• 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
• • Channel?(pore) proteins
  • Carrier proteins?change shape?to transport a substance across the membrane e.g. protein pumps and electron carriersCarrier proteinsThere are two types:
  • 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