Cell wall is the outermost protective cover of cell. It was first observed by Robert Hooke. In plants it is made up of cellulose, hemicellulose, pectin lignin, cutin suberin and silica. In a plant ,cell wall shows three distinct regions (a) Primary wall (b) Secondary wall (c ) Middle lamellae.
a . Primary wall :
(i) It is the first layer innner to middle lamellae, primarily consisting of loose network of cellulose microfibrils in a gel matrix.
(ii) It is thin ,elastic and extensible. in most plants the microfibrils are made up of cellulose oriented differently based on shape and thickness of the wall.
(iii) The matrix of the primary wall is composed of hemicellulose, pectin glycoprotein and water.
(iv) Hemicellulose binds the microfibrils with matrix and glycoproteins control the orientation of microfibrils while pectin serves as filling material of the matrix.
(v) Cell such as parenchyma and meristems have only primary wall.
b. Secondary wall :
(i) Secondary wall is laid during maturation. it plays a key role in determining the shape of a cell. it is thick ,inelastic and is made up of cellulose and lignin.
(ii) The secondary wall is divided into 3 sublayers `S_1,S_2 and S_3` where the cellulose microfibrils are compactly arranged with different orientation forming a laminated structure and the cell wall strength is increased.
c. Middle lamellae. :
(i) It is the outermost layer made up of calcium and magnesium pectate, deposited at the time of cytokinesis.
(ii) It is thin amorphous layer which cements two adjacent cells. It is optically inactive. (isotropic).
(iii) Plasmodesmata and pits : plasmodesmata act as a channel between the protoplasm of adjacent cells through which many substances pass through.
(iv) Moreover, at few regions the secondary wall layer is laid unevenly whereas the primary wall and middle lamellae are laid continuously such regions are called pits.
(v) The pits of adjacent cells are opposite to each other. Each pit has a pit chamber and a pit membrane.
(vi) The pit membrane has many minute pores and thus they are permeable. The pits are of two types namely simple and bordered pit.
