The structure and composition of the cell wall not only varies in different plants, but in different types of cells of the same plant. Often, there might be variety in the cell wall composition at different stages of growth of the cell.
The cell wall is very rigid and maintains the shape of the cell acting as a boundary. A unique feature of the cell wall is its permeability which is due to the presence of small pores.
2. Gross structure:
Three layers are distinguishable in the cell wall; these are the primary cell wall, middle lamella and the secondary cell wall. The primary cell walls of adjacent cells are separated by middle lamella. Secondary wall is deposited after a certain period of development.
The primary wall is secreted by the plasma membrane. In some instances like the leaves, fleshy stems, roots etc., the cells have only the primary wall and the middle lamella. The primary wall is formed during the early stages of growth and is about 1 to 3nm in thickness.
Chemically the primary wall is composed of cellulose, hemicellulose and pectic compounds. The young primary wall is plastic and is prone to changes. Often it may be lignified.
The middle lamella is the intercellular substance binding together the primary and the secondary walls.
The middle lamella is formed during cell division and is amorphous and colloidal in nature. Chemically the middle lamella is composed of pectin, cellulose, calcium and magnesium pectate.
The secondary wall is deposited on the primary wall after cell maturation. It is very rigid and does not alter its shape. The thickness varies from 5 – lOnm. The secondary wall usually has three layers viz., outer layer, middle layer and inner layer.
Chemically the Secondary wall is composed of cellulose, non cellulosic polysaccharides and hemicelluloses.
3. Ultra structure of the cell wall:
The cell wall is mainly composed of cellulose. Cellulose is a polymer of glucose molecules. Approximately each molecule of cellulose consists of about 3000 glucose residues joined together by 1-4 oxygen bridges.
These cellulose chains are oriented along the axis of the cell about 100 of this cellulose chains bundle together to form micellels or elementary fibrils. Each micelle is about 100 A in diameters many hundreds of these micelles of this stack together to form microfibrils. Each microfibril is 200A to 300A in diameter.
The micorfibril is composed of xylan, instead of cellulose. The secondary wall acquires different types of thickenings occasionally a tertiary cell wall may also be found as in the tracheids of some gymnosperms.
This is a very thin layer and is visible only under electron microscope. About 2000 of these microfibrils join together to constitute the macrofibril which is about 0.5 in width the macro-fibrils are visible under the light microscope.
In secondary walls, the microfibrils are more orderly and compact than in the primary walls. The space between fibrils is filled by an interfibrillar substance usually the matrix. The matrix is filled by non cellulosic polysaccharides – such as pectin, hemicellulose, xylans etc.
The primary cell wall of dicotyledons contains many complex polysaccharides such as xyloglucans, arabinogalactans and rhamnogalacturonans. These are linked to the cellulose fibril and to one another by covalent linkages.
In certain walls, a glycoprotein called extension is present. It is similar to collagen in being rich in 4 – hydroxyproline of which are linked, residues of arabinose and galactose. Extension has a long protein chain, to the hydroxy groups
Protoplasts of adjacent cells are connected to one another by means of thin cytoplasmic threads called plasmodesmata.
These pass through the small pores (pit fields) present in the cell wall. Tubules which are in continuity in the endoplasmic reticulum are seen within plamodesmata thus maintaining an effective intercellular communication.
5. Origin of cell wall:
At the completion of Karyokinesis, small vesicles of endoplasmic reliculum (phragmoplast) accumulate along the equatorial line to form the cell plate. The cell plate becomes thick by the addition of material from the golgi complex.
Plasmodesmata are left open in the cell plate to maintain communication. This cell plate forms the lamella. Later, on either side of the middle lamella, cytoplasmic fibrils start accumulating to form the wall.