Membrane Proteins (SL IB Biology)

• The phospholipid bilayer carries out the main function of the plasma membrane, providing a barrier to the movement of some substances into and out of the cell
• Additional functions are carried out by proteins in the membrane
• These proteins are grouped into two categories:
  • Integral
    • These are partially hydrophobic, i.e. they are amphipathic
    • They are embedded in the phospholipid bilayer
    • They can be embedded across both layers or just one layer
  • Peripheral
    • These are hydrophilic proteins
    • They are attached to either the surface of integral proteins, or to the plasma membrane via a hydrocarbon chain
    • They can be inside or outside the cell
• The protein content of membranes can vary depending on the function of the cell
  • E.g. 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

• Transport proteins allow ions and polar molecules to travel across the membrane
• There are two types: 
  • Channel proteins
    • These form holes, or pores, through which molecules can travel
  • 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

• 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 inside the cell

Immobilised enzymes

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

Cell adhesion

• Cell adhesion allows cells to attach to neighbouring cells within a tissue

Cell-to-cell recognition

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

Plasma membrane proteins diagram

Membrane proteins have multiple functions