Components of Cell Surface Membranes (AQA A Level Biology)

Phospholipids, Cholesterol, Glycolipids, Proteins & Glycoproteins

• 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

  • Fit between phospholipid molecules and orientated the same way (head out, tail in)

  • Are absent in prokaryotes membranes

• 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)