2.2.7 Scientific Research into Cell Organelles

The Fluid Mosaic Model:

• Biological membranes are described by the fluid mosaic model due to:
  • Fluidity: Phospholipids can move laterally, making the membrane flexible and self-sealing.
  • Mosaic: Proteins are scattered throughout the phospholipid bilayer like tiles in a mosaic.
• Structure:
  • Phospholipid bilayer:
  • Hydrophilic heads face outward, interacting with water.
  • Hydrophobic tails point inward, avoiding water.
  • This arrangement:
  • Allows lipid-soluble molecules to pass through.
  • Prevents the movement of water-soluble molecules.

Components of the Membrane:

1. Proteins:

• Intrinsic proteins span the bilayer and include carrier proteins and channel proteins that facilitate the movement of molecules across the membrane.
• Extrinsic proteins are found on the surface and provide mechanical support or act as receptors in conjunction with glycolipids.
• Functions:
• Aid transport of substances.
• Provide mechanical support.
• Act as receptors for hormones and other signalling molecules.

2. Cholesterol:

• Embedded within the bilayer.
• Functions:
• Increases rigidity by reducing the lateral movement of phospholipids.
• Prevents the leakage of water and ions due to its hydrophobic nature.

3. Glycolipids:

• Made of carbohydrates bound to lipids.
• Extend from the surface of the cell membrane.
• Functions:
• Act as cell-surface receptors for specific molecules.
• Facilitate cell adhesion to form tissues.

4. Glycoproteins:

• Made of carbohydrates attached to extrinsic proteins.
• Functions:
• Act as receptors for signalling molecules like neurotransmitters.
• Enable cell recognition.
• Aid cell adhesion to form tissues.

Significance of the Membrane:

5. Selective Permeability:

• Controls the entry and exit of substances such as ions, nutrients, and waste.
• Maintains the internal environment of the cell or organelle.

6. Communication and Interaction:

• Contains receptors for hormones, neurotransmitters, and signalling molecules.
• Enables cells to interact and adhere to form tissues.

7. Flexibility:

• The fluid nature allows membranes to change shape, important for processes like endocytosis and exocytosis.