Cell Membrane Structure All membranes throughout the cell have the same structure e.g. Cell surface membrane, endoplasmic reticulum, golgi apparatus….

Membrane structure The cell membrane is composed of: a phospholipid bilayer: Hydrophilic heads face outwards into the surrounding solution Hydrophobic tails turn away from the solution towards the centre of the membrane, proteins, scattered throughout the membrane: intrinsic proteins, are found in the membrane, either in one layer or across the 2 layers (transmembrane proteins) Extrinsic proteins are attached to the periphery of the membrane (external to the phospholipid bilayer).

Cholesterol molecules lie between the phospholipid tails. Glycocalyx extends from the outer surface of the membrane and contains polysaccharides attached to lipids (glycolipid) or membrane proteins (glycoprotein). The structure is called THE FLUID MOSAIC MODEL: Fluid as phospholipids can move sideways throughout the membrane Mosaic because of the appearance of the different molecules in the structure.

Component Function Further info: Phospholipid bilayer Forms backbone of membrane Selectively permeable Cholesterol Increases stability at high temperature Restricts sideways movement of phospholipids, making them less fluid Maintains stability at low temperatures Acts as a wedge between adjacent phospholipids, preventing them sticking together, making them more fluid Proteins Stability & support Anchors phospholipids Enzymes e.g. in ileum Prevents them being lost in secretion Adhesion sites Holds adjacent cells together Transport Channel protein create hydrophilic channels allowing polar molecules to pass, open or gated Carriers carry specific ions/molecules across membrane, either because it has charged groups (glucose) or moves them against a concentration gradient. Cell recognition (see glycocalyx) Receptors Antigens Glycocalyx Cell-cell recognition Similar cells recognise each other and bind to form tissues. This identifies individual cell types so also act as antigens. Receptor sites Complementary in shape to specific molecules. Important in hormone action (eg insulin only recognised by liver & muscle cells) & passage of neurotransmitters between neurones. Stabilise the membrane Glycoproteins & glycolipids can form hydrogen bonds with H2O molecules outside the membrane.