4.1 structure of the cell-surface membrane

• phospholipids form a bilayer that is permeable to water and polar molecules
• the function of the phospholipids in the membrane are to:

  1. allow lipid-soluble substances to leave and enter
  2. prevent water-soluble substances from leaving and entering
  3. make the membrane flexible and self-sealing
• phospholipid:

  1. hydrophilic phosphate head
  2. hydrophobic hydrocarbon tails
• some proteins in the membrane occur on the surface to give mechanical support or act as cell receptors
• some proteins span the phospholipid bilayer and are protein channels or carrier proteins
• protein channels form water-filled tubes to allow water-soluble ions to diffuse across the membrane
• carrier proteins bind to ions or molecules then change shape in order to move these molecules across the membrane
• functions of proteins in the membrane:

  1. provide structural support
  2. act as channels
  3. carrier proteins
  4. act as receptors
  5. help cells adhere together
• cholesterol molecules in the membrane are very hydrophobic
• functions of cholesterol in the membrane:

  1. reduce lateral movement of other molecules
  2. make the membrane less fluid at high temperatures
  3. prevent leakage of water and dissolved ions
• glycolipids are made up of a carbohydrate covalently bonded with a lipid
• functions of glycolipids in the membrane:

  1. as recognition sites
  2. maintains the stability of the membrane
  3. help cells adhere together
• the function of glycoproteins in the membrane are to:

  1. recognition sites
  2. help cells adhere together
  3. allows cells to recognise each other
• most molecules do not freely diffuse across it because many are:

  1. not soluble in lipids
  2. too large
  3. the same charge as the charge on the protein channels
  4. polarised
• the way all molecules are combined in the membrane is known as the fluid-mosaic model
• fluid-mosaic model:

  1. fluid because the individual phospholipid molecules can move giving the membrane flexibility
  2. mosaic because the proteins that are embedded in the phospholipid bilayer vary in shape, size and pattern