Root Pressure Theory

by Jason Amores Sumpter
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cohesion. Tension theory is the most widely accepted theory as to why water flows up through asylum. However, there are some other theories that try to explain this phenomenon, and they're not mutually exclusive with cohesion, tension. It's important to point that out. However, given all of the evidence, uh, it is most likely that cohesion. Tension is the dominant reason that water is flowing up. Silom thes uh, other things might contribute, but they're probably not. The main factor is the point. So I want to talk about one of these other theories route pressure theory, which basically says that positive pressure builds up in the root xylem due to increased absorption of water relative to transpiration. So remember I said that that idea of transpiration was gonna come back in terms of water moving up through the asylum. Well, here's where it comes back to haunt us, or it's just the start of it, really. Now, the reason all of this water is gonna build up in the root Silom is that ions are pumped into root silom, and this creates a negative water potential relative to soil. So it's gonna drive water into those roots asylum. Azzawi said before water enters via osmosis and the more water that builds up in the roots. I'll, um, you know, like the more water that builds up in our roots island, this little blue line that you can see traveling through the root there, the greater the positive pressure. Right? So the basic idea is that mawr water goes into the root Silom here than leaves the leaves, leaves the leaves, get it through transpiration. And there you know, there is some evidence for this. For example, stomach close at night. But roots continue to absorb waters of that water and ions from soil. And in fact, if you look at root pressure, it is highest in the morning due to this. And in fact, you can even sometimes see this phenomenon known as rotation, where water is forced out of leaves due to all of this pressure, and you can actually see a picture of dictation happening here. These water droplets are being squeezed out of the leaves due to this super high pressure. And, um, you know, this is the most common. This site is most common in mornings when pressure is the highest and like right before you know, the plants open, They're still mata and start transpiring again, you know? And they've had the whole night to suck up water and ions. Basically. So they're gonna be Cem. Let's say, Cem, factors that we need to cover in order to understand cohesion, tension theory and the movement of water through zeile. Um, in general, water has something some ability known as cap Hillary action or capital charity. This is the ability of liquid in this case, water what we're talking about to move through narrow spaces. And it's basically due to three three factors. We see capital a reaction as a product of three factors. One of those is adhesion. When this is the attraction between unlike molecules. So in the case of capital a reaction it's going to be the attraction between water and the different molecules that make up the tube. You can see an example of adhesion here, where these water droplets are clinging to the spider. Web cohesion is the attraction between, like molecules. So, in our, uh, you know, in terms of capital reaction, when we're talking about water, it's gonna be the attraction between water and itself. And you can see a nice example of cohesion. Here, let me get my head out of the way. The water beating up on the surface of these leaves is due to the fact that, uh, the leaves surfaces hydrophobic, and so water is going to want to cling to itself. Here, Uh, this is an example of cohesion because and, you know, you might have toe, you know, kind of look closely to see this. The water is actually forming orbs, right? So instead of the droplet having a flat bottom like that, the water is actually beated up in an orb like that. So it's lifting off the surface because the molecules are being attracted to each other in that droplet. Now, this cohesion will sometimes lead to what's known as a meniscus, which is a, uh, con cave surface boundary due to cohesion and adhesion. You can see an example of a meniscus here. That's the type of meniscus that water is going to form where it comes up on the sides like that. There are some liquids that will form a convex meniscus like that. Those tend to be heavier liquids than water. For example, mercury forms a convex a meniscus like that. Now, the last force that I want to talk about in terms of capital reaction is surface tension, and you can see an example of surface tension and straighten S t on this image for surface tension right here with this paper clip that is seemingly floating, though it's actually being held up on the surface of the water. It's not actually floating because it hasn't broken the surface. And basically, surface tension is the force between the water molecules at the air water interface. So the water is going to be attracted to itself that that air water interface and it's going to create, um, a tense like a tension across the surface, which is why this paperclip can just sit there on the surface without breaking it. So how does this all come into play in terms of capital? A reaction that you can see here with water has moved up the tube against gravity and now is higher than the surface level of the water it's in. Basically, adhesion pulls up from the container wall, right? So the adhesion between the water molecules and theme the tube wall is going to pull up surface. Tension is going. Thio pull up from the very surface, and then cohesion basically transmits the pole between all the water molecules. So as surface tension pulls up from the surface, that meniscus adhesion is going. Thio allow, you know, pull from the walls and cohesion is going to transmit that pulled all the water molecules in the tube. That is how we get capital a reaction. And with all of that in mind, let's actually flip the page and talk about cohesion, tension theory.