Diffusion

• Molecules that can diffuse in or out of cells:

  • Amino acids.
  • Water.
  • Glucose.
• Factors that affect the rate of diffusion:

  • Concentration gradient.
  • Temperature.
  • Surface area.
  • Distance to diffuse across.
• Why does higher temperature increase rate of diffusion:

  Increased temerparature would give the particles more kinetic energy which would make them move faster.
• Examples of exchange surfaces in humans:

  • Alevoli - allow oxygen and carbon dioxide to diffuse in and out of the body / blood stream.
  • Villi (small intestine) - allow nutrients such as glucose, amino acids, minereal ions etc to be absorved from small intestine to into the bloodstream.
• Features exchange surfaces have in common:

  • Large surface area - lots more molecules can diffuse at the same time.
  • Surfaces are very thin - substnaces only have to diffuse across a shorter distance.
  • Surfaces are permable - substances are able to pass through the membrane.
  • Good blood supply - maintains a strong concentration gradient as blood is constantly replaced.
• Features of alveoli, that make them good exchange surfaces:

  • Lots of alveoli - lots of molecules can diffuse across at the same time.
  • Surfaces are 1 alevolar and 1 capilary thin - Molecules have shorter diffusion pathway.
  • Surfaces are permable - oxygen and carbon dioxide can diffuse across.
  • Good air supply - Maintains a strong concentration gradient as the air in the alevoli is constantly replaced with oxygen rich air from the atmosphere.