Photosynthesis

• Leaf Structure

  
• Diffusion


  The random movement of substance from an area of high concentration (where there is lots of the substance) to an area of low concentration (where there is not much of the substance)
• Substances that move through diffusion

  • Oxygen
  • Carbon Dioxide
  • Water
  • Nutrients
• Factors affecting the rate of diffusion

  • Temperature
  • Surface Area
  • Concentration Gradient
• Higher temperatures


  Increase the kinetic energy of the molecules, making them move faster, therefore increasing the rate of diffusion
• Greater surface area


  Faster the rate of diffusion as there is more area over which diffusion can occur
• Greater concentration gradient


  Faster the diffusion
• The rate at which substances diffuse into and out of cells depends on several factors, most importantly the surface area of the cell and the volume of the cell
• As a cell grows larger (increases in volume)


  Its surface area to volume ratio halves
• Diffusion will be too slow if the cell is too large, and it will die as there is not enough surface area to allow in the oxygen that the cell needs
• There is a limit on the size of single-celled organisms that continually require energy for respiration
• Most organisms are multicellular and therefore require a much bigger surface area for exchange
• Having many cells packed together in the body does increase the volume but does not increase the surface area
• The body has developed specialised gas exchange surfaces in the lungs to enable the diffusion process to occur effectively
• Cells have become specialised to ensure the adequate transfer of substances through the circulatory system