Cell Surface Membrane

• Why is it called the Fluid-mosaic Model

  • Mosaic: proteins are scattered through the bilayer like tiles in a mosaic
  • Fluid: phospholipids are constantly moving

  
• The cell-surface membrane is a phospholipid bilayer
• What is meant by a phospholipid bilayer

  • Phospholipids are arranged into 2 layers:
  • One layer is non-polar with hydrophobic tails (fatty acids) pointing inwards
  • Second layer is polar with hydrophilic heads (consisting of glycerol and phosphate) pointing outwards

  
• The centre of the bilayer is hydrophobic

  This means small, non-polar, lipid soluble molecules can pass through the bilayer

  So large, polar, water soluble molecules must pass through transport proteins
• Some molecules must pass through transport proteins

  So, since molecules of some substances are unable to pass through it directly, passage into or out of the cell is controlled by proteins in the membrane
• Fluidity of fatty acids


  • The greater the proportion of unsaturated fatty acids, the more fluid it is because they are bent
  • The greater the proportion of saturated fatty acids, the less fluid as they’re linear and therefore more compact
• Cholesterol in the cell surface membrane

  • Cholesterol stabilises the phospholipid bilayer by binding to the hydrophobic tails, providing mechanical stability to the membrane by packing them closer together
  • This restricts movement of phospholipids, making the membrane less fluid and more rigid
• Cholesterol in the cell surface membrane

  • It helps maintain the shape of animal cells (as no cell wall)
  • especially for cells that aren’t supported by other cells
• Cholesterol in the cell surface membrane

  It has hydrophobic regions, so it’s able to create a further barrier to polar substances moving through the membrane
• The membrane being lipid soluble means that simple diffusion can take place through the phospholipid bilayer