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Studying for a test? Prepare with these 2 lessons on Crash Course: Biology and Ecology.
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Hank: Plants are freaking great because they have this magical wizard power that allows them to take carbon dioxide out of the air and convert it into wonderful, fresh, pure oxygen for us to breathe. They're also way cooler than us because, unlike us and every other animal on the planet, they don't require all kinds of Hot Pockets and fancy coffee drinks to keep them going. The only thing plants need to make themselves a delicious feast is sunlight and water; just sunlight and water. Paula Deen can't do that, and she makes fried egg bacon doughnut burgers. I'm telling you, this is surprisingly good. This is a different kind of magic, but part of this is plants. Everything in it, in fact everything that is in this [McDonalds], in fact, everything you have ever eaten in your life is either made from plants or made from something that ate plants. So let's talk about plants. (cheerful music) Plants probably evolved more than 500 million years ago. The earliest land plant fossils date back to more than 400 million years ago. These plants were lycophytes, which are still around today and which reproduce through making a bunch of spores, shedding them, saying a couple of Hail Marys, and hoping for the best. Some of these lycophytes went on to evolve into scale trees, which are now extinct, but huge, swampy forests of them used to cover the earth. Some people call these scale tree forests coal forest because there were so many of them, they were so dense and they covered the whole earth, that they eventually fossilized into giant seams of coal, which are very important to our lifestyles today. This is now called the Carboniferous Period. See what we did there? Because coal is made of carbon, so they named the epoch of geological history over how face-meltingly intense and productive these forests were. I would give my left eyeball, three fingers on my left hand, the middle ones so that I could hang loose, and my pinky toe, if I were able to go back and see these scale forests, because they would be freaking awesome. Anyway, angiosperms, or plants that used flowers to reproduce, didn't develop until the end of the Cretaceous Period, about 65 million years ago, just as the dinosaurs were dying out. Which makes you wonder if, in fact, the first angiosperms assassinated all the dinosaurs. I'm not saying that's definitely what happened. I'm just, it's a little bit suspicious. Anyway, on a cellular level, plant and animal cells are actually pretty similar. They're called eukaryotic cells, which means they have a good kernel, and that kernel is the nucleus. Not "nuculus". The nucleus can be found in all sorts of cells: animal cells, plant cells, algae cells. Basically, all the popular kids. Eukaryotic cells are way more advanced than prokaryotic cells. We have the eukaryotic cell and we have the prokaryotic cell. Prokaryotic basically means before the kernel, pro-kernel. Then we have the eukaryotic, which means good kernel. The prokaryotes include your bacteria and your archea, which you've probably met before in your lifetime. Every time you've had strep throat, for example. Or if you've ever been in a hot spring, or an oil well or something, they're everywhere: they cover the planet; they cover you. Like I said, eukaryotes have that separately enclosed nucleus, that is, that all-important nucleus that contains its DNA and is enclosed by a separate membrane, because a eukaryotic cell is a busy place. There's chemical reactions going on in all different parts of the cell. It's important to keep those places divided up. Eukaryotic cells also have these little stuff-doing factories called organelles, because we decided that we'd name everything something weird. But organelles, and they're suspended in cytoplasm, continuing with the really esoteric terminology that you're going to have to know. Cytoplasm is mostly just water, but it's some other stuff, too. Well, basically, if you want to know about the structure of the eukaryotic cell, you should watch my video on animal cells, which let's just link to it right here. Plant and animal cells are very similar environments. They control themselves in very similar ways. But obviously plants and animals are very different things, so what are the differences in a plant cell that makes it so different from an animal? That's what we're going to go over now. First, plants are thought to have evolved from green algae, which evolved from some more primitive prokaryotes. Something plants inherited from their ancestors was a rigid cell wall surrounding the plasma membrane of each cell. This cell wall of plants is mainly made out of cellulose and lignin, which are two really tough compounds. Cellulose is by far the most common and easy to find complex carbohydrate in nature, though if you were to include simple carbohydrates as well, glucose would win that one. This is because, fascinating fact, cellulose is in fact just a chain of glucose molecules. You're welcome. If you want to jog your memory about carbohydrates and other organic molecules, you can watch this episode right here. Anyway, as it happens, you know who needs carbohydrates to live? Animals. But you know what's a real pain in the ass to digest? Cellulose. Plants weren't born yesterday. Cellulose is a far more complex structure than you'll generally find in a prokaryotic cell, and it's also one of the main things that differentiates a plant cell from an animal cell. Animals do not have this rigid cell wall. They have a flexible