Companion cells are parenchymal cells found within the phloem of flowering plants that manage the flow of nutrients through sieve tubes. Companion cells deliver ATP, proteins and other nutrients to sieve elements. This is due to the cytoplasm of sieve tubes lacking typical cell maintenance structures.
Companion cells are similar to other nucleate plant cells, although they often have more ribosomes and mitochondria. While sieve elements are cells responsible for transferring sugars in the plant, they lack nuclei and other associated organelles. This means that they are reliant on smaller companion cells to do much of the metabolic work required for transporting sugars and other nutrients.
Three types of companion cells exist: ordinary companion cells, transfer cells and intermediary cells. Ordinary companion cells have smooth cell walls and lack connections to bodies other than the sieve tube. Transfer cells have folded walls and specialize in larger transfers of materials due to their adjacency to non-sieve cells. These cells also scavenge solutes from nearby cell walls. Intermediary cells have smooth walls and many connections to other types of plant cells. Albuminous cells have a similar function to companion cells, differing in that they are exclusively associated with sieve cells. The phloem's other parenchyma cells are typically assigned to food storage.