Components of Cell Surface Membranes (CIE AS Biology)

• The cell membranes of all organisms generally have a similar structure
• Cell membranes contain several different types of molecules:
  • Three types of lipid:
    • Phospholipids
    • Cholesterol
    • Glycolipids (also containing carbohydrates)

  • Two types of proteins:
    • Glycoproteins (also containing carbohydrates)
    • Other proteins (eg. transport proteins)

• Phospholipids:
  • Form a bilayer (two layers of phospholipid molecules)
  • Hydrophobic tails (fatty acid chains) point in towards the membrane interior
  • Hydrophilic heads (phosphate groups) point out towards the membrane surface
  • Individual phospholipid molecules can move around within their own monolayers by diffusion

• Cholesterol:
  • Cholesterol molecules also have hydrophobic tails and hydrophilic heads
  • Cholesterol fits between phospholipid molecules and orientated the same way (head out, tail in)
  • It is only found in eukaryotic membranes; it is absent in prokaryotes membranes

Cholesterol Structure Diagram

The structure of cholesterol shows that it has a hydrophilic region (the -OH group) and a hydrophobic region (a hydrocarbon chain). This gives the molecule a dual character that allows it to fit inside a phospholipid bilayer.

• Glycolipids:
  • These are lipids with carbohydrate chains attached
  • These carbohydrate chains project out into whatever fluid is surrounding the cell (they are found on the outer phospholipid monolayer)

• Glycoproteins:
  • These are proteins with carbohydrate chains attached
  • These carbohydrate chains also project out into whatever fluid is surrounding the cell (they are found on the outer phospholipid monolayer)

• Proteins:
  • The proteins embedded within the membrane are known as intrinsic proteins (or integral proteins)
  • They can be located in the inner or outer phospholipid monolayer
  • Most commonly, they span the entire membrane – these are known as transmembrane proteins
  • Transport proteins are an example of transmembrane proteins as they cross the whole membrane
  • Proteins can also be found on the inner or outer surface of the membrane, these are known as extrinsic proteins (or peripheral proteins)

Positions of Membrane Proteins Diagram

Membrane proteins (shown in light blue) can span the whole membrane (intrinsic) or just occupy one side of it (extrinsic).