I. Overview: Structure and Core Functions

1. Structural Characteristics

• Thickness and Composition: The plasma membrane is approximately 7.5 nm thick. It is fundamentally composed of a lipid bilayer, which houses associated integral and peripheral proteins.
• Leaflets: The bilayer consists of two distinct leaflets:
  • The inner leaflet faces the cytoplasm.
  • The outer leaflet faces the extracellular environment.
• Microscopic Appearance: When viewed with a transmission electron microscope (TEM), the plasma membrane exhibits a trilaminar (unit membrane) structure.

1. Key Functions

The plasma membrane is a multifunctional structure critical to cell life:

1. Integrity: It envelops the cell, maintaining its structural and functional wholeness.
2. Semipermeability: It acts as a selective barrier between the cytoplasm and the external environment.
3. Recognition: It enables the cell to recognize macromolecules and other cells, and to be recognized in turn.
4. Signal Transduction: It participates in converting extracellular signals into intracellular events.
5. Interaction Control: It helps control interactions between adjacent cells.
6. Electrical Potential: It maintains an electrical potential difference across the membrane.