The typical plant cell has three distinct regions. They are cell wall, protoplast and 3 vacuoles. A cell wall is the non living outer boundary of the cell of plants and fungi which encloses various living organelles and non living inclusions.
One of the major differences between animal cell and plant cells is the possession of true cell walls by the plants. The chief structural component of cell wall is cellulose. In addition to cellulose, pectic compounds, hemicellulose, lignin, suberin, cutin, protein, fats etc. may also found in cell wall.
A plant cell wall can be differentiated into three regions namely-middle lamella, primary wall, and secondary wall.
1) Middle lamella
It is chiefly composed of calcium and magnesium pectate and is the amorphous region between two cells. The formation of middle lamella begins in the telophase stage of the cell division when fragments of endoplasmic reticulum and golgi apparatus migrate and accumulate in the middle region to form a cell plate.
Middle lamella is a cementing layer between two cells and the two cells are separated when middle lamella is dissolved. The loosening of mature ripe fruits is due to the dissolution of middle lamella.
2) Primary wall
It is the first formed wall of the cell which is produced inner to the middle lamella. Primary wall is generally thin, elastic and extensible in growing cells. It grows by addition of more wall material within the existing one and becomes more rigid. Such a growth is called intussusception.
The meristematic cells in plants possess only primary wall. The primary wall consists of about 50% hemicellulose, 25% pectin cellulose and smaller amounts of pectic substances, proteins and non-cellulosic polysaccharides. The cellulose is present in the form of filamentous macrofibril consists of a group of approximately 250 parallelly arranged microfibrils and each microfibril consists of 20 micelles. Each micelle is a bundle of about 100 cellulose chains.
The other chemicals substances of primary wall such as water, hemicellulose, pectin and glycoproteins form an amorphous matrix or ground substance in which the microfibrils are embedded. Pectin is the filler substance of the matrix. Hemicellulose binds microfibrils with the matrix and glycoproteins control orientation of microfibrils.
The main function of primary cell wall is to provide mechanical strength and protection to the protoplast.
3) Secondary wall
Secondary cell wall is a thick wall laid inner to the primary wall by the deposition of cellulose. The secondary wall is composed of several layers which are successfully deposited one upon the other on the inner surface of the primary wall after cell enlargement stops. Such a type of growth is called accretion.
In addition to cellulose, the secondary wall also shows the deposition of pectin, lignin, suberin, hemicellulose, fats and waxes. Lignin is deposited in the secondary wall of xylem and sclerenchymatous cells. The strengthening of cell wall by the deposition of lignin is called lignification. The secondary wall of the cork and endodermal cells is impregnated with a fatty substance, suberin. It makes the cell wall impermeable to water and gases.
In many kinds of cells, especially in wood, the secondary wall layers are not developed at certain points. These thin areas on the cell wall are called pits through which delicate strands of protoplasm called plasmodesmata connect one cell with that of the adjacent cell. The plasmodesmata are mainly concerned with the transport of materials and conduction of stimuli between the cells.
Function of cell wall
- It provides a mechanical support and gives a definite shape and protection to the cell
- The cell wall being permeable, is capable of imbibing water.
- Physiologically, the primary wall is porous and considered as permeable membrane which allows both solvent and solute molecules to pass through it.
- Lignin deposition makes the cell wall strong and suberin deposition makes it impermeable to gases and water.
