Hello everyone. In this lesson, we are going to be talking about plant tissues, specifically the plant tissues dealing with cell walls. Okay, so we know that plants are pretty unique in comparison to animal cells given the fact that they have cell walls. So plants use cell walls as their primary means of support. We have a skeleton that supports our body, but do plants? No. Plants don't have a skeleton that supports their body. In fact, every single one of their cells is going to have a type of cell wall and different tissues of the plants are going to have thicker cell walls and provide more structural support. These are thicker cell walls and provide more structural support. These are primarily going to be the areas of the trunk and the branches in woody plants and these are going to have really thick cell walls.
So usually when we talk about the cell wall of a plant, we talk about it in its singular form. But actually, there are 2 types of cell walls that you find in plant cells. The primary cell wall and the secondary cell walls. The primary cell wall is going to be the outermost layer of the cell wall and this is going to be the 1st cell wall that forms in a developing plant cell. So that's why it is the newly formed cell wall. It's the very first one to appear. This is going to be a pretty flexible cell wall, so it's going to be thin and flexible, but still fairly tough. It is resilient, but it's definitely not super hard and strong. It's just thin and flexible with some strength to it. And it is going to contain things like cellulose, hemicellulose, and it's going to contain pectins, which are going to be branched polysaccharides, which are going to be sugars or carbohydrates cross-linked with calcium ions to form a semi solid gel. So that's why the primary cell wall is flexible because it's formed via this semi-solid gel in the pectin formation.
But the primary cell wall is made of cellulose fibers, Hemicellulose Fibers, which is just another type of cellulose, and pectins. All of these are carbohydrates. So all of these are going to be sugars. The Cellulose is going to be the main strength in the primary cell wall. The Hemicellulose are going to be fibers of Cellulose that connect to the larger fibers of cellulose and then pectin is going to be kind of like this gel-like matrix that encases the cellulose and hemicellulose. So that's going to be the flexible, but still pretty tough, cell wall that really you're more familiar with. This is going to be the cell wall that really provides that strength and that support.
This cell wall will only form when the cell is done growing. Do you guys know why? Because the secondary cell wall is really hard and rigid and structured, so if the secondary cell wall forms before the cell is done growing, the cell can't grow anymore. It is going to inhibit the growth of the cell. So the secondary cell wall forms last and it is within the primary cell wall. So if you're looking at a plant cell, the outermost cell wall is the primary cell wall, and the innermost cell wall which is going to be a bit thicker because it's got more structural support. This is going to be the secondary cell wall. And, it is going to form last and it is within the primary cell wall. So these are much, much thicker cell walls, and they are much less flexible. They are also going to be composed of Cellulose and Hemicellulose, but, the thing is there's a lot more cellulose fibers in the secondary cell wall than the primary cell wall so it's going to get a lot more strength and structure from the increased concentration of cellulose fibers. It's also going to contain Lignin, which is going to be a polymer of alcohols that acts to waterproof the, secondary cell wall and provide extra strength and support. So, the secondary cell wall is going to be made of cellulose, hemicellulose, and lignin, and some other molecules, depending on what that particular secondary cell wall needs to do in that unique tissue of the plant cell, but we're not going to go into that amount of detail. We're just going to go over the basics of the secondary cell wall.
Okay, so now let's talk about those Cellulose Fibers that actually make up these cell walls. So cell walls contain Cellulose Microfibrils, which are long Cellulose chains. You can almost think of them as like miniature fibers of Cellulose kind of like little hairs of Cellulose or chains of cellulose, and this is going to be what provides the true strength of the cell walls in general. This is really what supports the load of the plant and is really going to act as the skeleton that the plant does not actually have. So I know I've talked about hemicellulose, but hemicellulose is also a polysaccharide. It's just a little bit different than cellulose in its sugar composition, but it does the same general thing and it's going to attach to cellulose fibers and hold them in place and add some more strength.
So, let's look at this image that we have here. So, this is going to be an example of the cell wall. You can see that we are going to have Cellulose Microfibrils, and these are going to be these really long blue large strands, and they are going to be crossed over each other to provide strength. And then, we're going to have the hemicellulose, which you can see in green. They're much smaller, but they are holding the cellulose fibers together. And then, you can also see that we have pectin. Pectin is going to be utilized to form that matrix, that gel-like matrix, and it's going to actually encase the cell walls. Now, this is going to be the primary cell wall as you can see by the diagram. This is going to be the primary cell wall. Some cells only have a primary cell wall. As you can see, this cell only has a primary cell wall. There is no secondary cell wall in this image, but secondary cell walls look extremely similar in their composition to primary cell walls. They just have a lot more cellulose, and they have lignin. So they're a lot thicker, they have a lot more cellulose, but they look really, really similar.
Now, you may be wondering what is a middle lamella? A middle lamella is actually a pectin layer. So, a pectin layer between cells. So this layer right here is actually the layer between 2 cells. It's a pectin layer, and it's basically utilized to adhere 2 cells together. So what that would look like is if you have this plant cell and if you have this plant cell, the pectin layer or the middle lamella would be in the middle here, and it would be holding those 2 plant cells together. So the middle lamellae is right here, holding those 2 plant cells together. So that is an image of what a generalized primary cell wall is going to look like. Now, let's go down and talk about some unique characteristics. So, the cell wall provides certain characteristics that are unique to plant cells. Obviously, plant cells are pretty unique in their cell wall composition and, like I said, the cell wall is going to prevent cellular growth. That is why the secondary cell wall will only form once the cell has fully matured because secondary cell wall is not flexible and will not allow plant cell growth. And that's because cellulose fibers are really strong and they don't really bend well all that much, so they are going to limit the growth of the cell. And the cell wall does provide support to the and it does maintain the overall shape of the cell and overall shape of the plant. So it maintains cell shape. Now, it's also going to help maintain osmotic pressure, which is also called hydrostatic pressure, and this is very important for plants. Whenever they don't have a high enough osmotic pressure,](https://www.example.com/) they wilt and they kind of fall over. So the cell wall helps provide and maintain some regularity for that osmotic pressure and it's also going to be called turgor pressure. This is another word for it and basically, turgor pressure is how much water pressure is inside of the cell. And you can tell what the turgor pressure is because when the water pressure inside of the cell is very high, it's going to plump up the cell. And when it's very low, the cells kind of kind of shrivel in a little bit. So that's why plant cells wilt when they don't have enough water. Their cells shrivel and they kind of fall over because there's not enough pressure to hold them up. And this is going to be based on the differing water concentrations inside and outside of the cell.
Now, let's talk about one more thing before we end this lesson. Let's talk about plasmodesmata. So we know that plant cell walls are extremely hard and extremely difficult to move or bend or grow, so how do things get to the cell past the cell wall? How do certain chemicals or transporters or proteins get into the cell if the cell wall is in the way. Where there are going to be special areas called plasmodesmata, which act as connections between the cytoplasm of adjacent plant cells, and they go through the cell wall. You can see some examples of plasmodesmata down here. You can actually see these holes in the cell walls of adjacent plant cells, and this is going to allow these cells to exchange material. So, this is going to be the cell wall in this image, and you are going to see the plasmodesmata. So, plasmodesmata is a channel through the cell wall, which is going to allow certain molecules and signals to pass between these cells.
Okay, everyone. That's all I have today for about cell walls. Let's go on to our next topic.
