CELL MEMBRANE

Cell is enveloped by a thin membrane called cell membrane or plasma membrane which allows only selective passage of substances. Membranes are made up of lipids, proteins, and small amounts of carbohydrates. Different membranes have different ratios of protein and lipid. Proteins in the cell membrane are of two types: peripheral proteins and integral membrane proteins. Peripheral proteins exist on the surface of membranes, and they are attached by ionic and polar heads of lipids. Peripheral proteins can be easily removed from the membrane. Integral membrane proteins span across the membrane from one side to another and are attached by hydrophobic bonds. Some of the integral proteins are involved in the transport mechanism. The lipid elements of membrane are composed of phospholipids, glycolipids, and cholesterol. Both polar and non-polar groups are present in the phospholipid molecules. They form a bilayer. The lipid bilayer model was proposed by Davson and Danielli in 1935. According to this model, the membranes are made up of a lipid bilayer holding globular proteins on its either surface by ionic and hydrogen bonds. Structure of the membranes was described as fl uid–mosaic model by Singer and Nicholson in 1972. According to this model, the phospholipids are arranged in a bilayer with hydrophobic ends (fatty acid tails) oriented towards the centre of the bilayer while their hydrophilic ends are pointed outside. The cholesterol content of the membrane maintains fl uidity. The structure of a cell membrane showing that it is composed mainly of a lipid bilayer of phospholipid molecules is as shown in