Definition - Cell membrane is the thin, elastic and semipermeable living membrane that surrounds the protoplasm of a cell is called cell membrane o plasma membrane or plasma lemma.
Existence of cell membrane was first observed by Nageli and Cramer in 1856.Then in 1931 it was named as plasma lemma by Plowe.It is find in each and every living cells including both plants and animals. It is formed due to modification of peripheral layer of cytoplasm.
Structure of the cell membrane- Cell membrane in some cases (renal tubules, intestinal mucosa ) form finger like projections called microvilli, phagocytic vacuoles, cilia, flagella, branched tubular structure called lamella are observed.
Ultrastructure of the cell membrane - it is found due to the study of cell membrane under electron microscope. In 1935 Danielli and Davson define the cell membrane as trilamellar structure which is made up of lipoprotein (lipid and protein). Average thickness of the cell membrane is 7.5 nm where each of two protein layers are about 20 angstrom and the middle layer is 35 angstrom thick. The lipid layer is made up of phospholipid molecules. This layer is called bimolecular phospholipid layer.This layer has two end one is hydrophobic and another is hydrophilic.
1972 Singer and Nicolson described fluid mosaic model of cell membrane. According to fluid mosaic model of cell membrane –phospholipid bilayer contains discontinuous mosaic of globular proteins . The matrix is in fluid form so the randomly distributed proteins are relatively free to move.Different organisms have different ratio of protein – lipid ratio. Structurally membrane contains two types of protein. One of them is intrinsic and another is extrinsic protein. Extrinsic proteins are called peripheral protein which is associated with the membrane surface. They are dissociable and situated outside the membrane. Intrinsic proteins are called integral proteins which are partially penetrating throughout the thickness.
According to function membrane proteins can be divided into three groups- structural protein (backbone of the membrane), carrier proteins (transport molecules), enzyme or receptors protein (catalytic function).
Functions of cell membrane are –
1. It provides proper shape of the cells of animal .
2. It also acts as only covering of cell boundaries.
3. Cell membrane can separates protoplasm from the extra cellular fluid so that it can maintain the individual identity of the cells and protects the cytoplasmic organelles.
4. In an animal tissue ,the cell membrane forms linkages or places of close contact between the adjacent cells which may help in structural and functional integrity.
5. Cell membrane also have the capacity to respond to the stimulus of the living cells.
6. Some of the biomolecules much as chemical , molecular receptors are sensitive to sensitive to the hormones, neurotransmitters and regulate the function of target cell.
7. It also acts as medium of transport (semipermeable) membrane and enhance the materials from protoplasm and extra cellular fluid.
8. Membrane transport are of two types , passive transport (it is governed by concentration gradient), active transport ( energy dependent process of transport against concentration gradient).