Video Transcript
The cohesion–tension theory of
water movement aims to explain how water is transported through the xylem. Which of the following best
explains what is meant by cohesion in water movement? (A) Cohesion refers to the forces
of attraction that occur between the water molecules and the walls of the xylem. (B) Cohesion refers to the forces
of attraction between the molecules of water inside the xylem. (C) Cohesion refers to the pull of
water through the xylem, as water vapor diffuses from the stomata. Or (D) cohesion refers to the
movement of water from the soil into the root, from an area of low solute
concentration to an area of high solute concentration.
Do you know why water transport is
so important for plants? One reason that plants need water
is that water is a reactant in the chemical reaction with which plants produce their
own food, photosynthesis. Photosynthesis generally happens in
the leaves of the plant. Water therefore has to be somehow
transported from the roots, where the plant takes up the water, to the leaves of the
plant. So how does this happen?
The question states that the
cohesion–tension theory of water movement aims to explain this water
transportation. This theory suggests that water is
pulled up from the roots as it evaporates off the surface of leaves through a
process called transpiration. Transpiration is the process
through which water is lost by evaporation into the atmosphere. This loss of water causes a
so-called transpiration pull that pulls the water from the roots through the xylem
vessels to the leaves. You can imagine that the
transpiration pull is caused by a person pulling on a string at the top of a plant,
thereby moving the string through the plant.
Now, you might argue that water is
a liquid and therefore does not behave like a solid string. Interestingly, water in the plant’s
xylem vessel does exactly that. The reason for this phenomenon are
cohesive and adhesive forces. Cohesion describes forces between
two or more water molecules. Adhesion, on the other hand,
describes the attraction between water molecules and the surrounding walls of the
xylem. Both of these forces cause the
water to form a continuous water column, which, like a string, can be pulled up the
xylem.
Now that we have reviewed the main
components of water transport and its importance for photosynthesis, let’s take a
look at the provided answer choices.
Answer choice (A) is incorrect
because it is adhesion that describes the forces of attraction that occur between
the water molecules and the walls of the xylem.
Answer option (B) states that
cohesion refers to the forces of attraction between the molecules of water inside
the xylem. This seems to correctly describe
cohesion. But let’s take a look at the
remaining answer choices just in case.
Answer choice (C) is incorrect
since this actually describes the process of the transpiration pull. Finally, we can exclude answer
option (D). It is osmosis that refers to the
movement of water from the soil into the root, from an area of low solute
concentration to an area of high solute concentration.
After reviewing each answer choice,
we determined that the best answer to our question, “Which of the following best
explains what is meant by cohesion in water movement?,” is given by answer choice
(B). Cohesion refers to the forces of
attraction between the molecules of water inside the xylem.
