part_of part of part of adjacent to adjacent to adjacent_to companion cell A parenchyma cell (PO:0000074) that is adjacent to a sieve tube element (PO:0000289) and arises from the same phloem mother cell (PO:0000400) as the sieve tube element. Companion cells are connected to sieve tube elements through plasmodesmata and function in phloem loading of small molecules, including photosynthate (either sucrose or raffinose-family oligosaccharides). May participate in phloem sucrose loading (GO:0009915). Molecules can be loaded into the companion cells from surrounding cells through either symplastic (via plasmodesmata) or apoplastic (transmembrane) transport. Unlike sieve tube elements, companion cells retain their nuclei and other organelles at maturity. A sieve tube element may have one or more or no companion cells associated with it. This term should not be used for other types of parenchyma cells (PO:0000074) that do not arise from the same phloem mother cell as a sieve tube element. Companion cells are found in angiosperms, as compared to albuminous cells (PO:0025412), which are found in non-angiosperm vascular plants. Companion cells may be specialized as transfer companion cells (PO:0025458), ordinary companion cells (PO:0025459), or intermediary companion cells (PO:0025460). Multiple types of companion cells may be found in the same plant. PO:0000071 PO_GIT:442 c&#233lula acompa&#241iante (Spanish, exact) minor vein companion cell (exact) plant_anatomy 伴細胞 (Japanese, exact) parenchyma cell sieve tube element phloem