The channels in microscopic form that traverse the walls of a plant cell are known as plasmodesmata or plasmodesma based on plural or singular forms. Few algae also have these structures apart from the land plants. The cell walls take care of the permeable proteins, but plasmodesmata regulates the transport of all intracellular substances.
There are two types of plasmodesmata the primary and secondary, these are formed when the cell division occurs and cell matures. The plasmodesma is form when some part of the endoplasmic reticulum gets trapped in the middle lamella. When new cell is formed the structures turn out to be primary connections between cells. Few pits are formed in the walls and then the adjacent cells pair up. Some non dividing cells also show presence of plasmodesma in their cell walls, these are secondary structures.
In general one plant cell will have about 103 and 105plasmodesma linking adjacent cells together. These are 50-60 nm in diameter. The plasma membrane area of the plasmodesma is seen in a continuous extension and is somewhat similar to phospholipid bilayer.
The molecule transport trafficking occurs through a small space filled with fluid and covered by plasmalemma in the plasmodesma. Various small molecules can easily pass off, but the proteins need more complex mode of transport. There is no clear know-how of this process and just one hypothesis is put forth. It suggests polysaccharide callose forms along the plasmodesmata in the shape of a collar. The diameter is considerably reduced and so permeability is controlled.
Desmotubule is one structure with endoplasmic reticulum which runs between adjacent cells. Transport of proteins is also known to occur through this channel. Many modes of active transport are seen to occur through the cells. All kind of transport interaction occurs through localized desmotubules.
Plasmodesma is one important part of the plant cell that ensures healthy transport and exchange of materials in the cell body.