 For those that are here today, people are taking a longer than usual weekend, I think. The first point up there is important, Monday's a holiday, right? So there's no lecture on Monday, there's no discussions on Monday. What you can do is you can go to a discussion on Tuesday, anyone. There are discussions on Tuesday. If you go to a discussion and there end up being 40 people in the room, the GSI is going to give priority to the students that are regularly scheduled in that discussion. So you might have to sit on the floor or hang from the ceiling, whatever. So take advantage of that opportunity because those of you who don't have any discussion on Monday are going to sort of fall behind, you're not going to have a discussion until the following Monday. There are also no labs next week because of the Monday holiday that throws a mess into our schedule and we have to cancel labs for the week. Again, office hours today, 9 to 10 in 2084 VLSB. Some people actually came over to my office in Koshland looking for me for office hours. I don't hold my office hours there. My office is about this big. Even 2084 is getting small compared to the number of people coming. But that's where the office hours are. I'll have them on Tuesday morning as well. Okay, so today I'm going to talk in some detail about the structure of the eukaryotic cell and I'm going to be jumping around a little bit in the figures. So I hope you will bear with me. They're not going to go in a nice straight order because I've sort of changed the way I discuss this subject. But I think you ought to be able to get through them. I want to just start with some general comments and the general comments refer to figures 39 and 40 in the handout on page 18. Figure 39 shows you a typical eukaryotic cell and it shows you the large number of membrane bound components that exist in that cell. And some of the functions of these components are given in the table at the bottom of that page, number 40. And we will be talking more about the functions of the specific organelles in some detail. I've put up two tables of numbers and these are just not anything you're going to have to memorize or anything like that. Just to indicate to you the general composition of the cell compartments in a typical mammalian cell. And when we talk about a typical mammalian cell, we use a liver cell as the sort of model. So if you look at a liver cell, 54% of the volume is a cytosol, this solution soluble phase of the cell. And then there's a substantial, a large percentage of the cell is occupied by mitochondria. These are the energy transducing organelles and then smaller amounts of some of the other membrane bound components. The rough endoplasmic reticulum we'll talk about in detail, the smooth endoplasmic reticulum and the Golgi, the nucleus, peroxisomes, lysosomes. And if you put it on a number basis, don't ask me how they do it. I don't imagine anybody counted these but they must have done some calculations. Again, you can see the predominance of mitochondria in a liver cell compared to many of the other components. Although there are substantial high numbers of both lysosomes and peroxisomes. Now all cells differ in us, ourselves. Certain cells have certain functions and you might expect that these components are going to vary depending on the specific function of the cell. So here I've found some numbers where you can compare the percent of the total cell membrane in a liver cell versus a pancreatic cell. A pancreatic cell is actively engaged in secretion of proteins. And what you see are very high amounts of the ER membrane, the rough endoplasmic reticulum as it's called compared to a liver cell. And when you look at mitochondria, a very high concentration of mitochondrial inner membrane. And you'll understand what that means probably later today or on Wednesday relative to what you find in a pancreatic cell. Pancreatic cells are not actively that actively engaged in making proteins in contrast to a liver cell. So the cell type and its composition is going to vary depending on the particular function. Okay. When we start looking at the cell, the first thing we see is the membrane that surrounds the cell. The cell membrane or as it is called the plasma membrane. And I've given you some information previously about the general, what you might expect the general structure of this membrane to be. I'm not going to talk in great detail about the cell membrane structure because I have a full lecture on membranes coming after cell structure. But you can see there's a phospholipid bilayer which is the major feature of this membrane and the major feature of most biological membranes. But in addition to that, there are proteins stuck in this membrane and they perform absolutely essential processes like transport of molecules across the membrane. The interactions in this membrane are primarily hydrophobic because of that phospholipid bilayer. You know, all those fatty acid side chains that you can see. Some of the interesting features of the structures of these proteins we'll talk