AS Biology - Cell Structure - Exchange Across Cell Membranes

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Diffusion & Passive movement

  • Diffusion is the net movement of particles (molecules/ions) from an area of higher concentration to an area of lower concentration
  • Molecules will diffuse both ways, but the net movement will be to the area of lower concentration.
  • This continues until particles are evenly distributed throughout the liquid or gas
  • The concentration gradient is the path from an area of higher concentration to an area of lower concentrations.
  • Particles diffuse down a concentration gradient
  • Diffusion is a passive process - no energy is needed for it to happen
  • Particles can diffuse across cell membranes as long as they can move freely through the membrane. E.G. oxygen and carbon dioxide molecules are small enough to pass through spaces between phospholipids

Rate of diffusion

The rate of diffusion can vary depending on the following:

  • The concentration gradient - the higher it is, the higher the rate of diffusion
  • The thickness of the exchange surface - the thinner it is (the shorter the distance the particles have to travel), the faster the rate of diffusion
  • The surface area - the larger it is (e.g. of the cell membrane), the faster the rate of diffusion


  • Is the diffusion of water molecules across a partially permeable membrane, from an area of higher water potential (i.e. higher concentration of water molecules), to an area of lower water potential
  • Water potential is the potential (liklihood) of water molecules to diffuse out of or into a solution
  • Pure water has the highest water potential. 
  • All solutions have a lower water potential that pure water

How cells are affected

  • Water moves in or out of a cell by osmosis
  • How much moves in or out depends on the water potential of the surrounding solution
  • Animal and plant cells behave differently in solutions

Animal cells:

  • Hypotonic solution: solution with a higher water potential than the cell.
    -Net movement of water molecules is into the cell and causes the cell to burst
  • Isotonic solution: solution with the same water potential as the cell
    -Water molecules pass into and out of the cell in equal amounts
    -The cell stays…


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