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Companion cell

(botany) Any of the metabolically-active parenchyma cells associated with a sieve tube element in the phloem tissues of angiosperms, and whose function is to provide energy to the sieve tube element during translocation
The phloem is the vascular tissue that carries out the function of translocation (i.e. the process of transporting photosynthate materials from the photosynthetic plant organs to various parts of the plant). The phloem is comprised of the following major components: (1) sieve element, (2) companion cell, (3) phloem sclerenchyma, and (4) phloem parenchyma.
The companion cells are specialized parenchyma cells in the phloem tissues of the angiosperms. They are nucleated living cells with several ribosomes, plastids, and mitochondria. Their nuclei and nucleoli are relatively large (an indication of being metabolically active). They are closely associated with the sieve tube elements. They connect to them via the plasmodesmata. They provide energy to the sieve elements during food transport (translocation).
A companion cell and the associated sieve element have an ontogenic relationship, which means that they arose from a common progenitor cell. The companion cell though is smaller and narrower than the sieve tube element.
There are two types of companion cells: the ordinary companion cells and the transfer cells. The transfer cells differ from the ordinary companion cells in having plasma membrane infoldings, which increase the surface area that permit larger areas of transfer.
Companion cells are present only in angiosperms. Pteridophytes and gymnosperms do not have companion cells although some of them have a cell with similar function (called albuminous cell) that is closely associated with the sieve cell. However, there is no ontogenic relationship between an albuminous cell and the sieve cell.
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