 So now we've learned about the background, to some extent, the machinery that the plant is going to use. Then we have to think about the plant itself. What is it? Well, it's basically a support structure for chloroplasts. That's what it's there for. It's rather like the selfish gene view. You can argue that plants exist only to support the chloroplasts that are within them. So we have typically a support structure. Well, I suppose we start at the very bottom and we've got the roots, which we've already remarked are essential for pulling the nutrients out of the soil that the plant uses to make the machinery, which is photosynthesis as well. Then there's a trunk. That typically holds the plant up. To be honest, it's not always quite clear what the function of a trunk is. In many cases, it helps to keep the leaves above predation, or it may move them up into sunlight, where the photosynthesis becomes more effective. But if you take it to the extreme and you look at, for example, why some trees are tall and others are short, it's not at all apparent why that's the case. So there are some subtleties involved in that. So that trunk is mostly made out of carbohydrates, which at the very beginning we learned came from the seed, but all the other synths then have been produced by the photosynthesis plant itself. So you've got the trunk and you've got the branches, and they serve to distribute the leaves. And the leaves are simply organs to photosynthesize. They're little structures to house chloroplasts. So the trunk and the roots bring the water up that the plant needs, and the leaves themselves are broad and flat and optimized to capture sunlight and to give a high surface area through which CO2 can be brought into the mechanisms that will capture it and convert it into sugars. The leaves themselves, typically they're sort of a bit of a compromise between structural strength and making the best out of the resources that are available. Of course, the majority of the leaf is made out of carbohydrates, but there are other materials in there such as lignins as well, which start their lives also as compounds that are synthesized from calm dioxide water. So the leaf has little holes in the bottom bit that are called stomata. And these stomata serve to allow calm dioxide into the chloroplasts, where the sunshine will illuminate the chlorophyll within them and can be converted to energy. The holes in the bottom of the leaf, the little stomata, they lead into usually a rather spongy tissue called mesophyll, where much of the chloroplasts are. Though they're also in the upper part of the leaf just below the surface, the light penetrates fairly deeply into the leaf, and often the maximum photosynthesis is well into the thickness of the leaf. Now what do the little holes do? Well, one of the things they do is lose water. And the water actually, losing water is not really an essential part of the plant's function. The plant spends a lot of effort and energy to draw water out of the soil, up through the branches and out through the leaves. Why does it do it? Well, the water cools the leaves a bit, which on some circumstances can be useful, but mostly it's because they want to capture CO2. To capture CO2, the little stomata in the bottom of the leaf have to be open, and when they're open, typically a plant loses something like 400 times as much water as the CO2 that it gains. That's partly because water being a lighter molecule diffuses faster than CO2 does, and partly of course because there's a lot more water than CO2 for which we can be grateful. Something like 25,000 ppm of water versus at the moment about 400 ppm of CO2. But the plant doesn't want to lose the water, it's just stuck with that situation. And some plants, such as cacti, will go out of their way to avoid that situation. So a cactus is a bit cunning about the way that it separates its photosynthetic processes. The stomata open at night and the cactus captures CO2 through the open stomata while the temperature is lower and the relative humidity is higher, so it loses less water. It then stores the compounds that it's created with the stored carbon dioxide, closes its stomata and waits for sunshine. And then when the sunshine comes, it moves into the light processes of photosynthesis and it starts creating ATP, NADPH and all those other substances that will be necessary to drive the dark cycles that continue in the night and actually synthesize the sugars.