Companion Cells in Plants: Function & Concept

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  • 0:00 Phloem Function
  • 1:15 Companion Cell Interactions
  • 2:27 Three Kinds of…
  • 3:36 Lesson Summary
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Lesson Transcript
Instructor: Jeremy Battista
Many of the functions in plants do not happen simply on their own; they need some help. In the case of the phloem, companion cells are the helping agent. Read on to learn how.

Phloem Function

In plants, there are two main types of vascular tissue that help the plant get water and nutrients. The xylem makes sure water gets around the plant from the roots. The phloem helps to transport sugars and other essential nutrients. The phloem works to take the products of photosynthesis (sugar) away from the leaves and transport it around the plant, specifically to the roots and other places in the plant where there is no photosynthesis taking place. This is basically the plant's way of making sure that the non-green portions of the plant receive food from the leafy green portions.

It is this function that makes the phloem so absolutely essential to the plant. If you can picture the leaves as the stomach of the plant, the phloem would be the bloodstream of the plant. You eat the food, it digests in your stomach (like the sugars 'digest' in the leaves) and gets transported for use as an energy source in your body (like the phloem transports sugars all around the non-green parts of the plant).

Found in the phloem cells of a plant are companion cells. Companion cells are a type of parenchyma cell. Parenchyma cells are parts of the ground tissue of plants. The cell walls of these cells are typically thinner than most other cell walls. These cells make up the majority of the plant.

Companion Cell Interactions

In the phloem, the sugars and other organic compounds that are transported are done so through tubes that are connected in chains called sieve-tube members. These have pores in them that allow materials to flow through them. Alongside the sieve-tube member is the companion cell. They are connected to the sieve-tube members through plasmodesmata.

In this image you can see the sieve tube members, companion cells, and plasmodesmata of the plant cell
plant cells

The plasmodesmata are interconnected tunnels through the cell wall of a plant. This allows the cells to share cytosol, as well as sugars and other materials. This would work in with our idea of the phloem, as the companion cell could share the sugars transported by the phloem, specifically the sieve-tube members.

The reason for the connection between the companion cells and the sieve-tube members is, as stated earlier, the plasmodesmata allow materials to flow between the cells. The sieve-tube members are alive, yet they lack organelles like the nucleus or ribosomes. Due to the lacking of certain organelles, the cell needs to rely on the organelles from the companion cells. The companion cell will carry out all the metabolic functions of the sieve-tube member. Without the companion cell, the sieve-tube member would die, ceasing phloem function, and thereby killing the plant.

Here is an excellent view of the sieve-tube member attached to the companion cell via the plasmodema.
Companion Cell

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