, a third region above the line, and a small thing, a fourth region above the line, and then something like that. These regions, if you look at the sequence of the protein, each one of these regions contains 20 to 30 hydrophobic amino acids. And the prediction is these represent the membrane-spanning regions, or represent membrane-spanning regions in this protein. So you take the graph at the top of figure 66, and you end up with a model which is shown below that, where there are four membrane-spanning regions. This little loop is shown as a region which is connecting two of the membrane spans. This stuff at the end is the C-terminus of the protein. So this has been a very, very powerful technique in providing information as to whether you have a membrane protein or not. If you find in the structure of a specific protein, a region where you've got 25 very hydrophobic amino acids, it's very likely that you have a membrane-spanning region in that particular protein. Now, all of this was sort of hypothetical for a large number of years, because we did not have the actual structure of any membrane protein. And it was, I think, 1986, 1985. This is about 30 years after the structure of hemoglobin and myoglobin were done, a long time after. There was a group in Germany, group of biophysicists, who isolated from a photosynthetic bacterium a membrane protein complex. Not only did they isolate it, but they were the first one to crystallize a membrane complex. It happened to be from photosynthesis, but that's neither here nor there. They crystallized this complex, and they determined the x-ray crust structure. They determined the complete three-dimensional structure of this complex. And this is a schematic. Can I have the? Yeah, OK, great. Can you see that? Well enough. I'll move it down a little bit. OK, let me say a couple of words about this. This work was done by Dyson Offer and Miquel. These tubes represent membrane helices. There are four proteins in this complex. So there are one, two, three, four, something like 10 or 11 membrane-spanning regions. There are regions outside the membrane. These little black shaded boxes represent chlorophyll molecules in this structure. There's another region down here that is not in the membrane. This is the presumed, this region here, hydrophobic core of the membrane. And when this came out, this was an amazing result. I mean, it was just totally unprecedented. Here was the structure of a membrane protein. You couldn't do this, but it was done. It gave an enormous amount of information on a number of subjects. One, photosynthesis, which we'll talk about later, much of our thinking about photosynthesis and how photosynthesis occurs was generated out of this structure when we had the actual structure of this complex. But also about membranes and membrane components. For example, I said that these interactions are hydrophobic. And there are these 25 or 30 amino acids that have been predicted to form membrane spans. Well, each one of these membrane spanning regions contained about 25 amino acids. They were all hydrophobic. There's a picture in one of their original papers where they show the number of charged amino acids in this structure. And each one is sort of spotted. There's a little light, a little bright light. There are hydrophobic amino acids here. Hydrophilic amino acids here. This is totally black. There's not one hydrophilic amino acid in the membrane portion of that structure. So they worked this structure out in about 1983 or so. Big paper came out in Nature, and everybody was absolutely amazed because you weren't supposed to be able to do this. You were not supposed to be able to crystallize membrane proteins because of a number of different reasons. It took the people in Stockholm who like to recognize significant work all of, I think, two years to give them the Nobel Prize in Chemistry because they got it right this time. Sometimes they don't get it right. An enormous breakthrough. And they not only determined this structure, but they worked out techniques for the crystallization of membrane proteins. So that membrane proteins still, we don't have as much information as we do on soluble proteins. But we have maybe, I don't know, 50. 50 is probably too many. Structures of membrane proteins. But lots of very important membrane proteins. And the basic principles that were first put forward by Dyson Offer and Miquel about why, what will cross a membrane and how are these interactions have been upheld. So that's what we know about membrane structure and membrane protein structure. And it's a lot better than it used to be, is all I can say. It used to be pretty bad, but we really do have a lot of information. The rest of the hour today, and then I'll finish it up on Friday, we're going to talk more about one of the important functions of proteins in membranes. And that is transport. As I said, membranes cannot be impermeable. They have to allow molecules to cross in both directions. So you have to have proteins in membranes that are allowing things like sugars, iron transport, very important, and even larger molecules like proteins and things like that to get into cells and to get out of cells. In terms of small molecules, which are metabolites, like let's say sugars, amino acids, and ions, there's a very simple principle that's held in terms of what will cross a membrane. And I think you probably understand this already. So figure 71 shows a membrane, and it shows that some molecules, which are hydrophobic, for example, are going to cross that membrane pretty easily. But large, uncharged molecules such as sugars, which are pretty hydrophilic, and small molecules such as ions, potassium ions, sodium ion, chloride ion, these things you would not expect to cross membranes very readily. There are several ways things can cross membranes. The first is simple diffusion. If I have a membrane, and I have a high concentration of a solute on one side of the membrane and very little on the other side of the membrane, you know that these things will cross the membrane. And they will cross quickly if the molecule is hydrophobic, less quickly if the molecule is hydrophilic. The higher the concentration on one side, the faster they cross. So if I were to plot rate of movement versus solute concentration, it's going to be linear. The more I have on one side, the faster it will go. But that's dependent on the property of the particular solute. Hydrophobic one will go very fast. So that is called simple diffusion. Cells, for the most part, do not use simple diffusion to allow molecules to cross membranes, because generally it's slow. And you often do not have a high concentration on one side moving in the direction of the gradient. So cells firstly have a mechanism, or involve a mechanism, which we will call facilitated diffusion. Facilitated diffusion is protein mediated. The facilitators here are proteins, so it involves proteins, protein mediated. The molecules are moving, however, from high concentration to low concentration. And that's why we still are calling it diffusion. The kinetics, however, of this process are not linear like this, so this is simple. What you see is something like this. This is facilitated. So that's the way facilitated diffusion works versus simple diffusion. This behavior, which you will see again when we talk about enzymes, shows something which is known as saturation. The rate increases. Usually it increases rather rapidly, and then it slows down. At this point, we are still moving molecules in, but the rate is not increasing. This is a phenomenon which is known as saturation. And these curves show saturation behavior because you have a fixed number of molecules in a membrane. Let's say we're transporting glucose into the cell. There are a fixed number of protein molecules that can transport glucose. So when you start off at low concentrations of solute, the reaction occurs quickly. You have a lot of protein and very little solute. But when you go up in concentration, you get to a point where every protein molecule is working. So by increasing the solute concentration, I'm not going to get any increase in rate. This is similar to what we'll talk about when we talk about enzymes. So you see it now, and you're going to see it next week as well. If you look at figure 73, you can see that that summarizes the kinds of protein molecules that are involved in facilitated diffusion. And another kind of transport. You can have molecules which are called carriers where the molecule being transported actually binds to a protein. In the membrane, that binding may cause the protein to change shape. And that allows that molecule to move across the membrane. There's some type of interaction. There are, however, channel mediated diffusion processes where basically it's almost like the nuclear pore. I said the nuclear pore is a hole in the membrane. A channel is a hole in the membrane. And the hole can be of various sizes. You can have a big hole or a little hole. That's where the specificity for what gets transported comes from. A channel that will transport sodium may not transport potassium or vice versa because of the dimensions of the particular hole. If you go back a couple pages, I skipped this figure. But if you go back and you look at figure 65, figure 65 is a potassium channel. And you can see there are a number of membrane-spanning regions. But the top down view shown on the right shows that the potassium actually goes down a channel. And you can visualize that if that channel was much smaller, potassium might not fit. If it was much bigger, something else would fit. So there are actual channels within the membrane which allow the transport of molecules. In all of these cases, the facilitated diffusion, the molecule diffusing is being moved with its concentration gradient. It's going from high concentration to low concentration. On Friday, we'll talk about the situation where it's the opposite, where you have to move things against the concentration gradient, much more complicated. OK.