The diagram provided shows an
outline of a phloem vessel. How are companion cells adapted for
their role of actively transporting substances in and out of the sieve tube
members? (A) They contain many mitochondria
that provide energy via photosynthesis. (B) They contain many mitochondria
that provide energy via cellular respiration. (C) They contain a large vacuole to
keep them turgid and more stable. Or (D) they contain many nuclei
that provide energy via photosynthesis.
Before we answer this question,
let’s remove the answer choices so we have more room to work with.
Just like humans have blood vessels
to transport substances around the body in the blood, some plants have vessels for
transporting substances too. They are called xylem and phloem
vessels. You may recall that plants carry
out photosynthesis, a chemical reaction in which light energy is used to convert
carbon dioxide and water into glucose and oxygen. The water required for
photosynthesis is absorbed in the roots and then transported up the plant in the
xylem vessels. The glucose produced by
photosynthesis is primarily converted into sucrose and transported around the plant
in the phloem vessels.
The question asks us about the
phloem, so let’s discuss the structures of the phloem in more detail. The main transport vessel of the
phloem is the sieve tube, which is made of sieve tube elements stacked end on
end. Sieve tube elements are living
cells whose nuclei and organelles have broken down to leave a hollow space
inside. This space allows dissolved
substances, such as sucrose, to easily flow through the sieve tube.
The end walls of the sieve tube
elements are partially broken down to form what are known as sieve plates. The sieve plates contain pores
through which dissolved sucrose can pass, allowing it to travel easily from one
sieve tube element to the next.
Closely associated with each sieve
tube element is a specialized cell known as a companion cell. The companion cells are connected
to the sieve tube elements via narrow channels called plasmodesmata, through which
sugar molecules can be transported into and out of the sieve tube.
Now we have a clear picture of the
structure of the phloem, let’s discuss how substances are moved throughout the plant
via the phloem.
Let’s assume that it is dark and
the leaves of the plant require sugar which is stored in the roots of the plant. First, complex sugars which are
stored in the roots are converted into sucrose. The sucrose then diffuses into
cells surrounding the phloem. Next, the sucrose is actively
transported into the companion cells. From there, it can diffuse into the
sieve tube elements via the plasmodesmata.
The low water potential generated
in the sieve tube elements causes water to move into the phloem by osmosis. This generates a high turgor
pressure, which transports sucrose up or down the sieve tube by a process called
mass flow. And the sugars will eventually end
up in the leaves where they are needed. Note that sucrose has to be
actively transported from the cells where the sugar is stored and diffuses to into
the companion cells.
You may remember that active
transport requires energy and probably can imagine that the transport of sugars into
the companion cells needs a lot of energy. Among other adaptations, companion
cells therefore contain many mitochondria. This allows them to carry out
cellular respiration at a high rate and, hence, release lots of energy.
If we return now to our answer
choices, we’ll be able to find the correct one. We have learned that companion
cells contain many mitochondria, which provide the energy to actively transport
substances in and out of the sieve tube members. So (A) and (B) could be our correct
If we study these answer choices in
more detail, we can see that answer option (A) claims that mitochondria provide
energy via photosynthesis. This is incorrect, as
photosynthesis is the process in which light energy is captured and stored in
glucose molecules. Furthermore, photosynthesis happens
in chloroplasts, not mitochondria. Mitochondria, otherwise known as
the powerhouse of the cell, are the cell organelles which provide energy via
Therefore, the correct answer is
given by answer option (B). They contain many mitochondria that
provide energy via cellular respiration.