Video Transcript
Which of the following best
explains the theory of water movement by root pressure? (A) Root pressure is the force that
pulls water into the xylem from the roots. (B) Root pressure forces water into
the cell walls of the phloem to be transported around the plant. (C) Root pressure is the bonding of
water molecules to the walls of the root hair cells. Or (D) root pressure is the force
that pushes water from the roots vertically up the xylem.
This question asks us about root
pressure, which is one of the theories of water movement in plants. Let’s discuss the importance of
water movement and how root pressure was thought to cause the water to move in order
to answer this question correctly.
You might already know that plants
need to be watered regularly in order to survive. One reason for this is because
plants require water to make their own food through a process called
photosynthesis. This occurs in the leaves of plants
and requires carbon dioxide, water, and energy in the form of sunlight.
Many scientists were interested in
how water could move from the soil and roots up to the leaves where photosynthesis
occurs. This becomes especially interesting
when we remember that some plants, like the California redwoods, can grow up to be
400 feet, respectively, 122 meters tall. Water needs to move against a lot
of gravitational force in order to reach these heights.
Many different theories were
proposed, and one of them was the theory of water movement by root pressure. Root pressure is a positive upward
pressure that develops in the xylem vessels in the root. This occurs due to the absorption
of water into the roots by osmosis. When the water pressure in the
roots rises because more and more water enters the roots, water is pushed up the
xylem vessels. Note that the theory of water
movement uniquely by root pressure has shown to be insufficient. That is because root pressure is
generally low. Root pressure alone could not push
up water to the top leaves of, for example, the earlier mentioned redwood trees.
Now that we have reviewed the
theory of water movement by root pressure, let’s take a look at our answer
options. Answer choice (A) might look like a
good answer at first. However, you might remember that
transpiration is responsible for the pulling effect. Root pressure, on the other hand,
provides an upward push, and this answer option is therefore wrong. It is also important to remember
that root pressure pushes water into the xylem, and not the phloem, as stated in
answer choice (B). The phloem is responsible for
carrying sugar and metabolic products, not water. The function of root hair cells is
to absorb water, which is the reason that root pressure is building up. However, the water molecules
generally pass through their cell walls and don’t bond to them.
The best answer option is option
(D). Root pressure is the force that
pushes water from the roots vertically up the xylem.