membrane that frees them up to move around and eat plants and stuff. However, the cell wall gives structure to a plant's leaves, roots and stems, and also protects it, to a degree. Which is why trees aren't squishy, and they don't giggle when you poke them. The combination of lignin and cellulose is what makes trees, for example, able to grow really, really freaking tall. Both of these compounds are extremely strong and resistant to deterioration. When we eat food, lignin and cellulose is what we call roughage because we can't digest it. It's still useful for us on certain aspects of our digestive system, but it's not nutritious. Which is why eating a stick is really unappetizing and like your shirt. This is a 100% plant shirt, but it doesn't taste good. We can't go around eating wood like a beaver, or grass like a cow, because our digestive systems just aren't set up for that. A lot of other animals that don't have access to delicious donut burgers have either developed gigantic stomachs, like sloths, or multipe stomachs, like goats, in order to make a living eating cellulose. These animals have a kind of bacteria in their stomachs that actually does the digestion of the cellulose for it. It breaks the cellulose into individual glucose molecules which can then be used for food. Other animals, like humans, mostly carnivores, don't have any of that kind of bacteria, which is why it's so difficult for us to digest sticks. But there is another reason why cellulose and lignin are very, very useful to us as humans. It burns, my friends. This is basically what would happen in our stomachs. It's oxidizing. It's producing the energy that we would get out of it if we were able to, except it's doing it very, very quickly. This is the kind of energy, like this energy that's coming out of it right now, is the energy that would be useful to us if we were cows. But we're not, so instead, we just use it to keep ourselves warm on the cold winter nights. (blows air) Ow; it's on me; ow. Anyway, while we animals are walking around, spending our lives searching for ever more digestable plant materials, plants don't have to do any of that. They just sit there and they make their own food. We know how they do that. They do it with photosynthesis. Another thing that plant cells have that animal cells just don't have are plastids, the organelles that plants use to make and store compounds that they need. You want to know something super interesting about plastids? They and their fellow organelles, the mitochondria, that generate energy for the cell, actually started as bacteria that were absorbed into plant cells very early in their evolution. Like maybe some [produ-select] cell absorbed a bacteria and it found that instead of just digesting that bacteria for the energy that it had, it could use that bacteria. That bacteria could create energy for the cell or convert light into lovely glucose compounds, which is crazy. Nobody's really precisely sure how this happened, but they know that it did happen because plastids and mitochondria have double membranes: one from the original bacteria, and one from the cell as it wrapped around it. Cool, huh? Anyway, the most important of the plastids are the chloroplasts, which convert light energy from the sun into sugar and into oxygen, which the plant doesn't need, so it just gets rid of it. All of the green parts of a plant that you see: the leaves, the non-woody stems, the unripened oranges, are all filled with cells that are filled with chloroplasts, which are making food and oxygen for you. You're very welcome, I'm sure. Another big difference between plant cell and an animal cell is the large central vacuole. Plant cells can push water into vacuoles, which provides turgor pressure from inside the cell, which reinforces the already stiff cellulose wall and makes the plant rigid, like a crunchy piece of celery or something. Usually, when the soil dries out or the celery stalk sits in your refrigerator for too long, the cells lose some water, turgor pressure drops, and the plant wilts or gets all floppy. So the vacuole is also a kind of storage container for the cell. It can contain water, which plants need to save up, just in case, and also other compounds that the cell might need. It can also contain an export stuff that the cell doesn't need anymore, like wastes. Some animal cells also have vacuoles, but they aren't as large, and they don't have this very important job of giving the animal shape. Now, let's do this. Let's just go over the basics of plant cell anatomy. One, they have a cell wall that's made out of cellulose, and so it's really rigid and not messing around. Two, they've got a nucleus in its own little baggie that's separate from all the other organelles. This is basically the headquarters of any eukaryotic cell. It stores the genetic information for a plant and also access the cell's activities director, telling it how to grow, when to split, when to jump and how high, that sort of thing. Animal cells have this kind of nucleus, too, but prokaryotes don't, which is why they're stuck, hanging around in oil wells and stuff. Three, they've got plastids, including chloroplasts, which are awesome, green, food-making machines. Four, they have a central vacuole that stores water and other stuff, and helps give the cell structural support. And so, stack these cells on top of one another, like apartments in an apartment building, and you had a plant. All of these unique features are what makes it possible for plants to put food on our table and air in our lungs. So next time you see a plant, just go ahead and shake its hand. Thank it for its hard work and its service.