Diffusion

• Substances may move into and out of cells across the cell membrane by diffusion
• Diffusion is the movement of particles of any substance in a solution from an area of high concentration to an area of low concentration
• Substances transported in and out of cells by diffusion

  • Oxygen and carbon dioxide in gas exchange
  • waste product urea from cells into the blood plasma for excretion in the kidney
• Factors that affect the rate of diffusion

  • Concentration gradient: the bigger the concentration gradient, the faster the rate of diffusion
  • Temperature: higher temperatures means faster rate of diffusion as the particles have more energy so move around faster
  • surface area of the membrane: the bigger the surface area of the membrane, the faster the rate of diffusion because more particles can pass through at once
• • Single celled organisms have a large surface area to volume ratio
  • this allows enough transport of molecules into and out of the cell to meet the needs of the organism
• • Multicellular organisms have a smaller surface area to volume ratio so not enough substances can diffuse from their outside surface to supply their entire volume
  • they need some sort of exchange surface for efficient diffusion. The exchange surface structures have to allow enough of necessary substances to pass through.
• Ways the small intestine is adapted for exchanging surfaces

  • walls are folded and have millions of projections called villi to give a massive surface area
  • villi have very thin walls
  • villi have a rich blood supply
• Ways alveoli in the lungs are adapted for gas exchange

  • thin walls
  • a rich blood supply
  • large surface area
• Ways plant leaves are adapted for gas exchange

  • openings in epidermal tissue called stomata let gases in and out of the leaf
  • air spaces inside leaf allow gases to diffuse in and out of cells
• Ways the structure of fish‘s gills help them to exchange gases with water

  • gills are made up of lots of gill filaments which are covered in lots of folds called lamellae
  • this gives the gills a large surface area, which increases the rate of diffusion between gases between the fish’s blood and water
• Fish gill lamellae adaptations

  • thin walls
  • rich blood supply
  • blood flows through them in the opposite direction to water flowing over them
• The effectiveness of an exchange surface is increased by

  • having a large surface area
  • a membrane that is thin, to provide a short diffusion path
  • (in animals) having an efficient blood supply
  • (in animals for gaseous exchange) being ventilated