Cell Membranes

Electron micrograph studies revealed that cell membranes are 7-8 nm. wide and have three layers. Cell surface membranes are 45% lipid, 45% protein and 10% carbohydrate. According to the fluid-mosaic model of cell membrane structure, individual protein molecules shift and move on a fluid bilayer of phospholipid. They vary in function and structure, but they all contribute to the mechanical strenght of membranes. Proteins can be:

• Intrinsic (widht of membrane): some may act as carrier molecules, and other as enzymes.


• Extrinsic (confined to outer or inner surface): many of them combine with carbohydrate groups to form glycoproteins that act as chemical receptors, together with glycolipids. Some proteins attach to the cytoskeleton to anchor the membrane.

Functions of cell surface membrane

• Control of the passage of materials in and out of the cell
  
• Recognition of other cells


• Receptor sites for hormones and neurotransmitters


• Transmission of nerve impusles


• Insulation of nerves

Functions of membranes inside cell

• Acting as a reaction surface
  
• Acting as an intracellular transport system
  
• Providing separate intracellular ccompartments, isolating different chemical reactions.