字幕列表 影片播放 列印英文字幕 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 donut burgers. I'm telling you this is surprisingly good. This is a different kind of magic. But you know, part of this is plants! And everything in it, in fact, everything that is in this McDonalds in fact, everything that you have ever eaten in your life is either made from plants, or from something that ate plants. So, let's talk about plants! Plants probably evolved more than 500 million years ago. The earliest land-plant fossils date back 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 forests" because there were so many of them and they were so dense and they covered the whole Earth and they eventually fossilized into giant seams of coal, which are very important to our lifestyles today. So this is now called the Carboniferous Period. See what they did there? Because Coal is made out 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 were freaking awesome. Anyway, Angiosperms, or plants that use 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 saying it's a little bit suspicious. Anyway, on the 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 "new-cue-lus." And the nucleus can be found in all sorts of cells. Animal cells, plant cells, algae cells. You know, basically all of 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. And then we have eukaryotic, which means "good kernel!" The prokaryotes include your bacteria and your archaea, 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 covered the planet. They cover you! But like I said, eukaryotes have that separately enclosed nucleus. That all important nucleus that contains its DNA and is enclosed by a separate membrane Because the 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. I guess we decided we would 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. 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. What are the differences in a plant cell that makes it so different from an animal? Well 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, and something plants inherited from their ancestors was a rigid wall surrounding the plasma membrane of each cell. So, this cell wall of plants is mainly made 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, although if you were to include simple carbohydrates as well, glucose would win that one. And this is because, fascinating fact: cellulose is 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 will generally find in a prokaryotic cell, but it's also one of the main things that differentiates a plant cell from an animal cell. Animals cells don't have this rigid cell wall--they have just 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 it also protects it to a degree. Which is why trees aren't squishy and 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 in 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. However, other animals that don't have access to delicious donut burgers have either developed gigantic stomachs like sloths or multiple stomachs like goats in order to make a living eating cellulose. These animals have a kind of bacteria in their stomach 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. But 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. Ah! 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. And 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. Ow! It's on me! Ow! Ahh! Anyway, while we animals are walking around, spending our lives searching for ever more digestible plant materials, plants don't have to do any of that. They just sit there and they make their own food. And you 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, organelles that plants use to make and store compounds that they need. And you wanna 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 protist-like cell absorbed a bacteria, and it found that instead of digesting that bacteria for the energy that it has, 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 both 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 chloroplasts, which convert light energy from the sun into the sugar and into oxygen, which the plant doesn't need, so it just gets rid of it. All the green parts of a plant that you see -- the leaves, the non-woody stems, the unripened oranges -- are all filled with cells which are filled with chloroplasts, which are making food and oxygen for you. You're very welcome, I'm sure. Another big difference between a 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 soil dries out or a 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 vacuoles are also kind of a 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 and export stuff 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. So now, let's do this. Let's just go over the basics of plant cell anatomy: 1. They've got a cell wall that's made out of cellulose and so it's really rigid and not messing around. 2. 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 all the genetic information for the plant and also acts as 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. 3. They've got plastids, including chloroplasts, which are awesome green food-making machines. 4. They've got a central vacuole that stores water and other stuff and helps give the cell structural support.