about. One that I'll point out because it's pointed out in this figure is that some of the proteins which are actually inserted into the membrane. So they are facing the outside and the inside. They have carbohydrate groups attached to them. They have sugar molecules attached to them. And that's shown to the right of that figure. And the sugars are important in basically identifying that cell as possibly a liver cell versus a pancreatic cell. Okay. I'm not going to talk more about membranes. We'll come back to it. The next cell component I want to talk about in a little more detail is the nucleus. And the nucleus is, I've already mentioned, generally the most conspicuous organelle in the cell. Those of you who've been in lab this week I'm sure have seen nuclei and some of the cells you've looked at. There is an envelope that surrounds the nucleus, which is known as the nuclear envelope. It's a phospholipid bilayer structure, but it's unusual in the sense that it has pores. There are actually holes in this envelope, nuclear pores. And these nuclear pores are rather large. So what's the purpose of the pores? The pores allow things to go in and out of the nucleus. Now I will make this very clear, this term I'm going to use. I'm going to say almost, almost put it in a box, almost all of the DNA of a cell, the genetic material of a cell, is in the nucleus. And by almost I mean not all and I will tell you later where there is another place or other places that have DNA. But almost all of the DNA is in the nucleus. It's organized into linear chromosomes depending on the organism. You might have 46 chromosomes. You might have 24 chromosomes. And those chromosomes, those proteins are associated with proteins. So you've got DNA protein complexes in the nucleus. What's the structure, what's the function of the nucleus? Well, obviously the genetic material is there. The information that directs all of the cell's activities is in the nucleus. It doesn't ever leave the nucleus. As an introduction to what's coming next, I've put this up and I'm sure everybody has seen this before. The information in DNA is transcribed into RNA. That occurs in the nucleus. There are different kinds of RNA that are made in the nucleus. Two of them that we will be concerned with will be ribosomal RNA. RNA that's found in the ribosomes of the cell, which we'll talk about next, and messenger RNA, which has the information that gets converted into proteins. That occurs by the process of translation that does not occur in the nucleus. It occurs primarily in the cytosol of the cell. If the DNA is being converted into RNA, RNA is being synthesized in the nucleus, and that RNA is being translated outside the nucleus, that means the RNA, the so-called messenger RNA, is leaving the nucleus through these nuclear pores. That's one of the points about the pores. RNA can go out through these pores. Ribosomes, the key component involved in the translation of proteins are these particles. I call them particles. If you have an earlier edition of Campbell, there were about seven editions, I think, where Campbell called ribosomes organelles. Ribosomes are not organelles because they're not surrounded by a membrane. They are particles. Ribosomes are the little factories that synthesize proteins. I am not going to talk about how this occurs. It's a very complicated series of reactions that I think must cover at least one of Bob Fisher's lectures. We understand how the ribosome does this, but for our purposes, it's not important how the ribosome actually synthesizes proteins. The key here is you require ribosomes to synthesize proteins. If you have an organelle that has ribosomes, ribosomes can be synthesized in that organelle. If you do not have ribosomes within an organelle, then there has to be some other mechanism for taking the proteins, which are, for example, being made in the cytosol and getting them into these organelles. And this is this problem of protein targeting that I alluded to last time. Ribosomes are enormously complex structures. This is a big table. Again, there's only one point I'm trying to make here, and that is ribosomes are extremely large, complicated particles. They are composed of two parts called a large subunit and a small subunit. Each of these subunits contains RNA and protein, RNA and protein, and RNA and protein. So the ribosomal subunits, the large subunit, has three different RNA molecules. These are called ribosomal RNAs, 49 proteins, etc., 133. The ribosomal subunits are assembled in the nucleus. What happens is the RNA is synthesized in the nucleus. Proteins, which are made in the cytosol of the cell, are transported into the nucleus. They associate with the ribosomal RNA, and these ribosomal RNA subunits move out of the nucleus into the cytosol of the cell. Very complicated trafficking here. The large subunit and the small subunit are made separately. They associate with each other when they start to get involved in the synthesis of a protein. Now, one of my favorite little tables is table 44, figure 44. It shows