Semi-Permeable Membrane

Definition

• A semi-permeable membrane, also known as a selectively permeable membrane, allows certain molecules or ions to pass through while blocking others.

Example: Phospholipid Bilayer

• Structure:
  • Composed of phospholipids with a phosphate head and two fatty acid tails.
  • Arranged in a double layer (bilayer).
  • Hydrophilic phosphate heads are exposed to water inside and outside the cell.
  • Hydrophobic tails are hidden on the inside of the membrane.

Molecule Transport

Diffusion

• Small, non-charged molecules (e.g., oxygen, carbon dioxide) can cross the membrane freely without energy.
• Molecules move down their concentration gradient (from high to low concentration).
• Process:
  • Movement occurs by slipping between phospholipid heads and through hydrophobic tails.

Assisted Transport

• Ions and polar molecules require assistance to cross the membrane due to difficulty.
• Carrier Proteins:
  • These proteins, often termed as "pumps," help transport these molecules.
  • Utilize energy to move molecules from low to high concentration (against the concentration gradient).

Key Concepts

• Selectivity: Ability of the membrane to allow certain substances to pass while blocking others.
• Concentration Gradient: The difference in concentration across the membrane which drives diffusion.