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Synonyms:
Plasma membrane (De Robertis, 1965), Plasma-lemma (J.Q. Plowe, 1931), Unit membrane (Rorbertson, 1959).
The term cell membrane was originally used by C. Nageli and C. Cramer (1855). Plasma Membrane of neuron (nerve cell) is called neurolemma while that of haemolysed RBC is called red cell ghost.
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The plasma membrane of muscle cell along with based lamina is called sarcolemma.
Definition:
Plasma membrane is a living, ultrathin, dynamic elastic semipermeable membrane that encloses the protoplasm of a cell.
Location:
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It is the outermost boundary of all living cells. But prokaryotes and plant cells generally have an additional cell wall outside the plasma membrane. In addition to Plasma membrane, eukaryotic cells contain intracellular membrane surrounding, the vacuole and organelles. The plasma membrane and the intracellular membranes together called as biological membranes of bio-membranes.
Chemical Composition:
Chemically plasma membrane is a molecular assembly of lipids (20- 40%), proteins (60-75%) and carbohydrates (1-5%). The carbohydrates found in the form of glycoproteins or glycolipids and restricted only to the outer surface of plasma membrane. The lipids and proteins are held together by non-covalent interactions.
The membrane lipids of plasma membrane are of 3 major types:
(a) Phospholipids,
(b) Glycolipids, and
(C) Sterols.
All of them are amphipathic or amphiatic molecules because they possess both hydrophilic (polar) and hydrophobic (non-polar) ends. Majority (80%) of the phospholipids are neutral (e.g. phosphatidylcholine, phosphatidylethalamine and spingomyelin) and the rest phospholipid s are acidic or negatively charged (e.g. phophatidylionositol, phosphotidylserine etc.). The glycolipids may be cerebroside or ganglioside. Sterols found in the membrane may be cholesterol (in animals), stigmasterol, β-sterol (in plants) and ergosterol (in microbes). All lipids are symbolically represented with a polar head and two fatty acid tails.
The Membrane proteins are two types: integral or intrinsic (~ 70%) and peripheral or extrinsic (-30%). Nearly all known integral proteins span the lipid bilayer, while peripheral proteins are superficially attached by electrostatic and hydrogen bond interactions. Membrane proteins have various roles-mechanical, transport, enzymatic etc.
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Carbohydrates are mainly branched or un-branched oligosaccharides present only on the outer face of plasma membrane. In many protists and animal cells they form a cell coat (= glycocalyx) on the outer face of plasma membrane which protect the underline plasma membrane.
Structural Models of Plasma Membrane:
1. Lipid bilayer Model (Gorter and Grendell, 1926):
The plasma membrane of erythrocyte is a continuous lipid bilayer structure.
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2. Sandwich model or ‘Protein-Lipid-Protein’ model (Danielli and Davson, 1935): According to this model plasma membrane is a trilamellar structure with a middle lipid bilayer sandwiched between two continuous layers of protein (Fig. 3.5).
3. Unit membrane model (Robertson, 1959):
This model is the interpretation of electron on microscopic image on myelin along the line of Danielli- Davson model.
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According to this model all biological membranes have the same basic structure:
(a) The average thickness is about 7.5 nm (75A).
(b) They have a characteristic trilamellar (3-layered) structure,
(c) The three layers include a central lipid bilayer (3.5 nm) sandwiched between 2 protein layers (each-7.5 nm) (Fig. 3.6).
4. Fluid mosaic model (Singer and Nicolson, 1972):
The authors described the model as “protein icebergs in a two dimensional lipid sea”.
This postulates:
(i) That the biological membranes are quasi-fluid (semi-fluid) structures in which both lipids and integral proteins are free to move laterally as well as within the bilayer (Fig. 3.7).
(ii) That the lipids and proteins are arranged in a mosaic manner.
(iii) The integral or intrinsic proteins are embedded in the lipid bilayer while the extrinsic or peripheral proteins are superficially attached on both surface of the membrane.
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(iv) The exoplasmic face (E-face) of the cell membrane often possesses carbohydrate chains or oligosaccharides. They are bound to both proteins and phospholipids producing glycoproteins and glycolipids respectively. The carbohydrate coat present on the E-face of plasma membrane constitute glycocalyx or cell coat. The oligosaccharides gives a negative charge to outer surface. They act as cell surface markers, receptors, blood grouping etc..
Evidences supporting Fluid-mosaic model:
(a) Branton (1968) conformed the mosaic nature of proteins by studying freeze-fracture electron microscopy of the plasma membrane that revealed randomly distributed pumps and depressions (Fig. 3.8).
(b) Frey and Edidin (1970) experimentally demonstrated the fluid nature of plasma membrane b) fusion of a mouse cell with a human cell to yield a hybrid cell called heterokaryon or cybrids. This cell fusion can be induced by agents called fusogen (e.g., Sendai virus, polyethyleneglycol, electric shock etc.) (Fig. 3.9).
