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.
