Video Transcript
The cohesion–tension, or
cohesion–adhesion, theory of water movement aims to explain how water is transported
through the xylem. Which of the following best
explains what is meant by adhesion in water movement? (A) Adhesion refers to the forces
of attraction between the molecules of water inside the xylem. (B) Adhesion refers to the pull of
water through the xylem, as water vapor diffuses from the stomata. (C) Adhesion 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. Or (D) adhesion refers to the
forces of attraction that occur between the water molecules and the walls of the
xylem.
Before we start to answer this
question, let’s first remove the answer choices so that we have some space to work
with. Most vascular plants, like this
one, absorb water from the soil into their roots. This water then moves through the
xylem vessels up the plant stem and into the leaves. In the leaves, water is either used
as a reactant for photosynthesis or it evaporates through the stomata, which are
specialized structures on plant leaves that allow gas exchange between the leaf
tissues and the atmosphere surrounding them.
It was unclear for a long time how
this water is transported through the xylem against the pull of gravity. The cohesion–adhesion theory, which
is sometimes called the cohesion–tension theory, gives an explanation as to how this
transport happens. When water evaporates from the
leaves and exits the stomata in the form of water vapor, a tension or pull on the
water in the xylem vessels arises. This creates a chain reaction of
pulls all down the column of water molecules that line the xylem vessel, drawing
water upward from the roots. It is the biochemical forces of the
water molecules that allow water to move against the downward pull of gravity up
through the plant.
Let’s discuss the biochemical
forces of these water molecules in more detail. The water molecules in the xylem
vessel are strongly attracted to each other. And hydrogen bonds form between
nearby water molecules, which makes them stick together. This stickiness between water
molecules is called cohesion, and it’s been represented in this diagram by pink
dashes between the water molecules. Cohesion between water molecules in
the xylem vessels is complemented by adhesion, which in this diagram has been
represented by orange dashes between the water molecules and the xylem vessel
walls.
It is the stickiness of hydrogen
bonds in water molecules that also help them to stick or adhere very tightly to the
cell walls of the xylem vessels. Both cohesion and adhesion are
related to sticking together, but the objects they’re sticking to are very
different. While cohesion occurs between
different water molecules, adhesion is the attraction between each water molecule
and the xylem vessel walls. The cohesion between different
water molecules helps to form a continuous column of water, while adhesion helps
this column of water to move upward against the downward force of gravity.
Cohesion and adhesion therefore
work together to pull water molecules from the roots, through the xylem vessels of
the stem, and out of the plant through the stomata on the leaves. With this knowledge, we can now
explain what is meant by adhesion in water movement. The term “adhesion” is best
described by answer option (D). Adhesion refers to the forces of
attraction that occur between the water molecules and the walls of the xylem.
