Question

What's the role of companion cells?

Answer 1

Cells are the essential building blocks of all living things. Our physical body consists of the trillions of cells. They supply structure for the body, absorb nutrients from food, convert those nutrients into energy, and perform specialized functions. Cells have so many parts, each with a special function.

Complete answer:
Companion cell may be a type of cell found within the phloem of flowering plants. Each companion cell is typically closely related to a sieve element. Its function is uncertain, though it appears to manage the activity of the adjacent sieve element and to require part in loading and unloading sugar into the sieve element.
Companion cells move sugar and amino acids into and out of the sieve elements. In 'source' tissue like leaf companion cells use transmembrane proteins to require up sugar and amino acids by transport. Loading is a setup of a water potential gradient that facilitates movement of the sugar.
These cells are found within the phloem of the flowering plants. They shift sugar and amino acids in and out of the sieve elements. Within the leaf (source tissue), the companion cells make use of the transmembrane proteins for the uptake of sugar and amino acids by transport.

Note:
The companion cells contain nuclei and enormous numbers of ribosomes and mitochondria. Phloem parenchyma cells are present at the border of the sieve tubes and are liable for the transportation of sugars. They bear nuclei and live cells.