Video Transcript
The diagram provided shows a
simplified outline of the movement of water through a plant. Through what transport vessel does
the majority of water move? (A) Vein, (B) phloem, (C) cell
walls, or (D) xylem.
To answer this question, let’s go
through the different answer options and discuss what the purpose of each of the
different mentioned structures is.
A vein, when used in the context of
transport vessels, is a type of blood vessel that’s found in animals like
humans. Veins form part of the human
circulatory system that are responsible for transporting blood back toward the heart
from the body tissues. A different sort of vein can also
be found in leaves. While leaf veins do play a role in
transport, they are not transport vessels themselves. As the question is specifically
asking us the transport vessel through which water moves, we can eliminate option
(A).
While the phloem and the xylem are
both transport vessels that are found in plants, the cell wall is a rigid structural
layer that surrounds plant cells. Cell walls are responsible for
providing structural support and protection to plant cells, but cell walls are not a
transport vessel. So this option can also be
eliminated.
As we mentioned, the phloem and the
xylem are both transport vessels, but they differ in their structure and
function. The phloem transports substances
like sugars and amino acids both up and down the plant stem. While these sugars and amino acids
will be dissolved in water, so some water will be involved in phloem transport, it
is not the main function of the phloem to transport water.
The xylem, on the other hand, is
responsible for transporting the majority of water molecules in a plant. It also transports dissolved
mineral ions. The water molecules and ions are
generally absorbed from soil into the roots of a plant and then are transported from
the roots into the plant stem. Unlike in the phloem, where
substances can move both up and down the plant, the xylem only transports water and
mineral ions up the plant from the roots to the leaves and flowers and any other
organs that might require them.
If we take a look back at the
diagram provided by the question, we can see that water is only moving up the plant
from the roots to the leaves. This helps us to confirm that the
phloem is not the vessel through which the majority of water moves, as it transports
sugars and amino acids both up and down the plant stem. Instead, the role of the xylem is
to move the majority of water through the plant.
