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Question Video: Describing Adaptations of Companion Cells Biology

The diagram provided shows an outline of a phloem vessel. How are companion cells adapted for their role of actively transporting substances in and out of the sieve tube members? [A] They contain many mitochondria that provide energy via photosynthesis. [B] They contain many mitochondria that provide energy via cellular respiration. [C] They contain a large vacuole to keep them turgid and more stable. [D] They contain many nuclei that provide energy via photosynthesis.

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Video Transcript

The diagram provided shows an outline of a phloem vessel. How are companion cells adapted for their role of actively transporting substances in and out of the sieve tube members? (A) They contain many mitochondria that provide energy via photosynthesis. (B) They contain many mitochondria that provide energy via cellular respiration. (C) They contain a large vacuole to keep them turgid and more stable. Or (D) they contain many nuclei that provide energy via photosynthesis.

Before we answer this question, let’s remove the answer choices so we have more room to work with.

Just like humans have blood vessels to transport substances around the body in the blood, some plants have vessels for transporting substances too. They are called xylem and phloem vessels. You may recall that plants carry out photosynthesis, a chemical reaction in which light energy is used to convert carbon dioxide and water into glucose and oxygen. The water required for photosynthesis is absorbed in the roots and then transported up the plant in the xylem vessels. The glucose produced by photosynthesis is primarily converted into sucrose and transported around the plant in the phloem vessels.

The question asks us about the phloem, so let’s discuss the structures of the phloem in more detail. The main transport vessel of the phloem is the sieve tube, which is made of sieve tube elements stacked end on end. Sieve tube elements are living cells whose nuclei and organelles have broken down to leave a hollow space inside. This space allows dissolved substances, such as sucrose, to easily flow through the sieve tube.

The end walls of the sieve tube elements are partially broken down to form what are known as sieve plates. The sieve plates contain pores through which dissolved sucrose can pass, allowing it to travel easily from one sieve tube element to the next.

Closely associated with each sieve tube element is a specialized cell known as a companion cell. The companion cells are connected to the sieve tube elements via narrow channels called plasmodesmata, through which sugar molecules can be transported into and out of the sieve tube.

Now we have a clear picture of the structure of the phloem, let’s discuss how substances are moved throughout the plant via the phloem.

Let’s assume that it is dark and the leaves of the plant require sugar which is stored in the roots of the plant. First, complex sugars which are stored in the roots are converted into sucrose. The sucrose then diffuses into cells surrounding the phloem. Next, the sucrose is actively transported into the companion cells. From there, it can diffuse into the sieve tube elements via the plasmodesmata.

The low water potential generated in the sieve tube elements causes water to move into the phloem by osmosis. This generates a high turgor pressure, which transports sucrose up or down the sieve tube by a process called mass flow. And the sugars will eventually end up in the leaves where they are needed. Note that sucrose has to be actively transported from the cells where the sugar is stored and diffuses to into the companion cells.

You may remember that active transport requires energy and probably can imagine that the transport of sugars into the companion cells needs a lot of energy. Among other adaptations, companion cells therefore contain many mitochondria. This allows them to carry out cellular respiration at a high rate and, hence, release lots of energy.

If we return now to our answer choices, we’ll be able to find the correct one. We have learned that companion cells contain many mitochondria, which provide the energy to actively transport substances in and out of the sieve tube members. So (A) and (B) could be our correct answer choices.

If we study these answer choices in more detail, we can see that answer option (A) claims that mitochondria provide energy via photosynthesis. This is incorrect, as photosynthesis is the process in which light energy is captured and stored in glucose molecules. Furthermore, photosynthesis happens in chloroplasts, not mitochondria. Mitochondria, otherwise known as the powerhouse of the cell, are the cell organelles which provide energy via cellular respiration.

Therefore, the correct answer is given by answer option (B). They contain many mitochondria that provide energy via cellular respiration.

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