IB Biology SL

Revision Notes

1.3.2 Membrane Proteins

Membrane Proteins

  • The phospholipid bilayer carries out the main function of the plasma membrane – to control the movement of substances into and out of the cell
  • The other functions are carried out by proteins in the membrane
  • Plasma membranes are globular proteins
  • These proteins are grouped into two categories:
    • Integral – these are partially hydrophobic and therefore are embedded in the phospholipid bilayer (either in both layers or just one)
    • Peripheral – these are hydrophilic and so are temporarily attached to either the surface of integral proteins (inside or outside the cell) or connected to the plasma membrane via a hydrocarbon chain
  • The protein content of membranes can vary depending on the function. Membranes of the mitochondria and chloroplasts have the highest protein content with their many electron carriers

Membrane protein functions

  • Membrane proteins carry out many functions: transport, receptors, cell adhesion, cell-to-cell recognition and immobilized enzymes


  • Transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane
  • There are two types:
    • Channel (pore) proteins
    • Carrier proteins
      • Carrier proteins change shape to transport a substance across the membrane e.g. protein pumps and electron carriers
  • Each transport protein is specific to a particular ion or molecule
  • Transport proteins allow the cell to control which substances enter or leave


  • Receptors are for the binding of peptide hormones (e.g. insulin), neurotransmitters or antibodies
  • The binding generates a signal that triggers a series of reactions

Immobilized enzymes

  • Immobilized enzymes are integral proteins with the active site exposed on the surface of the membrane (can be inside or outside the cell)

Cell adhesion

  • Cell adhesion allows tight junctions to be formed between cells

Cell-to-cell recognition

  • Glycoproteins act as cell markers or antigens, for cell-to-cell recognition (eg. the ABO blood group antigens are glycolipids and glycoproteins that differ slightly in their carbohydrate chains)

1.3.2 Examples of the functions of membrane proteins 1, downloadable AS & A Level Biology revision notes1.3.2 Examples of the functions of membrane proteins 2, downloadable AS & A Level Biology revision notes

Examples of the functions of membrane proteins

Exam Tip

As you go through the Biology course you will learn specific examples of how membrane proteins are used so try to make the links, this will help you remember in the exams.

Author: Catherine

Cate has over 20 years’ experience teaching Biology to IGCSE, IB and A-level students in seven different countries across Asia, Europe, North America and the Middle East. This has given her a fine appreciation of different cultures, places and teaching methods. Cate has a keen interest in producing Biology revision resources that will help students engage with the subject.

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