What is the function of companion cells in phloem transport?

Companion cells assist in the transport and regulation of nutrients within the phloem.

Companion cells are a critical component of the phloem, the vascular tissue responsible for the transport of organic nutrients in plants. They are found adjacent to sieve-tube elements, the primary conducting cells in the phloem. The main function of companion cells is to support these sieve-tube elements and facilitate the movement of substances in and out of them.

Companion cells and sieve-tube elements are derived from the same mother cell and remain closely connected throughout their lifespan. This close connection allows for the direct transfer of materials between the two cell types. Companion cells are metabolically active, possessing a large number of mitochondria and ribosomes. This allows them to produce ATP and proteins necessary for the functioning of the sieve-tube elements, which lack these organelles.

The transport of nutrients in the phloem, also known as translocation, is an active process that requires energy. Sucrose, the primary sugar transported in the phloem, is actively loaded into the sieve-tube elements by companion cells. This creates a high concentration of sucrose in the sieve-tube elements, causing water to move into them by osmosis. The resulting increase in pressure drives the flow of sucrose and water down the sieve tube.

Companion cells also play a role in the regulation of nutrient transport. They can adjust the rate of sucrose loading and unloading in response to changes in the plant's environment. For example, if a plant is exposed to cold temperatures, the companion cells may slow down the rate of sucrose loading to conserve energy.

In summary, companion cells are essential for the functioning of the phloem. They assist in the transport and regulation of nutrients, providing the energy and materials necessary for these processes. Without companion cells, the sieve-tube elements would not be able to carry out their vital role in nutrient transport.