ribosomes, but it also shows... Oh, I have to make a point about this. I'm sorry, prokaryotic ribosomes and eukaryotic ribosomes although doing the same thing, synthesizing proteins, differ. They must differ in structure in some way. The eukaryotic ribosomes are much bigger than the prokaryotic ribosomes. Interestingly, prokaryotic ribosomes, bacterial ribosomes as we call them, react with various antibiotics such as streptomycin or tetracycline, and they will stop protein synthesis and obviously kill bacteria. If you have a bacterial infection, it's very likely that your doctor will give you something like streptomycin to wipe it out. So there must be some slight difference in the structure of these two ribosomes, although they do the same thing. The structure of ribosomes, the actual three-dimensional structure of these very large complexes of proteins and RNAs has actually been worked out by electron, I'm sorry, by x-ray crystallography. And I'm not sure if it was last year or the year before, but very, very recently there were three groups who received the Nobel Prize in Medicine and Physiology for elucidating the complete structure of a ribosome. This is the largest biological thing for which we have a structure, four million molecular weight. This thing has 80 different proteins in it. So this is really a major accomplishment and has allowed people to really understand what happens in a ribosome, protein synthesis, in great detail. Within the cell, if you look at Figure 44, you can see ribosomes exist in two forms. They exist as what we call free ribosomes, which is what we'll talk about, free, which are in the cytosol, and bound, which means they are attached to a membrane. So there are two populations of ribosomes depending on where they are localized. The ribosomes that are free and the ribosomes that are bound are the same ribosomes. It's just that some of them sometime get stuck on a membrane and we'll talk more about that. That is the endoplasmic reticulum side of that figure. They get stuck on this membrane, they synthesize proteins, and then they're released from the membrane. When they're released, the two subunits of the ribosome dissociate and they go back into a pool of large subunits and small subunits. As they start to synthesize proteins, large and small subunits come back together. If you look at the middle figure on page 21A45, it shows the large subunit, the small subunit, coming together, becoming associated with a messenger, RNA, and a protein is synthesized. But a bound ribosome is not different in any way from a free ribosome other than it is bound part of the time. What we're going to talk about now is the sort of left, the right-hand portion of figure 45 at the bottom. What happens when you synthesize proteins on free ribosomes in the cytosol of the cell? The proteins which are synthesized here, I'm going to eliminate this so you don't get confused. They are synthesizing proteins that end up, that remain in the cytosol. Remember I mentioned last time, there's a lot of metabolism that goes on in the cytosol. So a lot of proteins are made in the cytosol and they just stay there. Proteins destined for mitochondria are synthesized on free ribosomes. Proteins destined for the chloroplast are synthesized on free ribosomes. Proteins that go into the nucleus are synthesized on free ribosomes. And proteins that go into paroxysomes are synthesized on free ribosomes. So you've got a number of different sites to which proteins are targeted and all of these proteins are made in the cytosol. And that's what's shown on the right-hand side of Figure 45 at the bottom. You can see the nucleus, the mitochondria, the chloroplast, the paroxysome and the cytosol. These proteins are synthesized and somehow now we have to try to understand how do these cytosol proteins end up in one, two, three, four, five, six different places. The key to this came when it was discovered that proteins, as they're synthesized, and let's see if that's shown anywhere. Not as well as I'd like it to be shown. It doesn't matter. As they're being synthesized, there is a series of amino acids at the end terminal, generally the end terminal, which acts as a signal. And if you look at Figure table 47, it shows you a number of different regions that I've just talked about, and it shows you what you find at the end terminal end of these proteins. So a protein that's going into the mitochondria has a large number of amino acids. It looks like it's almost 20 amino acids that is attached at the end terminus, a protein that's going into the peroxisome has three amino acids that's attached, for example. And these are signals. By that I mean the peroxisome will have a receptor on its outer membrane that can recognize these three amino acids and say, yeah, you can come in, but you can't because you don't have the right amino acids. Contrary, the mitochondrial receptor can recognize this series of amino acids. I don't remember how many, X and Y amino acids for the chloroplasts. So these three organelles, the proteins all have a signal