What is cell wall?
In the year 1665 while observing cells in the sections of cork Robert Hooke discovered the cell wall. As a definition, it may be said that the thick and rigid, non-living covering present just outside the plasma membrane of plant cells is called cell wall. Cell wall is not found in animal cells but it is present in all plant cells except the gametes and the cells of a few lower categories of plants.
Origin of cell wall
An important role in the formation of cell wall is played by Endoplasmic reticulum and Golgi body. During cell division, after the nuclear division is complete, fragments of endoplasmic reticulum and Golgi body called phragmoplasts become arranged in a row at the equatorial region. These phragmoplasts then blend to form a continuous membranous structure called cell plate. The cell plate secretes pectin and gives rise to the middle lamella. As a final point, some complex chemical compounds produced within the protoplasm of the plant cell as a result of metabolism are deposited on the intracellular side of the cell plate and that is the origin of cell wall.
Structure of cell wall
The cell wall may be relatively thinner in some cells e.g., parenchyma cells, or thick in others e.g., the cells of sclerenchyma and vasc'ular tissue. The cell wall may be either smooth or unequally thickened. There are minute pores on the cell wall through which cytoplasmic connection is established between the neighboring cells. These cytoplasmic bridges are called plasmodesmata . In matured cells, the cell wall is composed of three layers that are formed in succession from outside inwards. Those are as follows:
i) Middle lamella,
ii) Primary cell wall
iii) Secondary cell wall.
The thin layer of jelly-like viscous intercellular matrix present in between two adjacent cells is known as middle lamella. It is a common layer between the adjacent cells that binds them like a cementing material. It is a colloidal material composed by mainly of pectin and some amount of protein. This layer has the property of elasticity and is formed first during the cell division.
1. Primary cell wall
The primary cell wall is called the layer just beneath the middle lamella. This is the outermost layer of an individual plant cell. The primary cell wall is formed after the formation of middle lamella by deposition of protoplasmic secretion making materials on the inner side of the middle lamella. The thickness of it gradually increases from 1 to 3 um, with growth of the cell and after completion of primary cell wall formation; the cell does not increase in size. This layer is thin, plastic and permeable and composed of mainly cellulose. Small amounts of pectin, lignin and suberin are also present in the primary cell wall.
2. Secondary cell wall
The thick and rigid layer underneath the primary cell wall is known as secondary cell wall. This layer is deposited on the primary cell wall inside of it after the growth of the cell has ceased. The secondary cell wall is generally seen in dead cells and vascular tissues. In the cells of meristematic tissue and cambium, this layer is absent; naturally primary cell wall is the only layer present inside the middle lamella of these cells. Though the secondary cell wall is made up of cellulose mainly, however, some amount of other complex materials such as, hemicellulose, pectin, lignin, etc., are also present in it.
Functions of the cell wall
The following are the main functions of the cell wall of the plants:
i) As the cell wall is rigid, it protects the cell from external injuries.
ii) It gives a definite shape of the cell.
iii) It gives mechanical support to the cell.
iv) Being porous to water and salts, the cell wall allows entry and exit of these materials to and from the cells.
v) It helps to maintain intercellular connections through plasmodesmata.
vi) In the epidermis of various parts of the plants, the cell wall becomes water proof due to deposition of cutin and suberin. This helps to reduce evaporation and loss of water from the plant body.
vii) In the woody parts of the plants, the cell wall becomes hard due to deposition of lignin and imparts rigidity and strength to the plant.