Biology: Cell Walls

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We're on the outside of the cell. Think about it. We've seen this whole endomembrane system; we've seen the whole central dogma; we've seen these endosymbionts in there; and we've seen this whole idea of an internal skeleton in many, many cells. We talked, for example, about microfilaments - these thin things that cause cells to contract when they contract, and then, when they expand, to allow expansion of the cell. We talked about the whole idea that there are intermediate filaments, things that contain keratin, and keratin is tough, and, therefore, will give extra tensile strength to a cell. We talked in a lot of detail about microtubules; these things are just fabulous, these hollow tubes that are made out of these protein dimers - really exciting stuff there. In fact, if you remember, the first thing we talked about with that is the centriole, and it has three sets of microtubules arranged in a whole sequence of 9 all the way around a hollow structure called the centriole - and that's, of course, when you cut that in cross-section.
And, of course, we talked about the 9 plus 2 ratio in a cilia and a flagellum - great stuff there. Think about it. Even though a cilia and a flagellum is different than a centriole, we see some evolutionary significance here to the fact that, wow, arranged in series of 3 fused tubes and then attached, and arranged in series of 2 and attached. We could even take this a step further. I could tell you about the basal body; the basal body's kind of cool. At the bottom of every centriole and flagellum, at the base of it, is this thing called the basal body.
One cool story to link things and just make you think, because, man, it's way beyond us here, but a sperm fertilizes an egg to produce a zygote. Well, here's something for you: You, up till now, thought, "Ah, I know what happens there. The nucleus of the sperm enters and the two nuclei fuse, and you get an offspring." Well, that's true. But something you may not have known is that at the base of the flagellum is a structure called a basal body. You know that now, but guess what? The basal body also enters the egg. Now, remember, that's a basal body; it's not necessarily part of the flagellum, it's the base of the flagellum. And guess what? That basal body becomes a centriole. How cool is that?
So it's all linked, it all comes together. But that's not what I want to talk to you about right now. What I want to talk to you about right now is another supportive mechanism in this cell. I want to start talking about, well, what about when cells meet cells? And I want to start out with a structure called cell walls. Now, it's another case of publicity, publicity. When you think of cell walls, I'm sure you think of plants. But other organisms, or unicellular organisms, have cell walls too. For example, bacteria - prokaryotic cell walls - consist of a chemical called peptidyl glycans. These are polymers. You remember that word - many units - of modified sugars, so it's a bunch of sugars linked together. But they're cross-linked by very short polypeptides. So think about this. Their cell walls of prokaryotes are going to be literally a bunch of sugar molecules cross-linked in a very rigid situation.
Well, what's the significance of that? Well, it's a great story, but remember, if we're talking about cell junction, prokaryotes are unicellular, and junctions don't occur there. But don't forget, plants are not the same thing as prokaryotes, and their cell walls are not the same. Similarly, I know you know about chitin, and chitin fungi have cell walls. And in the old days fungi were considered plants because, microscopically, they seem to have cell walls. But the bottom line is their cell wall is made out of chitin.
Well, that's not what I want to talk to you about today. I want to talk to you about cell walls in plants, at least a little bit. You guys know a little bit about something called cellulose. You remember cellulose. Let's talk about a plant cell wall and what happens. I always used to wonder, when a plant cell divides, it must grow a new cell wall somewhere, like in the middle. And, if that's true and cell walls are so tough, then how does the plant cell ever get bigger so it'll divide again? Well, if you're asking that question, that's a darn good question. So we need to talk a little bit about what cell walls are made out of.
Cell walls are made out of a substance called cellulose. Cellulose, you remember, I'm sure, is a polymer of monosaccharides; it's insoluble. And you remember why, you remember the whole idea with the particular types of alpha and beta linkages differentiating the different types of starches. And you remember that the cellulose has the beta linkages, and with those beta linkages, you get an insoluble, high-tensile strength compound. Well, all of that being said, I haven't answered the question: How do plant cells grow?
Well, let's take a look at what happens in a growing plant cell. Sure enough, we do get the formation of new cell walls, and what begins to happen is, indeed, you start to get these vesicles forming that begin to form a new cell wall. And we're going to call that the primary cell wall. Now, the primary cell wall consists of basically cellulose fibers that are - it's very, very flexible, and it's just getting laid down. So, now, the plant - and, again, picture a plant cell as a three-dimensional cube, and this thing is forming and has the ability to grow in directions wherever this new cell wall is forming - what's happening here with this new cell wall is it's flexible and can allow growth, can allow expansion of both of these cells out from each other. So the thing can indeed grow, but, eventually, we're going to have to harden this thing off so it forms a cell division called a cell wall. And that's where the cellulose comes in, but that's where the glue comes in too.
Did you ever hear of pectin? Remember pectin - pectin's that stuff that we use in fruits to gel them all up - it's a concrete kind of material in cells. Well, as soon as the plant cell begins to get to its secondary stage of growth, or where it's ready to stop growing, pectin is laid down in the middle. So going back to our cells, and the beginning of the growth of this new cell wall, we start to find that we're going to form something, this hardening substance, and this is going to be what the pectin is all about. So we're going to get the beginning of this, and then we're going to move and the pectin is going to start forming so that we get this thing to begin to hard.
Now, some cells stop there, but some cells move on. And some cells are going to form a secondary wall. Now, in a secondary wall, what we're going to get is a thickening, and it can be laid down, actually, in several layers. So what's going to happen here is you may get 1, 2, or 3 layers of additional material laid down on this thing, in between the plasma membrane of the plant cell and the primary and secondary cell wall. So it's a thickening, and that is what makes up wood, that's how we make wood. So any plant that's going to be woody is going to have that extra material laid down; it's made out of other chemicals than cellulose - for example, lignin - which gives it this tough, woody feel to it. But, man, does that add structure to the cell. Now, do I want to call this a skeleton? I don't really like that whole idea of a skeleton, because we think of skeletons as internal things, and even when you think of an animal, you think of an exoskeleton, it's not the same thing. Very simply, it's why we call it a cell wall - it is merely around the outside of the cell.
There's one more thing I've got to tell you that is so cool, and this is going to surprise you. We picture plants as being boxes, and within those boxes, we picture cells. And remember, all cells have a plasma membrane. Oh, this drives me nuts. When my students say, "Plant cells have cell walls and animal cells have cell membranes," I just - child abuse comes to my mind; adult abuse comes to my mind. But you know what? Don't get that in your head. Plant cells have plasma membrane; cell membranes too. But even this diagram makes it look like cells live in isolation. No.
One last thing I want to tell you about is something called the plasmodesmata. Plasmodesmata occur in plant cells. In fact, here's what's going to happen - by the way, "desmos" - let's dissect the word - "desmos" means to bind. If I take a look at two plant cells, we're going to find a very, very interesting thing. Indeed, there are situations in the plant cells where there is a union between the plant cells; and what happens is, what you can do is you can actually have, within this opening, cell membrane from one plant cell continuous with the next one, which gives this idea of maybe the ability for two plant cells to communicate. Now, there's a thought for you: This whole idea of plant cells having the ability to exchange cytoplasm should be mind-boggling to you - that's great stuff. And here's an artist's rendition of this. Here's my plant cells - look at that big old vacuole, don't forget that - and there is my plasmodesmata. And you can see, from this diagram here, here's the water vacuole, and there is the cell membrane, the yellowish, continuing on through across those plasmodesmata. So, once again, this whole idea of cells in life - active, dynamic. And if you think plant cells are connected, wait till you see animal cells. But animal cells is another story, and that's going to come later on.
Cell Biology
The Cytoskeleton
Cell Walls Page [1 of 2]