Cell Wall

Most Plant cells possess a cell wall, and this feature distinguishes them from animals cells. The cell wall is a nonliving structure which is formed by the living protoplast. These are the middle lamella the primary cell wall and the secondary cell wall. Occasionally a tertiary cell wall may be present.



The middle lamella is formed between adjacent cell walls during cell division. It consists of a comparatively thin layer of intercellular material. It is a viscous and jelly-like substance and acts as a cementing material between the primary cell walls of adjacent cells. The middle lamella is composed of pectin, cellulose, calcium and polymers of various types. Pectin is a hydrophilic colloidal substance.

The Primary cell wall is formed during the early stages of growth and development. It is composed chiefly of cellulose, hemicellulose and pentic compounds. The primary cell wall is elastic, and undergoes extension with the growth of the cell. In many roots, flesly stems, fruits and leaves the cells contain only the primary cell wall and the middle lamella.  The cellulose fibrils of the primary cell wall are loosely interwoven, with a degree of polymerization of 8,000 to 12,000 glucose units. They are about 100A in diameter.

The Secondary cell wall is laid down on the primary cell wall when the latter has finished its growth. It is found only in certain mature and highly specialized cells. After the formation of the secondary cell wall commonly the protoplasm disappears in many cell types. The secondary cell wall commonly has three layers. These are known as the outer layer the middle layer and the inner layer. The secondary cell wall consists of layers of closely packed micro-fibrils. The succeeding layers are laid down at different angles to one another. In green plants the microfibrils are most often of cellulose, although they may contain other polysaccharides as well. In some algae there are many fibrils of xylan and mannan.