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Plant cell walls

The cell wall surrounds the plant cell, providing both structure and protection. Plant cell walls are made up mostly of cellulose, and also include hemicellulose and pectin. Plant cells connect directly to one another via tunnels in their cell walls called plasmodesmata.

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Video transcript

- We've talked a lot about cells in general, but what I thought I would do in this video is focus on plant cells, and in particular focus on the cell walls of plant cells. So this right over here, this is a drawing of a plant cell. And the thing that might jump out at you immediately is instead of drawing it as just kind of a roundish shape like that, the way I've drawn a lot of other cells, I've drawn this as kind of a cubic structure, or a rectangular prism, and that's because plant cells can have a structure like that. And so the next question is, well, what gives them that shape? What allows them to form that, kind of, cubic rectangular prism shape? And the answer is, it's the cell wall. So this is the cell wall. So let's make sure we can orient ourselves properly in this picture. So clearly, if I didn't have this cut-out, all I would be seeing is the outside. All I would be seeing is the cell wall. But we've cut it out, and we can see the different layers. We have the cell wall on the outside. Right below that, right below that we have the cellular membrane, or the plasma membrane. So that's the cellular membrane. Cellular membrane, right under that. And then under that, the cellular membrane is containing the cytoplasm. And inside of the cytoplasm we have all sorts of things. This big thing that is taking up a lot of the volume inside of this plant cell, that's a vacuole, which we have described in other videos. Vacuole. It's the combination of this internal pressure, things like the vacuole and, just frankly, the pressure from all of the fluid inside the cell pushing outwards, plus the cell wall kind of holding it all in. That's what gives plants their structure. That's why a plant is able to grow, and be upright. A plant is able to grow and be upright. So that's my drawing of a plant. I actually have a plant in my room that I'm looking at right now, and it's able to grow and be upright. And so you have the cell wall, you have the cellular membrane, you have the other organelles, I have some chloroplasts here, key for photosynthesis, our good friends mitochondria. We have our nuclear membrane, or I should say this yellow thing is the inner nuclear membrane. It has the DNA inside. Then you have the endoplasmic reticulum, kind of containing that. The rough ER, containing the ribosomes or having the ribosomes on the membrane. The smooth ER, not having the ribosomes. Golgi apparatus, so that's a little bit of a review. But our focus here is on the cell wall. So let's go back to that. So if we zoom in on this, if we zoom in on the cell wall right over here, we can look at this diagram. And over here, it might be a little bit surprising to you. Because when I've always imagined a wall, a cell wall, I imagine something like a brick wall. Something that's impenetrable. But this drawing shows us something different. And just to be clear what's going on here, so this is our cellular membrane -- sorry I wrote cellular membrane, so right over here I have my lipid bi-layer. And then right on top of that, I have the cell wall. But you see, it isn't just a thick, like a brick wall, like something that is impenetrable. You see you have all these polysaccharide fibers running across it. So you have things like cellulose, which we saw as a polymer of glucose, arranged in a certain way. Hemi-cellulose, which has different types of monomers associated with it. We have pectin, which is another polysaccharaide. And all of these things -- you've actually probably eaten, if not today probably in the last week, when we talk about fiber in your diet, you're talking about things like the cellulose and the pectin. Things that your body can't digest. When you eat a plant you're getting it, because you're eating their cell walls. And it does cool things, like slows the absorption of glucose in your intestines. It absorbs water, so I guess you could say things pass a little bit easier. But the key thing here is this isn't a wall. This actually allows -- Or, it is a wall, It's officially the cell wall. But it's not a thick, impenetrable wall like you might associate the wall of the room that you're in. You can see that it has space for small molecules to flow. And it's really more like a mesh, or like a fabric. And so the cellular membrane actually has access to the fluid, and to the molecules, that are between the cells. And so just to be clear what we're looking at. This layer right here, this cellular membrane, that's the lipid bi-layer. This right over here, this is the cell wall. I'll do that in a different color. That is the cell wall. And then right above the cell wall, that's the space between the cells, which we cell the middle lamella. So the space between the cells we call the middle lamella. So this also is, right over here, is also the middle lamella. So all of that is interesting, but you might say, "Okay, well, how hard is a cell? I get that it's a mesh, but clearly the cells, the plants are able to stand upright. Is that because the cell wall provides all of that rigidity?" And the answer is, kind of. The cell wall is like this mesh. It helps the cells have their shape. But if you stop watering a plant, you're going to see it kind of wilt over. And that's because part of its ability to stand up is from the internal pressure of the cells. But also part of its shape is the actual cell wall. Now, some of you might be saying, "Well I've seen plants that are much, much more rigid than this plant that you've just drawn. What about things like trees? What about wood? Wood seems very rigid. In fact, so rigid that we can build actual walls out of wood." And the answer there is these more mature plants, actually once the cell has stopped growing and you have your cell wall, more layers of cellulose and other molecules can be built to form what's called a secondary cell wall. So this could be viewed as a primary cell wall. And then a thicker, secondary cell wall could be built, which gives much, much more rigidity. And so when you look at wood, what gives wood its structure, even if you were to take out all of the water, even if you were to dehydrate the wood, it's still going to have its rigidity, because the cellulose layers and the other molecules that are so thick that it's able to have its rigid form. Now that last thing I want to talk about. We've already seen that the cellular membrane has access to the molecules floating around between cells, but there's actually also direct tunnels between adjacent plant cells. And those direct tunnels I've drawn here on this, outside of the cell walls these little yellow circles, these are plasmodesmata. These are plasmodesmata. To get a better understanding of what they're like, imagine this is one cell. So I'm writing here Cell 1. And let's say this is Cell 2. Cell 2 right over here. And I have a cross-section. You see the plasmodesmata are these tunnels that form between not just the membrane of the cell wall and the plasmodesmata. It forms between the two cells. And so you can actually have a flow of cytosol and small molecules directly between these two cells. And if you want to get a little bit more involved in the structure, you have this kind of smooth endoplasmic reticulum pipe going through it. But I want to make it very clear. 'Cause a lot of times when you study biology it's all explained, it seems all neat and clean and textbook. But people are still studying exactly why do we have these things? What are they necessary? What gets transported across these things, and how are they able to transport, and under what conditions are they? So all of these areas, when you were to kind of dig one layer deeper than frankly I'm talking about, you're getting into an area of active research. So anyway, hopefully this whole thing gives you a little bit more appreciation for the wood around you, the plants, the house plants around you, and even the salad that you might have for lunch.