Plasmodesmata
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Plasmodesmata (singular, plasmodesma) are microscopic channels of plants facilitating transport and communication between individual cells. Unlike animal cells, plant cells are protected by an impermeable cell wall; and as such, plasmodesmata are required for intercellular activity. Cells can utilize both passive and active transport to move molecules and ions through the passage. A plasmodesma is constructed of three main layers, the plasma membrane, the cytoplasmic sleeve, and the desmotubule.
[edit] Plasmodesmal Plasma Membrane
The plasma membrane portion of the plasmodesma is a continuous extension of the cellular plasmalemma. It is similar in structure to most cellular phospholipid bilayers.
[edit] Cytoplasmic Sleeve
The cytoplasmic sleeve is enclosed by the plasma membrane and is an extension of the cytosol. Trafficking through plasmodesmata is assumed to occur through this passage. Smaller molecules (e.g. monosaccharides)and ions, can easily pass through plasmodesmata by diffusion without the need for additional chemical energy. It is unknown how the selective transport of larger molecules, such as proteins, occurs. One hypothesis is that the polysaccharide, callose, accumulates around the neck region of plasmodesmata causing its diameter to be reduced and controls permeability of protoplasms or substances.
[edit] Desmotubule
The desmotubule is an area of appressed endoplasmic reticulum that runs between the two cells. Some molecules have been known to be transported through this channel, but it is not thought to be the main route for plasmodesmatal transport.
Around the desmotubule and the plasmamembrane areas of an electron density material have been seen, these are often joined together by spoke like structures that seem to split the plasmodesma into smaller channels. It has been hypothesised that these structures may be composed of myosin and actin, which are part of the cells cystoskeleton. If this is the case these proteins could be used in the selective transport of large molecules between the two cells.