sequence which is recognized by something on the surface of the organelle and allows those proteins to be imported. So this is the first way you can sort out proteins and their trafficking or targeting. Now the nucleus is a little different. There is a, it shows in that figure, I think, yes, there is a sequence of amino acids that is in the proteins that go into the nucleus. But the import of proteins into the nucleus is protein mediated. What do I mean by that? I mean if you look at figure 48, you can see there's a protein and it has an NLS, which is a nuclear localization signal. It's got some amino acids at its end terminus generally and that signal allows it to react with a protein and this protein is known as importin. That doesn't mean it's important, it means it's involved in importing this protein into the nucleus. The protein plus this importing molecule take the protein into the nucleus, then there are some other things that happen that are irrelevant to our discussion, and the importin comes back out to pick up another protein. So again, there is a sequence of amino acids in all of these proteins, even the nuclear directed ones. However, the nuclear import requires an additional protein. Now you're going to ask me what happens to these sequences of amino acids? Do they remain on the protein? They don't. There are proteins inside the organelle that will cut off this sequence of amino acids. It's not required for the biological function of these proteins. This process that I've just described for mitochondria, chloroplast, peroxazones, and nucleus. Oh, by the way, what happens to cytosol proteins? Proteins that just stay in the cytosol. Do you expect them to have a signal sequence or not? No, they just don't come with a signal sequence, so they stay in the cytosol. This process is called post-translational import. Remember, translation is the synthesis of a protein from RNA to protein. And what you're doing here is, if you look at figure 45 again, is you're synthesizing the protein. The protein is actually released into the cytosol and then it gets into the organelle using these signal sequences. That's a lot of sort of new stuff, I think. There is another process which is called co-translational import, which involves the so-called bound ribosomes that I alluded to. There's a series of membranes in the plant in cells, which is known as the endomembrane system. The endomembrane system are membranes that are related by direct physical contact or by the transfer of components through vesicles from one section to another. Vesicles are basically membrane-surrounded sacs. So what you have is a movement of material, for example, from the rough endoplasmic reticulum to the Golgi complex by vesicles, from the Golgi complex to the plasma membrane by vesicles. The components that are part of the endomembrane system are the membrane around the nucleus, the nuclear envelope, this so-called endoplasmic reticulum, which I'll talk about in more detail, the Golgi complex, the plasma membrane, the lysosomes and vacuoles, which are primarily in plants. The way these proteins are targeted, it starts off with a signal sequence, but then there is a different mechanism that's used for targeting these proteins to these various places. So what you have in Figure 44, my little favorite figure, is this summary that shows the cytosolic proteins, the cytosolic proteins. However, it doesn't have chloroplasts, that's on the left, and then you have a system involving the endoplasmic reticulum, which involves also the Golgi complex and proteins moving through this system to their final destinations, which are the lysosomes, being secreted from the cells or being transferred to the cell surface for the synthesis of more plasma membrane. You've got to look now, you can either look at the bottom of Figure 45 at the left, yeah, look at the left. What one has in this system are some membranes that when you look at them, they have an unusual appearance. The endoplasmic reticulum is a membrane system that contains two regions. One is a smooth region, which is known as the smooth endoplasmic reticulum, and the second one is a region that has bumps on it, which is known as the rough endoplasmic reticulum. The rough endoplasmic reticulum has this appearance because it has ribosomes stuck on it. These are the so-called bound ribosomes that I was just alluding to over here, that I erased, bound ribosomes. The bound ribosomes are actually ribosomes which are sitting on the rough endoplasmic reticulum, and that's what's shown. You can see that in Figure 42, not very well. Figure 42 shows the endoplasmic reticulum. Membrane system, look in the book and you're going to see a much better picture of this. You can see there's one membrane region that's got a lot of particles on it. Those are the ribosomes, and then you've got a region which is particle-free. That's the smooth ER. One says ER. What happens in this system is that after protein synthesis is initiated in the cytosol, the ribosome is actually dragged to the surface of the rough ER