A cell is the structural, functional and biological unit of all organisms. It is a membrane-bound structure containing a cytoplasm and cytoplasmic structures. The membrane that surrounds the cell and separates it from the outside environment is a cell membrane. In plants, fungi, and some bacteria, the cell membrane is surrounded by another layer, called the cell wall.
A cell wall is a membrane of the cell that forms external to the cell membrane whose main role is to give cells rigidity, strength and protection against mechanical stress. It is found in cells of plants, bacteria, archaea, fungi, and algae. Most prokaryotes have cell walls; few exceptions are mycoplasma. Animals and most protists do not have cell walls.
Plant cell wall
In plant cells, the cell wall is a tough, rigid structure that may consist of the primary cell wall which is generally a thin, flexible and extensible layer composed of cellulose, pectin and hemicellulose, and a secondary cell wall which is a thick layer rich in lignin that strengthens and waterproofs the wall and is formed inside the primary cell wall that has stopped increasing in surface area when the cell is fully grown. In between the primary walls is a middle lamella which is a pectin-rich intercellular material that glues the adjacent cells together. The cell wall is very essential in plants as it helps resist osmotic pressure.
Prokaryotic cell wall
In bacteria, the cell wall is composed of peptidoglycan, and is essential to the survival of many bacteria. Bacteria have been classified into Gram-positive and Gram-negative based on the structure of the cell wall. Gram-positive bacteria possess thick cell wall consisting of many layers of peptidoglycan and teichoic acids. Gram-negative bacteria have relatively thin cell wall consisting of few layers of peptidoglycan
In archaea, the cell wall is characteristically lacking the peptidoglycan (except for a group of methanogens) and is composed of glycoprotein S-layers, pseudopeptidoglycan, or polysaccharides.
Algal and fungal cell walls
In fungi, the cell wall is composed of chitin and other polysaccharides whereas in algae, it is made up of glycoproteins and polysaccharides, and in certain algal species it may be composed of silicic acid.
The primary function of the cell wall is to provide rigidity and strength. Thereby, it protects the cell from mechanical stress. It prevents the entry of large molecules that may potentially harm the cell. In plant cells, it keeps the plant from bursting (osmotic lysis) when placed in a hypotonic solution as opposed to animal cells that lack it. When an animal cell is placed in a hypotonic surrounding (i.e. higher water concentration), the water molecules will move into the cell causing the cell to swell. If osmosis continues and becomes excessive the animal cell will eventually burst. In a plant cell, excessive osmosis is prevented due to the osmotic pressure exerted by the cell wall thereby stabilizing the cell. In fact, osmotic pressure is the main cause of support in plants. However, if a plant cell is placed in a hypertonic surrounding, the cell wall cannot prevent the cell from losing water. It results in cell shrinking (or cell becoming flaccid).
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