Cell is a basic structural and functional unit of life which plays a major role for the proper functioning of the organs of the organisms. It is a microscopic structure that can be found in every living organism. Though cell itself is a microscopic structure, there are many organelles that contribute a lot in the formation and growth as well as functioning of the cell. Among such organelles, cell wall is also the one.
Cell wall is the outermost covering of a plant cell secreted and maintained by the protoplasm. It is not found in animal cell. It is non-living and permeable in nature. It provides a definite shape and protection to the cell. Being permeable in nature, the cell wall allows the movement of useful substances (water and gases) between cell and environment as well as one cell to another cell. The cell wall consists of four layers namely middle lamella, primary wall, secondary wall and tertiary wall.
i) Middle Lamella: Formation of middle lamella starts in the telophase stage of cell division. It is a thin common layer present between the two adjacent cells. It is made up of calcium and magnesium pectate. The pectin layer of middle lamella can absorb water and act as a cementing material between two cells.
ii) Primary Cell Wall: It is synthesized by protoplasm on either side of middle lamella in a growing cell. It is 1-3 μ in thickness. It is made up of cellulose, hemicellulose, pectic substances. Cellulose is a polymer of thousands of glucose (a hexose sugar) molecules, hemicellulose is a polymer of both hexose and pentose sugar, while pectin is a polymer of uronic acids (derivatives of hexose sugars).
Cellulose is the principal material that forms the framework of the cell wall and occurs in the form of microfibrils while the hemicellulose molecules from the ground substances or matrix.
iii) Secondary Cell Wall: In fully mature cell, secondary wall is formed internal to the primary wall. Secondary wall is rigid, inelastic and 5 to 10 μ in thickness. It consists of lignin in addition to cellulose, hemicellulose and pectin. Lignin is a complex polymer of aromatic compounds. It makes the secondary wall very strong and rigid. It is usually formed in non-living cells such as fibres, tracheids, xylem vessels and other sclerenchyma.
iv) Tertiary Cell Wall: Tertiary cell wall is found beneath the secondary wall in xylem tracheids of gymnosperms. It is purely celluloses in nature. It contains xylon and mannon. It is very thin and gives additional strength to the cell wall.
There are some other substances such as cutin, suberin, wax, mucilage and silica usually deposited outside the cell. Cutin, suberin, wax make the cell wall impervious to water and prevent loss of water from cells.
The primary and secondary wall are discontinued at some places and form pits, which are known as plasmodesmata.