1.3.2 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

• 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

• 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)

Examples of the functions of membrane proteins