AQA A Level Biology

Revision Notes

2.4.2 Components of Cell Surface Membranes

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)

Exam Tip

Make sure you can draw and label all the above structures on a diagram of the fluid mosaic model of cell membranes.

You can use an annotated diagram to state the functions of the above structures.


2. and 3. Sketch of fluid mosaic model labelled, downloadable AS & A Level Biology revision notes


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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