Video Transcript
Which of the following best
describes the structure of the xylem? (A) The xylem is a long, continuous
tube of living cells. (B) The xylem is composed of many
living cells with pores in the cell wall. (C) The xylem is a long, continuous
tube of dead cells. Or (D) the xylem is composed of
many living and dead cells that form a solid tube.
This question is asking about the
specialized vascular cells found in plants called xylem. To answer it, let’s recall the key
facts about vascular tissues in plants, looking at xylem in more detail.
Plants are autotrophs, making their
own food by photosynthesis. Therefore, they start most food
chains and provide us with the oxygen we need to stay alive. Vascular plants are complex,
multicellular organisms, which means they are too large to rely on diffusion and
osmosis alone to move substances between cells. They need a good transport system,
which is made up of the vascular tissues xylem and phloem. Xylem transports water and mineral
ions from the roots to the leaves. And phloem transports nutrients,
most commonly sucrose and amino acids, to wherever they are needed in the plant. It is common to get the two types
of tissues mixed up. So let’s look at the differences
between them and how they are adapted for their function.
Xylem tissue is made up of two main
types of cells: tracheids and vessel elements, or members. These cells grow on top of each
other and end up forming long, hollow tubes like straws. But how does this happen? As the cells get older, they become
lignified. This means lignin, a waterproof
substance, is laid down in the cell walls. And this kills the cells, as
nothing can get in or out of them. The end walls of the cells then
break down, forming the long, hollow structure as shown in the diagram. The lignin provides support and
allows the xylem to withstand pressure changes as the water moves up the plant. These structural adaptations allow
the xylem to carry out its role of transporting water and mineral ions in one
direction, from the roots to the leaves.
Phloem tissue, on the other hand,
is mainly made of living cells, sieve tubes and companion cells. Sieve tubes form again by the cells
growing on top of each other. This time, however, the end cell
walls do not break down. Instead, they form pores, making
sieve plates. These allow the movement of
cytoplasm between the cells. The cells lose their nuclei, and
there is little room for organelles. This is where the companion cells
come in. They are connected to the sieve
tubes through small pores. And as their name suggests, they
assist the sieve tube cells. Companion cells provide energy and
any substances required by the sieve tubes to function properly. Between the two, they carry out
their role of moving the organic molecules up and down the plant to wherever they
are needed.
Now that we have reviewed the
structure of the vascular tissue in plants, we can return to the question, which
asks “Which of the following best describes the structure of the xylem?” We have seen that phloem is mainly
made of living cells with pores in their end walls, so we know the answer is not
(B). Xylem is made only of dead
cells. So we can discount any answers left
with living cells mentioned. This means we can cross out (A) and
(D). Therefore, the correct answer is
(C). The xylem is a long, continuous
tube of dead cells.
