Cohesion-Tension Theory

by Jason Amores Sumpter
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cohesion. Tension Theory says that evaporation from the leaves creates negative pressure or tension, and this tension pulls water up from the roots. Now leaves contain a humid air, and that will evaporate when the stone mata are open and the external humidity is less than 100%. So the conditions needed for that evaporation or transpiration is I'm gonna label it here are that this external humidity is less than 100% which it's almost certain to be now. The evaporation lowers the humidity in the mezza feel or that inner tissue in the leaves. So inside these leaves, the evaporation is going to result in lower humidity, and this causes water. Thio enter into the space in the Mazza Phil from the from the parent came a cells, right. It's going to enter that a plastic region. Now what's gonna essentially happen is the evaporating water is going to pull water, and from the parent came a cells, and this is going to create a steep menace. Guy menace, guys, the plural of meniscus. So many meniscus is, ah, steep menace. Sky will form at the air water interface in the leaf cell walls, and this is going to create that tension now each meniscus is small, right? But there's many of them and all their forces added together become significant. The tension from all these menace guy in in the leaf cells is going to pull water up from the roots, which is going to be assisted by cohesion and adhesion. So, looking at our diagram here, essentially, what's gonna happen is this water leaving is going to create a negative pressure potential on that negative pressure potential is going Thio cause water to be drawn up from the roots to the leaves. And this is because that force from the negative pressure potential, right that tension is going to be transmitted all the way along through the asylum all the way along through that water due to cohesion and adhesion. Right? So that's how those factors we talked about previously are going to influence this process. You might be wondering now, wait, you didn't mention surface tension Guys, those meniscus is That's the surface tension that is pulling up on the water that is in that surface. Tension is in those many menace guy, So this process is basically solar powered and the reason I say that is plants don't expend energy to create the upward force on the water. Uh, the sun heats up the atmosphere and that's going thio, uh, you know, in part facilitate this transpiration happening. It's that transpiration happening that's going to create this negative pressure and that negative pressure is going to pull water up from the roots. To be fair, it should be noted that plants do expend energy to take ions up into the roots. And this is what allows water to enter the root hairs via osmosis. So, you know, technically there is some energy expenditure coming into play that is, you know, going to have some impact on this process. But the main point is that this cohesion tension aspect is totally energy free for the plant. The you know, they don't have to expend any energy for that part of the process. And, you know, just to kind of illustrate the amazing force that this congenital rate, you know, keep talking about redwood trees. There's super super tall, this process, this cohesion tension can create a negative pressure significant enough to draw water up over 300 ft vertically. That is a massive amount of pressure. In fact, it's so much pressure that if the secondary cell walls of the vascular tissue were not lignin fied, they wouldn't be able to withstand it. You know, the plant would actually, like break itself trying to do this. It's just another reason that lignin fication or that the, uh that including lignin in the secondary cell walls, is so important to the vascular tissue of plants. All right with that, let's flip the page.