Diffusion

• Simple diffusion: the net movement of particles from an area of high concentration to an area of low concentration down a concentration gradient
• Simple diffusion is a passivprocess (does not require energy)
• Facilitated diffusion: verisimilitude substances require help of proteins to cross the phospholipid bilayer (ions, large polar molecules)
• Channel proteins:

  • water filled pores that allow charged substances to diffuse through the cell membrane
  • most are gated (can open and close) to stop ions moving freely, allowing control
• Carrier proteins:

  • can change between two shapes
  • the binding site of the carrier protein is open to one side of the membrane first, then open to the other side when the carrier protein switches shape
  • the direction the molecules diffuse depends on their concentration on each side of the membrane
  • occurs down the concentration gradient
• Factors affecting rate of diffusion:

  • steepness of concentration gradient
  • temperature
  • surface area
  • size
  • polarity