Transpiration-Cohesion-Tension Theory

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This video explains the practicality behind the transpiration-cohesion-tension theory in plants.
Properties of Water Link: • Properties of Water an...
** If there are any pictures used in this video, they are NOT MINE and I will not take credit for them. **
TRANSCRIPT:
Transpiration is when water vapor evaporates through the leaves of a plant. The transpiration-cohesion-tension theory explains how water can move up a plant and evaporate into the air, even against the forces of gravity. The properties of water play a major role in transpiration, since water has the power to overcome gravity in this type of situation. So let’s start with a simple plant. The roots of the plant first absorb the water in the soil, and this water moves upward through a vascular tissue in the plant called the “xylem”. The other vascular tissue found in the plant is called the “phloem”, and it transports sugars downward. So, in the xylem, the water is able to move upward because of two properties: cohesion and adhesion. I went over these properties in one of my other videos, but if you haven’t seen my Properties of Water video, then I’ll just go over it now. You can find the link in the description.
Cohesion is when water is attracted to itself. This is because water is a polar molecule, meaning that the electrons in one water molecule aren’t distributed evenly. Water is made of two hydrogen atoms and one oxygen atom, and it just so happens that oxygen is very electronegative, meaning that it has a greater affinity for electrons than the hydrogen atoms. That means the electrons are going to spend more time near oxygen, so oxygen will have a partial negative charge most of the time. When this happens, and a lot of water molecules are close together, they become attracted to each other like this.
Adhesion is similar, except it describes when water is attracted to something else other than itself. This could mean that the “something else” is the sides of the plant, since there are varying charges. This varying positive-negative attraction allows water to attract to the sides of the plant, and so water can cling on to the sides of the xylem. As water moves upward, it brings other water molecules with it due to cohesion, so it’s kind of like having a barrel of monkeys and each monkey is clinging onto another, pulling each other up.
Water then moves through the xylem into the leaves of the plant. The place where it evaporates from the plant is called the stoma, or stomata for plural. Stomata are little openings in the plant leaves, and they’re usually on the bottom side of the leaf. These two guard cells make up a stoma, and the guard cells can open or close it. It’s important to close the stoma at times because you don’t want too much water to evaporate from the plant, or the plant will end up having no water at all. But then again, the plant needs to have open stomata in order for CO2 to diffuse in. This CO2 can be used in photosynthesis to make sugars. That’s why plants have guard cells to regulate the opening and closing of stomata.
The guard cells bend like this when K+ ions enter the cells. In order to balance out the high concentration of K+ ions, water will follow and fill up the guard cells. This will cause the guard cells to become turgid, which means they get kind of stiff and curve out, opening the stoma. And when the K+ ions go out, this will cause the water to follow out too, making the guard cells go limp and close the stoma.
Well, how do the K+ ions know whether to go in or not? This is powered by a mechanism in the guard cells called the proton pump, which pumps H+ protons out of the cell through active transport, meaning ATP is needed to power the pumping. When H+ ions go out, K+ ions come in. That’s because a positive charge is going out, resulting in a more negative charge inside the cell, causing the K+ ions to diffuse inside through protein channels.
Now that you know about stomata, we can fit all of the pieces together. So the water that evaporates through the stomata of the plant is replaced by water coming up from the xylem, and all of these water molecules are pulling each other up by cohesion. Plus, there’s the force of adhesion that’s allowing the water to cling on to the sides of the xylem. That’s basically it for the transpiration-cohesion-tension theory, and thanks for watching!