Video Transcript
A student writes, “Adhesion in the
xylem refers to the formation of hydrogen bonds between different water molecules,
which results in the water forming a continuous column.” Are they correct? Why? (A) No, because adhesion results in
the water molecules being spaced far apart, not forming a continuous column. (B) No, because the bonds formed in
adhesion of water are ionic bonds, not hydrogen bonds. (C) No, because adhesion refers to
the formation of hydrogen bonds between the water molecules and the molecules in the
wall of the xylem. (D) Yes, because due to their polar
nature, water molecules easily form hydrogen bonds with other water molecules.
Water is pulled up through a plant
in the xylem vessels. There are intermolecular forces
that allow this process to occur. So what are these forces? And does the student correctly
describe one?
Water molecules in the xylem are
attracted to each other by hydrogen bonds. This is known as cohesion. Cohesion helps the water be pulled
up through the xylem. However, cohesion is not the only
intermolecular force at work. It is complemented by the force
that attracts the water molecules to the cells of the xylem wall. This force is called adhesion and
is, again, the result of hydrogen bonds. Note that although the
intermolecular forces at play in these two processes are the same, cohesion involves
water molecules sticking to each other. And adhesion involves water
molecules sticking to other cells.
So, the sentence written by the
student is incorrect. Cohesion in the xylem refers to the
formation of hydrogen bonds between different water molecules, which results in the
water forming a continuous column, not adhesion. The correct answer is given by
option (C). Adhesion refers to the formation of
hydrogen bonds between the water molecules and the molecules in the wall of the
xylem.
