Now I mentioned aquaporins and the movement of water. But why water? Why is it important for me to bring in the part about water going across the membrane? The importance for water, here, is for the mechanism of the plant, of the leaf, closing, of the plant movement. While the electricity is the signal, what is the mechanism that allows the leaves to close physically? What is the electricity doing to cause the leaves to close? And to understand this point, we're going to go back and look at mimosa again, the plant whose leaves close when I touch it. Now the reason the leaves close, and I'll give you the end answer first, and then we'll talk about the mechanism, is because of water being pumped out of the cells as a response to the touch. It's actually as a response to the electrical signal that's generated from me touching the leaves. To understand the mechanism that water causes the closing and opening of the leaves, we need to go back and review, for a second, the cell biology that we learned in one of the earlier classes. I talked about two organelles, two parts of the cell that are specific for plant cells. The first is the cell wall. This is the rigid structure that bounds all plant cells and holds inside of it the protoplast, the basic cellular structure of the membrane and everything within it. The second organelle is the vacuole. The vacuole is sort of like a warehouse of the cell. The central vacuole is a large bubble, membrane bound bubble, within the cell that contains various chemicals, dependent on the cell type. For example, the vacuole can contain poisons such as nicotine. The pH of the vacuole can be vastly different than the pH of the entire cell. For example, the acidic pH of lemon cells is because of the acid within the vacuole. And the vacuole can also hold vast amounts of ions, different salts. And the water amounts in the vacuole can vary so that the size of the vacuole can at times be small, and at times be large. The cell can regulate the size of a vacuole by pumping ions in or out of the vacuole, thereby causing water to come in or out of the vacuole. Now to understand this point, again, I want to illustrate the difference between an animal cell and a plant cell. If we take an individual animal cell and put it in what's called an isotonic solution. An isotonic solution is one where the ion concentration, the solute concentration, inside the cell and outside the cell is equal. There will be a free flow of water into the cell and out of the cell, and the size of the animal cell won't change. If you take the same animal cell and put it in a salt solution, in a solution that we call hypertonic. Under these conditions, the salt concentration outside the cell, the concentration of the solutes outside the cell, Is much higher than that within the cell. And then through a process called osmosis, water will then go across the membrane outside of the cell, causing the animal cell to shrivel, to shrink. This is in a futile attempt to change the concentration of the solutes outside the cell so that they'll be equal. If you take the same animal cell, now, and put it in what's call hypotonic solution, a hypotonic solution such as distilled water where there's no solutes, no salt ions available. Then the concentration of salts within the cell is actually higher. So then, through osmosis, water will come into the cell, again, trying to equalize the concentrations. And in this case, the volume of the cell will grow as more and more water goes into the cell, until it will eventually pop in a process called lysis. The same principles occur in a plant cell, but the results are different. In a isotonic solution, again, of course you'll have water freely going in and out of a plant cell. And this is a condition which is called flaccid. There's no pressure of the protoplast onto the cell wall. If you put the same plant cell, again, into a hypertonic solution, into a salt solution, water will go out of the plant cell, primarily from the vacuole, which will cause the protoplast to shrink. But the cell wall will remain the same size. Now if you put a plant cell into a hypotonic solution, into distilled water, for example. In this case, the water will go into the cell, primarily into the vacuole, causing the volume of the vacuole to grow, which will attempt to cause the protoplast to grow. But, the protoplast is bound by the cell wall. So the more water that goes into the vacuole, the higher the pressure in the cell becomes, and this increase in pressure leads to a condition which we called turgidity. Or we called it, the cell is turgid. It becomes erect because of the pressure on the cell wall, and this is what give plants its structure. This is what allow green plants to stand tall, because of the pressure of the protoplast which comes from the volume of the vacuole onto the cell wall.