2.4.3 The Cell Surface Membrane

Key Features of the Cell Surface Membrane:

1. Fluid Mosaic Model:

• The membrane is described as a fluid mosaic structure:
• Fluid: The phospholipids can move laterally, giving the membrane flexibility.
• Mosaic: Proteins of various shapes and sizes are embedded throughout the phospholipid bilayer.

2. Phospholipid Bilayer:

• Composed of phospholipids arranged in two layers:
• Hydrophilic heads face outward, interacting with water.
• Hydrophobic tails point inward, creating a barrier to water-soluble substances.
• Functions:
• Provides a barrier to polar molecules and ions.
• Allows lipid-soluble molecules to pass through.

3. Embedded Proteins:

• Proteins are scattered within the bilayer and perform various roles:
• Intrinsic proteins: Span the membrane and include carrier and channel proteins.
• Extrinsic proteins: Found on the surface and act as receptors or support structures.
• Functions:
• Facilitate transport (e.g., active transport, facilitated diffusion).
• Act as receptors for hormones and neurotransmitters.
• Provide mechanical support.

4. Cholesterol:

• Found within the bilayer, particularly in animal cells.
• Functions:
• Stabilises the membrane by reducing fluidity.
• Prevents the leakage of water and ions.

5. Glycolipids and Glycoproteins:

• Glycolipids: Carbohydrate chains attached to lipids.
• Glycoproteins: Carbohydrate chains attached to proteins.
• Functions:
• Act as cell-surface receptors for hormones, neurotransmitters, and antigens.
• Facilitate cell-cell recognition and adhesion.

Functions of the Cell Surface Membrane:

1. Selective Permeability:

• Controls the entry and exit of substances:
• Small, non-polar molecules like oxygen diffuse freely.
• Ions and large polar molecules require transport proteins.

2. Communication:

• Contains receptors for cell signalling molecules, enabling communication between cells.

3. Transport:

• Supports various transport mechanisms:
• Diffusion: Movement of substances from high to low concentration.
• Facilitated diffusion: Transport via protein channels.
• Active transport: Movement against the concentration gradient using ATP.

4. Structural Integrity:

• Maintains the cell's shape and provides protection.