Biology: Transport in Angiosperms: Transpiration

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You know, plants suck - they really suck. I mean, they really suck in the sense that we're talking about another regulatory mechanism in plants. You see, when you talk about this whole idea of plant homeostasis, we're not just talking about regulation, we're talking about transport, too. And plants, if you think about it, lack a heart. They lack a way to pump their juices around their body. Plants suck. Because the way they suck is they literally suck water from the ground, up the stem, out the leaf, and into the air. They are suction machines. In fact, it's so clever the way they've evolved, these things. You would think plants had a brain. Okay, but they do suck.
And here's the thing; they use a process called TACT. And TACT is just kind of, some initials to show you some of the things that they do. TACT stands for: Transpiration, Adhesion, Cohesion, and Tension. Transpiration, adhesion, cohesion, and tension. And that is the secret to way plants get water from the ground 200 feet high, if you happen to be a 200-foot plant.
Now let's take a look at how this works. And literally, it comes down to this. Plants have this suctioning ability from the ground, because of this simple process, and that process is evaporation. Plants evaporate water from their leaves. You see it all comes down to knowing the structure and function of leaves.
If you take a look at a leaf... You guys, you know, we've learned about leaves before; and if you want to look back on photosynthesis, you go! Because there's an awful lot we learned about leaves and cells back then. But the most important thing I want to talk to you about right now are these portions of the leaf right there.
You see, every leaf in its lower epidermis, unless you happen to be a water lily, and they're in your upper epidermis, has these holes. And the hole is for two things. One is for gas transport, and that's where we talked about them before. But the other is for evaporation, and if you think about it, how clever is a water lily. It would not be a smart thing to have a hole in the bottom of your leaf for evaporation of water, if it's in the water, now would it? But if you have the hole in the top, that's kind of cool. Anyway, but I want to talk with you about this.
So, in the lower epidermis you have these holes in the bottom of the leaf, and the object of these holes is to get water to flow out. Okay. And where's the water going to flow out of? Well, it's going to flow out of the vascular bundle particularly the xylem cells. So, what has to happen, is you have to, from the spongy mesophyl, evaporate a layer of water from here, okay, and it's got to out in this direction.
So, we definitely want to get the water and have it go out in that direction as it evaporates from the spongy layer. Okay? In addition, we than want the water to leave the xylem and somehow get into this layer to replenish that water. And then we want to get into this stem so that we can transmit the water to the xylem. And then up the xylem to the leaf, and then we want to get water in the root, and then we want to get water from the ground into the root. It all comes down to this simple process. The sun causes suction because of evaporation. And that is a process called transpiration. Transpiration is the loss of water from the leaf, the first part of TACT.
But to raise a column of water 200 feet is quite a feat. And lets talk about how that happens. It happens through the other parts of TACT: adhesion, cohesion and tension.
Let's go to adhesion. As each of these water molecules is pulled out, it is attached to another water molecule. Do you remember, there is a great lecture back there, well it's great material, I don't know how great the lecture is; but there's a great lecture back there on the qualities of water, and one of those qualities of water was adhesion. And adhesion is the ability of something to adhere to another substance and cohesion is the ability of two molecules to adhere, to stick to each other, okay: cohesive forces.
And so let's talk about the `AC' of TACT right now. Out comes the water, all right, and one water molecule must be bound to another water molecule, which must be bound to another water molecule, another, another, another, okay? And so therefore, and what causes that? If you remember what causes that, it's the fact that water forms hydrogen bonds, one water to another and, therefore, each water molecule is going to pull the other by these cohesive forces.
But that's not going to do me any good because let's face it. I can pull water up a certain ways, and eventually it's going to be too heavy and I'm going to break that column, all right. Just like stacking people from an airplane is an analogy I use. You know, and if one is holding on, eventually if you don't have anything else to hold on to and you're just holding on to the next guy's feet, those poor guys up near the top are eventually going to have too much weight. So we have to fight this idea of the column that we're pulling up and the hydrostatic pressure: the weight of the water. And so comes adhesion, because adhesion is what happens when you have the water attracted to the sides of the xylem.
You see, the sides of the xylem are the xylem of cells stacked one on top of the other. And these cells have hydrophilic cell walls. And so the water molecules are going to be stuck to those and then it's going to get pulled up and stuck, pulled up and stuck, so you have two forces now. You have the adhesive forces being pulled by the transpiration, and you have the cohesive forces of the water sticking to the side of the xylem. And everybody is hanging on for dear life. And this baby is going 200 feet. But that's not all. Because in a vacuum, which this whole tube is, there's no places where there's air that can get in. in a vacuum you have tension. And do you want to know something? This creates so much tension, that on a hot day in the summer, believe it or not, a tree can, you can actually measure, that the tree gets pulled in because of the contraction of its xylem, as that tension pulls on the sides of its xylem elements. That's incredible. And it's true.
So there's transpiration, adhesion, cohesion and tension. But this brings to us a problem. And the problem is this: If you have this long column that's thin, and xylem elements are very thin, and all of a sudden you get say, and it's full of water, and all of a sudden you get an air bubble in there, you're going to lose your tension. You're going to lose your continuity. And think of what happens in the winter. In the winter you get ice as the tree goes into dormancy. And you remember what ice is. Ice is less dense because of the trapped air. When spring rolls around, and transpiration tries to begin again, it's not going to work, because of these air bubbles.
So what's the solution to this problem? Here's the solution: you ready? The solution is: we make new xylems. Trees make new xylems. You know that. Remember spring xylems. Big, get smaller as the summer comes along. Than as soon as spring starts, we get new xylems. And that's how we get around that stuff. So we're not going to lose any of our tension. So you know, when someone says to you, "plant suck." You say, "That's right!" but say it with a smile.
Plant Systems and Homeostasis
Plant Transport
Transport in Angiosperms: Transpiration Page [2 of 2]