membrane. That's what's shown in Figure 45 to the left, and as the protein is being synthesized, it is being inserted through that rough ER membrane into an inner space, which is known as a luminal space. So you've got a thing that looks like this. It's a sac, and you've got a ribosome sitting on it, and the protein is inserted into this luminal space. Luminal. Did I spill that in any way like I wanted to? No. I'll just say lumen. Actually, this is to be perfectly accurate. I should show it in two parts, the large and the small subunit part. But the protein is not fully synthesized before it's inserted into this space, in contrast to our co-translational, our post-translational process. So what we have is a co-translational import. The protein is being synthesized and imported into this space as it is being synthesized. Once it's in, once its synthesis is complete, then the protein is present in this space and various things that I'll describe happen to it. So the two processes are very different. How does this ribosome get dragged to the surface and all of that, how does that happen? Well, figure 46 gives more detail of that part of the process. As the protein synthesis starts, again, in the cytosol of the cell, there's a signal sequence, again, on that protein. But in this case, the signal sequence is used for the emerging protein to bind to a signal-recognition particle, SRP. A particle that actually forms a complex with a newly synthesized protein drags that ribosome to the ER membrane. And then that thing leaves and the protein can be inserted somehow through the membrane and synthesis can be completed. And then the ribosome leaves. These ribosomes do not stay on the ER membrane. Once they're done making proteins, they leave the membrane and the two parts dissociate from each other. So you have to understand the difference between co-translational import, post-translational import. You have to understand which components are synthesized by this process and which component are synthesized by this process. Very, very important. Okay. What do we have happening next? We've got the rough ER where synthesis of proteins occurs. Smooth ER, there are no ribosomes. That's why it's smooth. What does the smooth ER do if it's not involved in the synthesis of proteins? The smooth ER is involved in a bunch of metabolic processes. It is actively the site of lipid biosynthesis. Actively, for example, steroids, fats are synthesized in the smooth ER. It's involved in carbohydrate metabolism, the breakdown of polysaccharides. It's involved in the detoxification of drugs or other compounds that the cell doesn't like having around. But if you remember that it's primarily where lipid biosynthesis occurs, that's pretty safe in terms of what the smooth ER does. What it does not do and what it is not engaged in is protein synthesis, and that's because there's no association of ribosomes with this part of the membrane. Also, you should know that by now that most of these organelles that I've talked about, such as the Golgi complex, lysosomes, are incapable of synthesizing proteins themselves, because they don't contain ribosomes. Chloroplasts and mitochondria are special and unique in this regard, but we will talk about them in more detail in a little while. Now, what happens to these proteins when they are in the lumen of the ER? There's a very complicated series of reactions that occurs. The first thing that happens is look at figure 50. I certainly hope you have your book because there's no way I'm going to draw this. What first happens is a large number of carbohydrate groups. 14, in fact. 14 carbohydrates are attached to these proteins. They're not attached to all the proteins because proteins that remain in the ER don't get this attached. And there is a specific amino acid in all of these proteins and asparagine, which binds this 14 carbohydrate group. Once these proteins have been glycosylated, glycosylated is a word you should become familiar with, glycosylated means addition of CH2O groups, carbohydrate groups, and you'll see this term again over and over. So now these proteins have been modified, but they've been modified by the addition of sugar groups. The proteins are then transferred from the rough ER to the Golgi complex, and you can see this in figure 49, for example, where the rough ER is shown at the top of the figure. The transfer of proteins from the rough ER to the Golgi involves what are called transport vesicles. These are vesicles, membrane-bound sacs. The sacs are formed by budding off the ER membrane. And as they bud off, they trap proteins inside. Those proteins move from the ER to the Golgi. They don't go anywhere else. So proteins in the lumen of the ER have two fates. They stay there, okay, stay, or they go to the Golgi via vesicles. The Golgi complex is an amazing body. The Golgi complex is basically the post-office of the cell. What the Golgi complex does is it takes proteins that comes in and it modifies them further. All the proteins that come into the Golgi, the modification in the ER is identical