Diffusion

• Diffusion - Net movement of molecules or ions from an area of high concentration to an area of low concentration
• Diffusion is a passive process (does not require any energy) as the particles are moving down their concentration gradient
• Diffusion often happens across a partially permeable membrane

  • Molecules can diffuse simply through the membrane if they are not too large or polar eg Oxygen and Carbon dioxide
• Simple diffusion limitation:

  • Plasma membranes are not readily permeable to molecules
  • Only small, non-polar molecules can diffuse across them easily
  • Charged ions and polar molecules do not diffuse easily because of the hydrophobic nature of the fatty-acid tails of the phospholipids in the membrane