2.4.2 Components of Cell Surface Membranes

1. Proteins:

• Location:
  • Intrinsic proteins: Span the entire phospholipid bilayer.
  • Extrinsic proteins: Found on the surface of the bilayer.
• Functions:
  • Aid the movement of substances across the membrane:
  • Carrier proteins: Transport specific molecules via active transport or facilitated diffusion.
  • Channel proteins: Provide pathways for ions and polar molecules.
  • Provide mechanical support.
  • Act as receptors for hormones and other signalling molecules when combined with glycolipids or glycoproteins.

2. Cholesterol:

• Structure:
  • Embedded within the phospholipid bilayer.
  • Hydrophobic, which makes it impermeable to water-soluble substances.
• Functions:
  • Increases membrane rigidity, reducing lateral movement of phospholipids.
  • Prevents the leakage of water and dissolved ions, maintaining the cell's stability.

3. Glycolipids:

• Structure:
  • Consist of a carbohydrate chain covalently bonded to lipids.
  • Extend from the membrane surface into the extracellular space.
• Functions:
  • Act as cell-surface receptors for specific molecules.
  • Enable cells to adhere to one another, forming tissues.
  • Play a role in cell recognition, e.g., recognising pathogens.

4. Glycoproteins:

• Structure:
  • Composed of carbohydrate chains attached to extrinsic proteins.
• Functions:
  • Act as receptors for neurotransmitters, hormones, and signalling molecules.
  • Facilitate cell-cell recognition, important in immune responses.
  • Allow cells to adhere to form tissues.

Key Roles of Membrane Components:

1. Transport: Proteins and phospholipids regulate the movement of substances in and out of the cell.
2. Communication: Glycolipids and glycoproteins enable cell signalling and recognition.
3. Stability: Cholesterol maintains membrane structure and prevents excessive fluidity or rigidity.