diffusion

• Diffusion:

  • The net movement of particles from an area of high concentration to an area of low concentration.
  • No energy required as it's a passive process.
  • Down conc gradient.
  • Diffusion happens in both solutions and gases as the particles are free to move randomly.
  • Diffusion of water is called osmosis.
• Diffusion in humans


  • Nutrients in small intestine diffuse into capillaries through villi
  • O₂ diffuses from air in alveoli into blood in capillaries
  • CO₂ diffuses from blood in capillaries into air in alveoli
  • Urea diffuses from cells into blood plasma for excretion in kidneys
  • Some proteins and big carbohydrates from food are too large to diffuse across a membrane and into the bloodstream so they get digested into amino acids and glucose which are small enough to diffuse. 
• Diffusion in fish

  • Oxygen from water passing over gills diffuses into blood in gill filaments
  • CO₂ diffuses from blood in gill filaments to water
• Diffusion in plants

  • CO₂ used for photosynthesis diffuses into leaves through stomata
  • Oxygen produced during photosynthesis diffuses out of leaves through stomata
• Factors affecting rate of diffusion

  • Difference in concentration (steeper concentration gradient=increases rate)
  • Temperature (higher temperature=increases rate)
  • Surface area of membrane (larger surface area=increases rate)