PLASMA MEMBRANE

All cell are covered by a thin plasma membrane which separated the cell conten from the extra cellular environment. Eukaryotic cells have membrane bounded organelles. The plasma membrane and the organelle membrane have the same basic structure. In 1972, S.J. Singer and G.L. Nicolson proposed
the fluid mosaic model of cell membrane consisting of a bilayer of phospholipid with various protein molecule embedded and attached to it. The membrane is about 7 – 7,5 nm thick. It consists of phospholipid bilayer . The polar hydrophilic heads of phospholipid point outwards, attrached to the aqueos surrounding and form hydrogen bonds with water molecules. The non polar, hydrophobic hydrocarbon tails face inwards, and are attracted to one another by hydrophobic interaction and Van der Walls interaction. There are extrinsic protein on the outer and inner surface of the membrane. The intrinsic protein are embedded partially in the membrane and some penetrate through the membrane. The various membrane of a eukryotic cell are different because only certain protein are unique to each membrane. The protein part that contain hydrophobic amino acids interact with the hydrophobic fatty acids chains to expel the water. The hydrophilic  part of the protein is attracted to the aqueous surrounding . Some of the protein and phospholipid conjugated with short branched oligosaccharides to form glycoprotein and glycolipids on the outer surface. More glycoprotein are formed than glycolipids. These are important for cell recognition. Many cells are surrounded by a glycocalix formed by oligosaccharides extending out on the outer surface. Cholesterol molecules are found between phospholipid molecules. The cholesterol molecules stabilise the membrane structure and regulate membrane fluidity. The membrane is supported by intra cellular protein filaments at the inner surface which act as cytoskeleton.

MEMBRANE FLUIDITY

Cell membrane are dynamic, fluid structures. The fluidity of the membrane is affected by its composition.

A.      The pospholipid molecules and some protein molecules can move laterally or change places and thus give the membrane fluidity.

B.      The protein are scattered in the fluid phospholipid bilayer giving a mosaic appereance and have different structure and functions.

C.      Unsaturated fatty acids have double bond which form kinks. These prevent close packing of the phospholipid molecules, increase fluidity and promote movement of substances across the membrane.

D.      An increase of temperature increase the fluidity of the mebrane. At moderate warm temperature, the cholesterol molecules reduce the free movement of phospholipid molecule and make the membrane less fluid. At low temperature, cholesterol molecules prevent the close packing of phospholipid molecule and slow down solidification of the membrane.

FUNCTION of MEMBRANE

Surface membrane form a protective barrier between cell conten and external environment and determine the shape of the cell. Form membranous organelles for example mitochondria the site of aerobic respiration, choloroplast which carry out photosynthesis and endoplasmic reticulum. Membranes are selectively permeable and regulate movement of substance in and out of the cell. Cell surface receptor protein are involved in signal transduction by converting an extrcellular signal to an intracellular one. Glycoprotein with branched oligosaccharides act as antigens. Glycolipid are involved in cell cell recognition. Folding of cell membrane enable the cell to carry out phagocytosis and pinocytosis. This enables the cell to obtain nutrient or to engulf and destroy foreign paricles. Cell adhesion protein join cell together forming tissue which carries out spesific function.