Cell membrane

Cell membrane

1. Fluid Mosaic model of cell membranes

   • The "fluid" phospholipid bilayer has proteins floating in it which forms a "mosaic"
     • "Fluid" - phospholipids are free to move within the layer which allows flexibility
       • "Mosaic" - proteins are embedded in the bilayer varying in shape, size and position

2. Types of proteins

   • Intrinsic (integral)
     • Transmembrane proteins
       • Have amino acids with hydrophobic R-groups on their external surface which keep them in place
         • Examples: channel and carrier proteins, glycoproteins and glycolipids
   • Extrinsic (peripheral)
     • Present on one side of the bilayer
       • Have hydrophilic R-groups on surface which keep them in place
         • Can be present on either layer and some move between layers
           • Form receptor sites for hormones with complementary shape

3. Channel and carrier proteins

   • Channel proteins: form a hydrophilic channel that allows passive movement of polar molecules and ions down a concentration gradient (diffusion)
   • Carrier proteins: used in facilitated diffusion and active transport. The shape of the protein changes and moves the molecule across the membrane

4. Glycoproteins

   • Proteins with an attached carbohydrate
     • Used in cell adhesion to form tissues
       • Used in cell signalling as a receptor when it will initiate a chain of events when chemical binds to the receptor
         • Examples: Receptors for acetylcholine at nerve cell synapses, receptors for peptide (protein) hormones
           • Helps cell to stay together, stabilise the membrane because it can make hydrogen bonds, act as antigen

5. Glycolipids

   • Lipids attached to a carbohydrate
     • Called antigens or cell markers
       • These signal to the immune system that the cell is 'self'
         • Identify foreign material as 'non self'