Which substance is the phloem
primarily responsible for transporting? (A) Carbon dioxide, (B) sugars, (C)
oxygen, (D) minerals, or (E) water.
To answer this question, let’s
start by understanding the transport system in plants. Transport systems, made up of
vascular tissue, are required by plants as they are too large to rely on diffusion
alone. These tissues are held in vascular
bundles in the stem, roots, and leaves of the plant. They allow water and minerals to be
carried from the soil to other parts of the plant and also solutes, such as sugars
and amino acids, from the leaves to wherever they are needed by the plant.
If we look more closely at a
vascular bundle, you will see it consists of three tissues: phloem, xylem, and
cambium. Cambium is a thin layer of
undifferentiated cells, which sits between the phloem and xylem. Each vascular bundle is supported
by a tough section of sclerenchyma tissue called the pericycle. Sclerenchyma tissue is made up of
dead cells which have a tough waterproof substance called lignin in their cell
Let’s focus on the xylem tissue to
start with. Xylem tissue carries water and
mineral ions from the roots to the leaves. It consists of two main types of
cell, xylem vessels and xylem fibers. Some parts of the xylem might
include cells called tracheids. These are similar to xylem vessels,
but they are tapered and closed and often longer and thinner. Xylem is regarded as a complex, or
compound, tissue, as it is made up of cells which are structurally and functionally
different as opposed to the simple tissues, such as parenchyma and collenchyma.
Xylem vessels are made up of
sclerenchyma cells stacked on top of each other. As the cells mature, they die and
their end walls break down to form long, continuous tubes. Sclerenchyma cells have lignified
cell walls, which make them waterproof, preventing leakage of water, and strong,
giving further strength and structure to the plant. The xylem fibers are also
lignified, giving further support to the vessels.
Now let’s focus on the phloem
tissue, the tissue our question is asking us about. Phloem is also a compound, or
complex, tissue. It is made up of four different
types of cells: sieve tube elements, or members, companion cells, fibers, and
sclereids. The role of the phloem is to carry
solutes, mainly sugars but also amino acids, from the leaves and other
photosynthesizing tissues to wherever they are needed by the plant. As these solutes are needed for
lots of different roles, such as growth of buds or root tips or storage, such as in
potato tubers, they need to be able to move both up and down the plant unlike the
water in the xylem. The sieve tube elements, or
members, are the cells that carry the solutes up and down the plant.
The cells are stacked on top of
each other, and perforations develop in their end cell walls, forming sieve
plates. These perforations allow certain
substances to flow through them. The companion cells are linked to
the sieve tube members by small gaps called plasmodesmata. Companion cells have lots of
mitochondria to help provide the energy required for the movement of the solute
throughout the phloem. The fibers and sclereids, not shown
here, help support the phloem.
Now that we have explored the xylem
and the phloem in depth, let’s take another look at our question. We know that the xylem transports
water and minerals, so we can cross out answers (E) and (D). Carbon dioxide and oxygen are
mainly exchanged in the leaf through stomata and air spaces, so we can rule out
answers (A) and (C). This leaves us with the correct
answer to the question “Which substance is the phloem primarily responsible for
transporting?” as (B), sugars.