Notes on Ultrastructure of cell wall, Plasmodesmate standard 11-12 B.Sc. sem-1 college

Ultrastructure of the cell wall:

Electron microscopy has shown that the cell wall is constructed on the same architectural principle which applied well in the construction of animal bones and such common building materials a fibreglass( Plastic + glass fibres ) or reinforced concrete (concrete+ metal frameworks), i.e. , strong fibre(e.g., cellulose microfibrils)  resistance to tension embedded in an amorphous matrix (comprising hemicellulose, Pectin and Proteins)resistant to compression.

In the primary cell wall, the fibres and matrix molecules are cross-linked by a combination of covalent bonds and non-covalent bonds to form a highly complex structure whose composition is generally cell-specific.

In fact, hemicellulose molecules (e.g., Xyloglucans) are linked by hydrogen bonds to the surface of the cellulose microfibrils.

Some of these hemicelluloses molecules are cross-linked in turn to acidic Pectin molecules (e.g., Rhamnogalacturonans) through short neutral Pectin molecules (e.g., Arabinogalactans).

In the multilamellar secondary cell wall, cellulose microfibrils are down in layer. the microfibrils of each layer running roughly parallel with each other but at an angle to those in other layers.

Plasmodesmata:

Plasmodesmata

Every living a cell in a higher plant is connected to its living neighbours by fine cytoplasmic channels, each of which is called a Plasmodesmata (desmos: ribbon, ligament: plural ).

Plasmodesmata which pass through the intervening cell walls.

Detail structure of  Plasmodesmata

The plasma membrane of one cell is continuous with that of its neighbour of each plasmodesmata.

A plasmodesmata is a roughly cylindrical membrane linked channel with a diameter of 20-40nm.

Running from cell to cell through the centre of most plasmodesmata is a narrower cylindrical structure, the desmotubule, which remains continuous with elements of the S.E.R. membranes of each of the connected cells.

Between the outside of the desmotubules and the inner face of the cylindrical plasma membrane is an annulus of cytosol, which often appears to be constructed at each end of the plasmodesmata.

These constructions may regulate the flux of molecules through the annulus that joins two cytosols(Gunning).

Plasmodesmata are formed around the elements of the smooth endoplasmic reticulum that become trapped during cytokinesis(of mitotic cell division) within the new cell wall that will bisect the parental cell.

Plasmodesmata function in intercellular communication, i.e., they allow molecules to pass directly from cell to cell.

For example, Plasmodesmata are especially common and abundant in the walls of columns of the cells that lead toward sites of intense secretion, such as in nectar-secreting glands(trichomes of abutilon nectaries).

  In such cells there maybe 15 or more Plasmodesmata per square micrometre of wall surface, whereas  therein often less then 1 per square micrometre in another cell-wall (Gunning and Hughes).