Movement of Nutrients in Plants 

For details of the structure of phloem cells click here

Sieve tubes are arranged end to end with their residual cytoplasms being connected through the holes in the adjacent walls. These connecting walls are called sieve plates

Each sieve tube element is associated with a companion cell and to which they are connected via plasmodesmata. 

Companion cells have large numbers of mitochondria so have the potential for high metabolic activity.

It is still unclear exactly what role the companion cells have but the potentially high metabolic rate, the plasmodesmata and the fact that sieve tube elements do not have the ability to synthesize ribosomes strongly suggest that they provide the sieve tube elements with proteins.

Some companion cells act as transfer cells moving organic solutes and inorganic ions into and out of the phloem tube. This are active transport processes so, again, the presence of many mitochondria makes sense. the inner surface of the walls of transfer cells (and hence the underlying cell membrane) are highly infolded providing a large surface area for the transport of nutrients. Thus they can act to move nutrients between the phloem tube and sinks and sources as required by the mass flow theory

For details of mass flow click here

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