Video Transcript
Which of the following best
explains the relationship between wind and the rate of transpiration? (A) An increase in wind speed
decreases the rate of transpiration as water molecules are moved away from the leaf
slower. (B) An increase in wind speed
increases the rate of transpiration as water molecules are moved away from the leaf
faster. (C) A decrease in wind speed
increases the rate of transpiration as water molecules are moved away from the leaf
faster. Or (D) a decrease in wind speed
increases the rate of transpiration as water molecules are absorbed by the leaf
faster.
This question asks us about
transpiration, which is the loss of water by evaporation from the leaves of a
plant. Let’s review some of the factors
that influence transpiration to help us determine how wind will affect this
process. First, we’ll remove our answer
choices so we have a bit more space.
Plants require water for
photosynthesis. Photosynthesis enables plants to
make their own food in the form of glucose, using carbon dioxide and water. This process also requires light
energy. The products of photosynthesis are
oxygen and glucose. The plant uses glucose molecules in
cellular respiration to provide energy. Photosynthesis largely takes place
in the leaves of the plant; these are adapted for photosynthesis as they are packed
full of chloroplasts and have a large surface area for absorbing sunlight.
The carbon dioxide required for
photosynthesis is absorbed through tiny holes in the leaves called stomata. Water is absorbed through the roots
and must make its way from the roots all the way up to the leaves in order for
photosynthesis to occur. Transpiration is required for this
movement to take place.
We have already described
transpiration as the evaporation of water from the leaves. As water evaporates, more water is
pulled up through the xylem, the vascular system that transports water through
plants. Water will evaporate through the
stomata. And as these water molecules
diffuse into the atmosphere, additional water molecules are pulled up to replace
them.
You may have already learned that
molecules move from regions of higher concentration to regions of lower
concentration through a process called diffusion. This means that a higher
concentration of water inside the leaf compared to outside ensures water molecules
move out of the leaf. If the concentration of water
inside the leaf is very high compared to outside the leaf, the rate of transpiration
will be higher.
The greater the difference in
concentration of the two regions, or the steeper the concentration gradient, the
faster diffusion will occur. When there are many more water
molecules inside the leaf compared to outside, the water molecules will quickly move
down their concentration gradient and out of the leaf. This means the rate of
transpiration will be high. Conversely, when the concentration
of water outside the leaf is similar to the concentration of water inside the leaf,
the water molecules will leave the leaf more slowly. There is not much difference in the
concentration of water inside the leaf compared to outside, so the rate of
transpiration will be low.
Now, consider the impact wind will
have on the water molecules surrounding the leaf. If it is a calm day, with little
wind, the water molecules leaving the leaf will remain close by, so the
concentration of water molecules surrounding the leaf will be higher. We have just seen how this will
make transpiration happen more slowly, as there is less of a difference between the
two concentrations.
On a windy day, however, those
water molecules that have evaporated from the leaf will quickly be blown away by the
wind, and the air surrounding the leaf will be replaced by drier air. We have just seen how a low
concentration of water molecules surrounding the leaf increases the rate of
transpiration.
Now that we have reviewed the
relationship between wind speed and transpiration, let’s have another look at our
question and answer choices. Our question asks us to select the
statement which best explains the relationship between wind and the rate of
transpiration.
We have just seen how high wind
speed moves water molecules away from the outside of the leaf, increasing the water
concentration gradient between the inside and the outside of the leaf, therefore
increasing the rate of transpiration. So, the correct answer is (B). An increase in wind speed increases
the rate of transpiration as water molecules are moved away from the leaf
faster.
