Cell Membrane | Fluid Mosaic Mod | NEET BIOLOGY

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CELL : STRUCTURE AND FUNCTIONS

Cell Membrane | Fluid Mosaic Model 

Cell Membrane

• The term was originally used by Nageli and Cramer (1855) for the membranous covering of the protoplast. 
• The same was named plasmalemma by Plowe (1931). 
• Plasmalemma or plasma membrane was discovered by Schwann (1838). Membranes also occur inside the cytoplasm of eucaryotic cells as covering of several cell organelles like nucleus, mitochondria, plastids, lysosomes, Golgi bodies, peroxisomes, etc. 
• They line endoplasmic reticulum, cover thylakoids in plastids or form cristae inside mitochondria. 
• Vacuoles are separated from cytoplasm by a membrane called tonoplast. 
• All membranes, whether external or internal are now called cell membranes or biomembranes. 
• They are quasifluid, elastic, pliable and film - like thin partitions over and inside cytoplasm. 
• Average thickness is 75 Å ( 50–100 Å ). 
• Biomembranes are selectively permeable for solutes but semipermeable for water. 
• They are dynamic in nature.
• Any injured part of the membrane is repaired within no time.

Fluid - Mosaic Model

• It is the most recent model of a biomembrane proposed by Singer and Nicolson in 1972. 
• According to this model, the membrane does not have a uniform disposition of lipids and proteins but is instead a mosaic of the two. 
• Further, the membrane is not solid but is quasifluid. 
• The quasifluid nature of the biomembranes is shown by their properties of quick repair, dynamic nature, ability to fuse, expand and contract, grow during cell growth and cell division, secretion, endocytosis and formation of intercellular junctions .
• Fluid - mosaic model postulates that the lipid molecules are present in a viscous bilayer as in lamellar model.
• Protein molecules occur at places both inside and on the outer side of lipid bilayer.

• protein icebergs in a sea of lipids.
• The internal proteins are called intrinsic or integral proteins while the external ones are known as extrinsic or peripheral proteins.
• The integral or intrinsic proteins account for 70 % of the total membrane proteins.
• They cannot be extracted from the membrane without disrupting the latter ( e.g. , with detergents ).
• The integral proteins pass into the lipid bilayer to different depths and establish hydrophobic bonds with lipid molecules.
• Some of the integral proteins run throughout the lipid bilayer.
• They are called tunnel proteins or transmembrane proteins.
• Transmembraneproteins may extend beyond the two surfaces as a single helix ( e.g . glycophorins ).
• The tunnel proteins individually or in a group form channels for the passage of water and water soluble substances. 
• The channels, however, possess selective properties for passage of different ions and other polar substances. 
• The proteins are held in their position by both polar (to hydrophilic heads of lipids) and nonpolar (to hydrophobic tails of lipids) side chains.
• The extrinsic or peripheral proteins are located superficially on the two surfaces of the membrane, more so on the cytosolic face than on the external face ( e.g. spectrin ). 
• The extrinsic proteins are attached covalently to phospholipid head or noncovalently to transmembrane proteins. 
• They can be separated with mild treat ment. 
• The proteins provide the structural and functional specific ity to the membranes. 
• Further, since the lipid bilayer is quasifluid, the membrane proteins may shift laterally and thence provide flex ibility and dynamism to the membrane. 
• Many membrane proteins function as enzymes . Some of them behave as permeases for allowing facilitated diffusion.
• A few proteins act as carriers because they actively transport different substances across the membrane. 
• Depending upon their role in active transport, carrier can be uniporters, symporters and antiporters. 
• Certain other proteins function as receptors for hormones, recognition centres and antigens. 
• Some lipids and extrinsic proteins present on the outer side possess small carbohyrate mol ecules to form glycolipids and glycoproteins. 
• They constitute glycocalyx or cell coat. 
• Conjugated oligosaccharides function as recognition centres, sites of attachment, anti gens, etc. 
• Oligosaccharides also provide negative charge to the outer surface. 
• Some workers propose the attachment of microfilaments to the membrane for stabilising the protein particles against lateral movement (Heslop - Harrison and Linskens , 1984 ).

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