Question Video: The Effect of Temperature on the Rate of Transpiration | Nagwa Question Video: The Effect of Temperature on the Rate of Transpiration | Nagwa

# Question Video: The Effect of Temperature on the Rate of Transpiration Biology • Second Year of Secondary School

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Which of the following best explains the relationship between temperature and the rate of transpiration? [A] As temperature increases, the rate of transpiration increases because water molecules are actively transported out of the leaf faster. [B] As temperature increases, the rate of transpiration increases because water molecules diffuse from the leaf faster. [C] As temperature decreases, the rate of transpiration increases because water molecules are actively transported out of the leaf faster. [D] As temperature increases, the rate of transpiration decreases because water molecules diffuse from the leaf slower.

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### Video Transcript

Which of the following best explains the relationship between temperature and the rate of transpiration? (A) As temperature increases, the rate of transpiration increases because water molecules are actively transported out of the leaf faster. (B) As temperature increases, the rate of transpiration increases because water molecules diffuse from the leaf faster. (C) As temperature decreases, the rate of transpiration increases because water molecules are actively transported out of the leaf faster. Or (D) as temperature increases, the rate of transpiration decreases because water molecules diffuse from the leaf slower.

First, let’s describe the relationship between temperature and the rate of transpiration. As temperature increases, the rate of transpiration also increases. Answer options (C) and (D) both suggest that the opposite relationship is true. So we can rule these out immediately as neither one of them can be correct. Answer options (A) and (B) both state the correct relationship, but only one of them has the correct explanation. So, let’s find out which one it is.

Here is a simple diagram of a leaf side-on. The cells of the leaf contain water because they need it as a reactant for photosynthesis and also because it’s produced as a byproduct of cellular respiration. The leaf also contains tiny pores predominantly on its underside, called stomata. The primary role of the stomata is to allow carbon dioxide to diffuse into the leaf as it’s another reactant for photosynthesis. But they can also allow water molecules to move out of the leaf. And it’s this process that’s known as transpiration.

Before water molecules move out of the leaf, they evaporate. You may recall that this is the process whereby a liquid, in this case water, changes state to become a gas. When water is in its gaseous state, it’s called water vapor. Because there’s a higher concentration of water vapor inside the leaf than outside, the water molecules move out of the leaf by diffusion. As the temperature increases, the water molecules gain more energy. This means they move around more and therefore they evaporate and diffuse out of the leaf faster. This is why the rate of transpiration increases as the temperature increases. We have therefore determined that the answer option which best explains the relationship between temperature and the rate of transpiration is (B). As temperature increases, the rate of transpiration increases because water molecules diffuse from the leaf faster.

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