Video Transcript
Which substance is the xylem
primarily responsible for transporting? (A) Glucose, (B) water, (C) carbon
dioxide, (D) sucrose, or (E) oxygen.
Many plants have vascular systems
composed of specific tissues that are used to transport different substances
around. The word “vascular” originates from
the Latin word “vascularis,” which refers to vessels or tubes. This is because the vascular
systems of different organisms are generally made up of a complex system of tubelike
vessels responsible for transporting various essential substances from one place to
another.
In vascular plants, the two main
vascular tissues are grouped together into regions called vascular bundles, as has
been shown here in the plant stem. These vascular bundles extend
throughout the plant from its roots, through the stem, to the leaves. These bundles consist of two
tissues: the xylem and phloem. Let’s discuss what the roles of
each of these tissues are.
Most plants are able to produce
sugars in the form of glucose by reacting carbon dioxide with water in the presence
of light energy through a process called photosynthesis. Glucose can react with oxygen in
all the living cells of the plant through cellular respiration. This process produces carbon
dioxide and water but, more importantly, releases energy for various cellular
processes.
As glucose easily reacts with
oxygen in cellular respiration, it is difficult to transport long distances. For this reason, glucose is
converted into another sugar called sucrose to be transported to the various cells
of the plant and then converted back into glucose when it is needed for cellular
respiration. Using this information, we can
eliminate answer choice (A), as we know that glucose is not transported around the
plant.
Sucrose and other solutes, like
amino acids, are transported in the phloem either up or down the plant stem to all
the cells that might require them. As this question is asking for the
substance that the xylem is primarily responsible for transporting and we know that
the phloem, not the xylem, transports sucrose, we can eliminate answer choice (D)
too.
While the sucrose and amino acids
transported via the phloem are dissolved in water, it is not the main function of
the phloem to transport water in bulk. That is the function of the
xylem. The xylem transports water and
dissolved mineral ions from the roots, which usually absorb them from soil, up the
stem to all the various parts of the plant that require them. This suggests that the correct
answer to this question is (B). But let’s double-check the last
couple of answer choices to make sure.
While there will be some dissolved
gases, like oxygen and carbon dioxide, present in both the phloem and the xylem, it
is not the role of the vascular system to transport these substances. Instead, these gases are constantly
exchanged between the plant and its external environment through pores in its leaves
and stems. So we can eliminate answer choices
(C) and (E).
With this information, we can
confirm the correct answer to this question. The substance that the xylem is
primarily responsible for transporting is (B), water.
