Video Transcript
What is the primary function of the companion cells in the phloem? (A) Maximizing the available surface area for the diffusion of gases. (B) Providing chloroplasts for a maximum rate of photosynthesis to take place. (C) Providing energy for the transport of substances in the phloem. Or (D) connecting the xylem and the phloem for the exchange of substances.
This question is asking about a specific vascular tissue found in plants, the
phloem. To answer it correctly, let’s review the key facts about the structure and function
of this tissue. 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.
Xylem and phloem make up most of the vascular bundle. Xylem carries water and minerals from the soil to the leaf. Phloem carries 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.
This question is asking about the phloem, so let’s now look more closely at this.
The phloem is composed of several types of cell, two of which are shown in this
diagram. The sieve tube elements are the cells that carry the solutes up and down the
plant. These are living, elongated cells which are stacked on top of each other. Perforations develop in their end cell walls, forming sieve plates which allow
substances to pass through them from one cell to the next. Sieve tube elements lose many of their organelles, including their nuclei at maturity
to allow the solutes to flow more easily through them. The companion cells are linked to the sieve tube elements by small gaps, called
plasmodesmata, which you can see here. As the name suggests, companion cells assist the sieve tube elements. In fact, the sieve tube elements are completely dependent on their companion
cells.
The transport of solutes in the phloem, a process called translocation, requires
energy. This energy is provided by a process called cellular respiration. Cellular respiration is the breakdown of carbon-containing compounds, usually
glucose, in a series of chemical reactions, releasing energy. Sieve tube elements do not contain mitochondria so cannot carry out aerobic
respiration. Companion cells, on the other hand, contain many mitochondria. And so it is these cells that provide the energy needed for translocation.
Now that we have explored the structure and function of the phloem and reminded
ourselves of the key facts, let’s return to the question. We now know the correct answer is (C). The primary function of the companion cells in the phloem is providing energy for the
transport of substances in the phloem.
