cellulose

Here's another polysaccharide, and this one is called cellulose. Uh, once again, this is another polysaccharide that uses glucose as its monomer. So, just like starch and and glycogen, they used glucose as their building blocks, so does cellulose. But there's another difference here. So, cellulose is what we call a structural polymer. It means it helps, it's a molecule that helps give things shape. And we can think of cellulose as a little bit like rebar that runs through concrete, that kind of holds it, holds concrete in shape.

Um, so cellulose is a material that plants build. So, it's built by plants and it's used to create the cell walls of plants. So, plant cells, um, to backtrack, all cells, whether they're from an animal or a plant, have a, uh, or a bacteria, they have a membrane, which is like a sort of floppy baggie that protects all the stuff on the inside of the cell. And then there's the membrane, and then there's the outer world. All right? So, that's all cells have a membrane. Your cells do, and plant cells do. But plant cells go an extra layer and they put a wall around the membrane. Animal cells don't do that. Animal cells are just like a baggie, just flopping around. But plant cells are like a baggie in a shoebox, and the shoebox is the cell wall, and the cell wall is made of cellulose.

So, cellulose is built of many, many, many hundreds of thousands, millions of glucose monomers that are all lined up and hooked together in long chains. Um, so, so far, it sounds like starch. But the difference is that in starch, the glucoses are lined up the same way, facing the same way, so that they, uh, they all hold hands facing down, so that there's a, um, they face the connections between the glucose rings are all facing down or hanging below the rings. In cellulose, the glucoses are alternately flipped. So, one's like this, and one's like that, and the next one's like this, and the next one's like that. Which means that when they hold the hands or when they link up, one is down, and the next one is up, and the next one holds hands down, and the next one holds hands up. So, there's a different arrangement down the chain. And you can see that here. Here's holding hands down, and then holding hands up, and then down, and then up.

So, you know, that again might seem really minor, like why does that even matter? Well, in our bodies, and in most mammals, in digestive systems, we cannot break these connections down. We can break down the ones where they're all holding hands facing down, but we can't break down the ones where they're flip-flopped like this. So, we cannot break this chain apart.

Um, so that has many, many implications. There's a couple of other things to note here. First of all, you can see here's one, um, chain at the top here, and then here's another chain, and here's another chain. So, there's three separate chains here in this diagram, and they're not connected together. But if you look here, and it's hard to see, but hanging down from this top chain, there's an OH, a hydroxyl group, right? And then pointing up from the chain underneath it, there's another hydroxyl group, right? And you should know that these hydroxyl groups are going to have partial charges because it's an O and an H. There's going to be a partial negative on the O's, and there's going to be partial positive on the H's. So, there's going to be attractions between these, um, functional groups that hang down and poke up, right? And once, uh, you can maybe see it a little better on this diagram here, you can see the dotted lines because remember, these are not covalent bonds, they're not permanent attachments, they're just weak attractions between these individual chains. And you can think of this like a rope that's made up of lots and lots of thin fibers. If you think about a rope from like the olden days, each individual fiber is very, very thin and not very strong. But when you stack all those fibers together and give it a twist, you get a really strong rope. So, the, the individual chains, you know, they're not, they're very thin and very wispy, not, not very particularly, um, strong. But when you put thousands of them all together, and then they're all weakly attracted to each other, so they all kind of stack up nicely, then you get a pretty strong fiber. And there's those fibers that build the walls of the cells, and it's those fibers that we cannot digest.

Now, can we chew through them? Yes. Like, if you think about when you eat celery, your crop, or when you eat broccoli, your, you can chew it up and you can kind of chew and break apart these fibers and kind of, um, break apart these hydrogen bonds and kind of smush apart the cell walls, you know? But, and then you can even burst the cells and release all the insides of the cells, that's all the kind of juice and the good stuff in like a tomato or a piece of sweet corn. But you can't break down the actual cellulose fibers. And this is the part of our diet that we can't digest, but super important that we call fiber, right? Dietary fiber, roughage, uh, is cellulose from the cell walls of plants. And, you know, you can't digest it because if you think about sweet corn, right? It goes in looking like sweet corn, and it comes out the other end looking like sweet corn. And that's the same for almost all plant material. Just sweet corn really stands out because of the yellow color, so you can see it floating in the toilet. But, uh, same for things like carrots and and broccoli and all those kinds of things. We can mush apart the cell walls, the fight, mush apart the fibers, but we can't break down these actual linkages.

So, this is another reason why high fiber foods like broccoli and stuff, they have very low calories because you, because you can't break this down, you can't get any calories out of it. It looks like it has glucose in it, you know, like sugar, right? But you can't get to the sugar because you can't break them apart. So, there's really no calories in a human for this. There's no calories really in it for many animals. So, animals that rely on plants for their diet, like a cow, really has to work hard to break down the, uh, to get any nutrition out of all the grass they eat. And they have very complicated stomachs, they really chew on it a lot, and then they have a really complicated amount of bacteria that live in their guts that help them break this down. But we don't have those bacteria for humans, so we can't break this down.

So, that is cellulose, a structural polymer of glucose made by plants, and a really important part of our diet as fiber.