Companion cells are specialized parenchyma cells that are closely associated with sieve tube elements in the phloem of angiosperms. They are involved in the loading and unloading of sugars and other metabolites into and out of the sieve tubes, which are responsible for the long-distance transport of these substances throughout the plant.

Companion cells are typically elongated and have a large nucleus and dense cytoplasm. They lack vacuoles and chloroplasts, but they do contain numerous plasmodesmata, which are small channels that allow for the movement of molecules between adjacent cells. The close association between companion cells and sieve tube elements allows for the exchange of nutrients and other substances, including water, ions, amino acids, and proteins.

Companion cells play a crucial role in the regulation of sugar transport in angiosperms. They are responsible for the uptake of sugars from the apoplast (the space outside the cells) into the sieve tubes. This process is driven by the proton gradient generated by the proton pumps located on the plasma membrane of the companion cells. The proton gradient creates an electrochemical potential that allows for the co-transport of sugars and other molecules into the sieve tubes.

Once inside the sieve tubes, the sugars are transported over long distances through the plant. When the sugars reach their destination, they are unloaded from the sieve tubes into the apoplast of the receiving tissues. Companion cells play a role in this process as well, facilitating the movement of sugars out of the sieve tubes and into the surrounding cells.

Overall, companion cells are essential for the sugar-transporting system in angiosperms, as they are involved in the loading and unloading of sugars into and out of the sieve tubes and regulate the movement of these substances throughout the plant.