Structures & Functions

• The cell surface membrane:

  1. Controls movement of substances into and out of the cell
  2. Is semi-permeable, and is a barrier to water-soluble substances, but allow passage of lipid-soluble substances
  3. Has a role in cell signalling, cell recognition, cell-to-cell adhesion
  4. Is a site for enzymes to catalyse reactions
• Fluid Mosaic Model:

  • Fluid - phospholipids and protein molecules are able to move about and diffuse sideways within its monolayer
  • Mosaic - proteins scattered within membranes
• Phospholipids:

  • 1 glycerol, 2 fatty acids, 1 phosphate (PO⁴-) group
  • has a polar hydrophilic head that forms H bonds with water and stabilises membrane
  • has non-polar hydrophobic tails that are insoluble and repel water, they stop water-soluble substances and allow passage to lipid-soluble, small substances, they also help maintain fluidity of membrane
  • can form micelles which are a circular monolayer of phospholipids
• Factors affecting membrane fluidity:

  1. Temperature - higher temperature, higher kinetic energy, more fluid
  2. Ratio of unsaturated to saturated fatty acids - more unsaturated FA, higher unsaturated:saturated ratio, more fluid, unsaturated FA has C=C which cause kinks, phospholipids are more loosely arranged
  3. Length of phospholipids tails - the longer the tails, the less fluid, as there is more surface area for interaction between tails
  4. Cholesterol
• Cholesterol:

  • small molecule with hydrophilic head and hydrophobic taol which allows it to fit between the phospholipid molecules
  • not found in prokaryotes
  • it regulates fluidity and stabilises the membrane, it also blocks the passage of very small ions through membrane
• Extrinsic/peripheral proteins: found in the inner or outer surface of the membrane

  Instrisic/integral proteins: extend into the hydrophobic core, and may be mobile or fixed, some span across the membrane (transmembrane protein)
• Transport proteins:

  • channel proteins - highly specific, the channel is water-filled, can be gated
  • carrier proteins - highly specific, conformational change occurs when it interacts with the ion/molecule, binding sites that alternatively open to one side of the membrane then the other
• Roles of membrane proteins:

  1. transport proteins
  2. enzymes
  3. receptor for cell signalling molecules
  4. anchoring cytoskeleton - maintaining cell shape
  5. cell-to-cell adhesion
• Glycolipids:

  • carbohydrates chains attached to phospholipids
  • interact with water to stabilise membrane structure - able to form H bonds with water molecules
  • cell-to-cell adhesion
  • cell recognition - glycolipids act as cell surface antigens/markers
• Glycoprotein:

  • carbohydrate chains attached to protein
  • same roles as glycolipids + 1
  • receptor for cell signalling molecules