The cells of all plants, bacteria and fungi have rigid,protective covering outside plasma membrane called “Cell wall”.
Cell wall is present in plant cells but its absent in animal cells.The cell wall was first observed by Robert Hooke in 1865 in cork cells.At beginning cell wall considered as unproductive but in later ages it was found that metabolically active and capable of growth and at least during its growth, contains protoplasmic material.
Formation of cell wall:
During telophasic stage of mitosis, the phragmoplast widens and becomes barrel shaped.At the same time,on equatorial plane the cell plate begins to form inside phragmoplast. After the formation of cell plate phragmoplast dissappers completelyThe cell plate undergoes some changes to form intercellular substances called as “middle lamella”.
Plant cell consist of mainly three parts:
- Middle lamella
- Primary wall
- Secondary wall
It is a thin amorphous cement like structure usually found between two adjacent cells.It is the fist layer deposited at the time of cytokinesis.It is optically inactive(Isotropic).It is made of calcium and magnesium pectates.Sometimes protein may also present.
It is the first formed wall of the cell produced inner to middle lamella.It is thin, elastic and extensible in growing cells.Its optically active.It grows by the addition of more cell wall material and the process is termed as Intussusception.Some cells like parenchymatous cells and meristematic cells possess primary wall alone.Primary wall consist of loose network of cellulose microfibrils embedded in a gel like matrix.Matrix composed of water, hemi-cellulose, pectin and glycopretein.Pectin is the filling material of the matrix.
After cell has reached maturity a thick secondary wall is laid inner to primary wall.It is laid in succession of three layers namely s1, s2, and s3.It grows in thickness by accretion.(i.e deposition of materials over the existing structures).S2 is the thickest layer which is central.But the number of layers may be different for different cells.Secondary cell wall is not uniform in all cells.This results in the different types of cells namely parenchyma, collenchyma, sclerenchyma, fibres, and tracheids.
The secondary wall consist of two continuous interpenetrating systems one of which is the cellulose microfibrils and the other continuous system of microcapillary spaces.These spaces may the filled with lignin ,cutin, suberin, hemi-cellulose and other organic substances.The chain molecules are arranged in bundles which are generally termed as micellae.Elementary fibrils composed of 36 cellulose molecules.It is mostly crystalline.
The cell wall is not totally complete around the cell.It is interrupted by narrow pores carrying fine strands of cytoplasm, which interlink the contents of the cells.These are called as “Plasmodesmata”.They form protoplasmic continuum called symplast and may consist of lined plasma membrane.It has simple and branched tubule known as desmotubule.
Pits are the areas of cell wall where secondary wall is not laid down.They are opposite and adjacent to each other.Each pit has a pit chamber and pit membrane.Pit membrane consist of middle lamella and primary wall.It has many minute pores and hence permeable.
Two types of pits: 1. Simple pits 2.Bordered pits
- Simple pits: In simple pits the width of the pit chamber is uniform.There is no secondary wall in simple pit.
- Bordered pit: In bordered pit the secondary wall partly overhangs the pits.
Generally each pit has its complementary pit lying exactly opposite to it in the wall of the neighbouring cell.Those pits form a morphological and functional unit called “Pit pair”.
Function of cell wall:
- It gives definite shape to the cell.
- It protects the internal protoplasm against injury.
- It gives rigidity to the cell.
- Prevents bursting of plant cells due to endosmosis.
- Walls of xylem vessels, sieve tubes etc. are specialized for longdistance transport.
- It takes part in cell defence and offence.