Structure of the Cell-Surface Membrane

• Phospholipids form a bilayer.
• Phospholipid’s hydrophilic heads…


  point to the outside of the cell-surface membrane, attracted by the water on both sides.
• Phospholipid’s hydrophobic tails…


  Point into the centre of the cell-surface membrane, repelled by the water on both sides.
• Lipid-soluble material moves through the membrane via the phospholipid portion.
• Functions of phospholipids in the membrane

  1. Allow lipid-soluble substances to enter and leave the cell
  2. Prevent water-soluble substances entering and leaving the cell
  3. Make the membrane flexible and self-sealing
• Two ways proteins are embedded in the phospholipid bilayer:

  1. Occur on the surface- either providing mechanical support, or acting as cell receptors (alongside glycolipids) for molecules such as hormones.
  2. Span it completely- protein channels and 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 (like glucose and amino acids), 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 transporting water-soluble substances across the membrane
  3. Allow active transport (through carrier proteins)
  4. Form cell-surface receptors for identifying cells
  5. Help cells adhere together
  6. Act as receptors, for example for hormones
• Are cholesterol molecules hydrophobic or hydrophilic?


  Hydrophobic. They play an important role in preventing loss of water and dissolved ions from the cell.
• What do cholesterol molecules do to the fatty acid tails of phospholipid molecules?

  Pull them together, limiting their movement and that of other molecules but without making the membrane too rigid as a whole.
• What are the functions of cholesterol molecules?

  • Reduce lateral movement of other molecules including phospholipids
  • Make the membrane less fluid at high temperatures
  • Prevent leakage of water and dissolved ions from inside the cell
• What are glycolipids made up of?

  A carbohydrate covalently bonded with a lipid
• What does the carbohydrate portion of a glycolipid do?

  Extends into the watery environment outside the cell and acts as a cell-surface receptor for specific chemicals.
• What are the functions of glycolipids?

  • Act as recognition sites
  • Help maintain the stability of the membrane
  • Help cells to attach to one another and so form tissues
• What are glycoproteins?


  Carbohydrate chains attached to extrinsic proteins on the outer surface of the cell membrane.
• Glycoproteins also act as cell surface receptors. They are specifically for what?

  Hormones and neurotransmitters
• What are the functions of glycoproteins?

  • Act as recognition sites
  • Help cells to attach to one another and so form tissues
  • Allows cells to recognise one another, for example lymphocytes can recognise an organism’s own cells.
• What is the model of the cell surface membrane called?

  Fluid mosaic model
• Why is the membrane known as the fluid-mosaic model?

  1. Fluid- individual phospholipid molecules can more relative to one another, so the membrane is flexible and constantly changing shape.
  2. Mosaic- the proteins are embedded in the phospholipid bilayer vary in shape, size and pattern.