Facilitated diffusion

• Facilitated diffusion - Diffusion involving the presence of protein carrier molecules to allow the passive movement of substances (normally large, polar or charged molecules) across plasma membranes

  • The proteins help to 'facilitate' the movement of the particles through the membrane
• Facilitated diffusion:

  This is still diffusion as particles are still moving down their concentration gradient, and it is still passive as no energy is involved

  • It is much faster than simple diffusion
• Carrier protein vs Channel protein:

  • Different proteins facilitate the diffusion of different large molecules
  • Different protein channels facilitate the diffusion of different charged particles
• Carrier protein vs Channel protein:

  • Carrier: Large molecules attach and then the protein changes shape releasing the molecule on the other side of the membrane
  • Channel: No changing shape, simple 'tunnel' which allows charged particles to cross the membrane
• Carrier protein - A protein completely spanning the phospholipid bilayer which bind to ions or molecules then change shape in order to move these molecules across the membrane

  
• Protein channels - Form water-filled hydrophilic channels across the membrane

  • Allow specific water-soluble ions to pass through
  • Selective - only opening in presence of a specific ion
  • The ions bind with the protein causing it to change shape in a way that closes it to one side of the membrane and opens it to the other side

  
• Facilitated diffusion:

  • Passive process - relies only on the kinetic energy of the diffusing molecules
  • Occurs down a concentration gradient, but occurs at specific points on the plasma membrane where there are special protein molecules - protein channels and carrier proteins
• Protein channels:

  • Allow specific water-soluble ions to pass through
  • The ions bind with the protein causing it to change shape in a way that closes it to one side of the membrane and opens it to the other side
• Carrier proteins:

  • When a molecule is specific to the protein that is present, it binds with the protein
  • This causes it to change shape in such a way that the molecule is released to the inside of the membrane