for all proteins. But what happens in the Golgi is some proteins are phosphorylated. Phosphorylated means that phosphate is added to them. Some sugar groups are removed. Some new sugar groups are added. If you want to see the details of what happens in the Golgi complex, you look at figure 51, which shows protein synthesis in the ER, and then a series of steps that occur in different regions of the Golgi complex where you modify the sugar group that was put on in the ER, in the lumen of the ER. And what you're doing when you do this is you're basically giving these proteins different addresses. For example, proteins in the Golgi that end up with a phosphate group, they will go to the lysosome. Proteins that may have, let's see, what does it say? A galactose group added to them. Maybe they will go to the cell membrane. Proteins that have mannose removed from that group will be secreted from the cell. It's as if each group of proteins has a different address, and that address is a chemical address. It's a chemical address based on the structure and modification of this large sugar group that has been put on in the ER. So the Golgi, firstly, modifies proteins. It modifies proteins so they can be sorted out and directed to different places. So not only is it a modifying place, but it's a sorting place. Once you modify these proteins, you've got to be able to sort them and deliver them to their final resting places, if I can call it that. Their final resting place being secreted from the cell, lysosome, plasma membrane, or remaining in the Golgi complex. So the Golgi is actually a rather remarkable complex that can do all of these things. Figure 52 gives you some schematic idea about the thinking in terms of how proteins get sorted out. For example, there must be a region in the Golgi complex where proteins that have a phosphate group, there must be a region that can recognize a phosphate group on these proteins, can collect these proteins in this region, and bundle them up again in a vesicle, a transport vesicle, and take them to lysosomes where they will become lysosomal proteins. Other proteins have to be bundled up, put in vesicles, which are known as secretory vesicles. These are proteins to be secreted from the cell. So there's got to be different regions in the Golgi that can do this, that can accomplish this. I'm not sure it's understood yet how all of this sorting goes on. It's a difficult problem to approach experimentally because you have to be able to sort of get inside the Golgi and understand what the different regions are and then look for potential receptors and things like that. But the general overall framework for this sorting is understood. What I think it's important for you to understand is how proteins are directed by co-translational import versus how proteins are directed by post-translational import. The two processes are really different. And if, for example, you look at some of these pictures and as I made a point already, I said that proteins in the plasma membrane have sugar molecules attached to them. Well, if they have sugar molecules attached to them, that means they are synthesized by which of these? How many vote for post-translation? Don't vote. Don't vote for that. That's wrong. Co-translation, because this is the process by which sugar molecules are attached. So these proteins that are inserted into the plasma membrane having sugar groups on them have to be synthesized and imported and moved by this co-translational process. Okay, so really what I've talked about, I've talked about the nuclear envelope, I've talked about the ER, I've talked about the Golgi, I've talked about the plasma membrane, lysosomes and vacuoles. I've talked about lysosomes, but I haven't told you what lysosomes do. Lysosomes are small organelles in cells that are involved in breaking down macromolecules. They break down polysaccharides, they break down lipids, they break down proteins, they break down nucleic acids. Lysosomes are the classic example of how and why you would compartmentalize a variety of like-minded activities. Lysosomes are very acidic organelles. The internal pH of a lysosome is about 5, 4, 5, very low. And that pH is maintained by pumping protons, moving protons across the lysosomal membrane into that organelle. If the enzymes of a lysosome were out in the cytosol of the cell and functioning, they would destroy the cell. They would break down all the proteins, they'd break down all the nucleic acids, they'd break down everything. So by taking this bag of enzymes, making them function at pH 5 protects the cell from having these enzymes doing things that are particularly bad. So if a lysosome is actually inactivated and the proteins are released, they're at pH 7 or 8, which is the presumed pH of the cytosol, they're inactive. So they can function very happily at an acidic pH and do their job. Vacuoles in plants are basically like lysosomes. They function like lysosomes. I used to tell people there were no lysosomes in plants and I was chastised by a colleague of mine who worked