2.5 - Membrane Permeability

Membrane Permeability and Cellular Boundaries

• Plasma Membrane Function: The plasma membrane is crucial for separating a cell from its surroundings and maintaining a stable internal environment.

• Selective Permeability

  • Mechanism: The membrane's selective permeability allows some substances to cross more easily than others, primarily due to its unique structure.

  • Role of Fatty Acid Tails: The hydrophobic fatty acid tails in the membrane core obstruct the passage of polar molecules.

  • Permeability to Molecules

    • Small Nonpolar Molecules: Molecules like CO2 and O2 can move freely across the membrane due to their nonpolar nature.

    • Large Polar and Charged Molecules: These cannot directly pass through the hydrophobic core of the membrane and require specific transport mechanisms.

    • Transport Proteins: Channel and transport proteins facilitate the movement of large polar molecules and ions across the membrane.

    • Small Polar Uncharged Molecules: These can pass through the membrane in small amounts.

    • Water Movement: Water molecules, though polar, can traverse the membrane through specialized channels known as aquaporins.

• Cell Wall in Plant Cells

  • Composition and Function: Unlike the plasma membrane, the cell wall is primarily made of cellulose—a polysaccharide—reinforced with other carbohydrates and proteins.

  • Structural Role: It provides structural support and acts as a boundary, also playing a role in controlling the permeability to certain substances.