on lysosomes in plants. He was very annoyed at me for saying that. But there aren't many lysosomes in plants. You have probably, if you've had lab and you've looked at plant cells, you have probably seen vacuoles. Vacuoles are very, very large organelles in plants and they contain a lot of the same enzyme activities that exist in the lysosomes of animal cells. They are also storage vessels, storage organelles. Plants tend to, under some conditions, accumulate large amounts of various materials like organic acids. We'll talk about that when we talk about photosynthesis and they will put those materials into the lysosomes within the cell. So lysosomes are, you can think of them as, you can think of vacuoles as lysosomes and you won't be far wrong. The two organelles I haven't talked about are ones that we will spend a great deal of time talking about later. So I will introduce them now and I'll come back to them later. And those are the chloroplasts and the mitochondrion. And we talk about them in the same breath because these are what we call energy-generating organelles. And they are probably the most unusual organelles in cells. By energy-generating organelles, what do I mean? Or energy-transducing organelles, what does that mean? These are the two organelles in the cell that generate ATP. And I'm sure everybody has at least once heard about ATP, I hope. ATP is basically the cell currency. It's what we pay for doing all of the biosynthesis and the biosynthesis within the cell that costs the cell money. And the money is ATP. ATP is generated by chloroplasts and mitochondrion. The chloroplasts and the mitochondrion are unusual organelles because not only do they have an outer membrane, but they have an inner membrane system, a complicated inner membrane system. And it's complicated because it's related to this complicated process of making ATP. So if you look at a mitochondrion, that's a mitochondrion. But what you see inside is a series of membranes like that. And I won't talk in detail about these membranes now because, as I said, we're going to spend almost two weeks on them. So this is a mitochondria, abbreviated MT. So there's an outer membrane and there's an inner membrane. And then there's these various spaces. There's a space between the two membranes, cleverly called the intermembrane space. We biochemists are so smart. And then there's another space inside this, which is known as the matrix. So there are really four different areas in this organelle. Chloroplasts are slightly more complicated. This is a chloroplast, outer membrane. There's a series of membranes that look like stacks of pennies in the chloroplast. And I won't go into the details of this now because it'll take me too long and we'll do it later. But you can see, again, there's a space between the outer membrane and this membrane, which is known as the stroma. I'll just put that up now. And then there's a space inside the membrane, which is known as the lumen. So these organelles are clearly more complicated than what I've talked about. If you look at the nucleus, there are no membranes inside the nucleus. There are no membranes inside paroxysones. Remind me, I have to talk about paroxysones. That's going to be next week. What is also unusual about chloroplasts and mitochondria that distinguishes them from all other organelles other than the nucleus, is both of these organelles contain DNA. So remember I had over there almost. Well, now you know why I put almost because the chloroplast and the mitochondria has its own DNA, it has its own RNA, and it has ribosomes inside the organelle. So what does this mean? These organelles can make some of their own proteins. They can't make all of their proteins. Approximately 70 to 80% of the proteins in both of these organelles are synthesized in the cytosol, and 10, that's probably too high. Let's say 80%. And 20% are synthesized in the chloroplast. The same is true here. 80% cytosol, 20% mitochondrial. So you have a very interesting situation where the nuclear genome, the information in the nucleus and the information within the organelle somehow have to coordinate with each other. For example, believe it or not, there are proteins in the chloroplasts, which I know more about, where they have tertiary structure. Everybody knows what that means. Number of subunits. Some of the proteins are made in the organelle and some of the proteins are synthesized in the cytosol and brought into the organelle. So that's a fairly complicated process of how do you control whether these proteins should be brought in if there aren't proteins there, et cetera, et cetera, et cetera. So the chloroplasts and the mitochondrion are unusual, and the composition of them, the properties of them, have led to various theories about where they came from. Why are these organelles different from all the other ones that we've talked about, and that's a great place to stop. So you'll all come back on Wednesday and I'll talk about the current thinking about chloroplasts and mitochondria and peroxazones. Have a nice weekend.