Exchange substances

• Single-celled organisms


  Gases and dissolved substances can diffuse directly into (or out of) the cell across the cell membrane
• Single-celled organisms


  • Have a large surface area compared to their volume, so enough substances can be exchanged across the membrane to supply the volume of the cell
• Multicellular organisms


  Have a smaller surface area compared to their volume, not enough substances can diffuse from their outside surface to supply their entire volume
• Multicellular organisms


  • Need some sort of exchange surface for efficient diffusion
• Exchange surfaces


  • Have to allow enough of the necessary substances to pass through
• Exchange surfaces


  • Are adapted to maximise effectiveness
• Adaptations of exchange surfaces

  • Thin membrane, so substances only have a short distance to diffuse
  • Large surface area so lots of a substance can diffuse at once
  • Lots of blood vessels, to get stuff into and out of the blood quickly
  • Often ventilated (e.g. alveoli)
• Air moves in and out
• Large surface area is a key way that organisms' exchange surfaces are made more effective
• Bacterial cell


  Can be represented by a 2 μm x 2 μm x 1 μm block
• Calculate the cell's surface area to volume ratio

  1. 2 μm x 2 μm x 1 μm block

  2. Surface area calculation

  3. Volume calculation

  4. Ratio calculation
• The job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioxide
• Alveoli


  • Millions of tiny sacs where gas exchange takes place
  • Have an enormous surface area (about 75 m² in humans)
  • Have a moist lining for dissolving gases
  • Have very thin walls
  • Have a good blood supply
• Villi


  • Millions of tiny projections in the small intestine
  • Increase the surface area to assist quick absorption of digested food into the blood
  • Have a single layer of surface cells
  • Have a very good blood supply
• Structure of Leaves


  • Allows gases to diffuse in and out of cells
• Carbon dioxide diffusion in leaves

  1. Diffuses into air spaces within leaf

  2. Diffuses into photosynthesis
• Underside of leaf


  • Exchange surface
  • Covered in small holes called stomata
• Gas exchange through stomata

  1. Carbon dioxide diffuses in

  2. Oxygen and water vapour diffuse out
• Stomata


  • Openings on leaf surface controlled by guard cells
  • Close to prevent water loss
• Leaf shape


  • Flattened to increase exchange surface area
• Cell walls in leaf

  • Form another exchange surface
  • Spaces between cells increase surface area
• Water vapour diffusion in leaves

  1. Evaporates from cells

  2. Escapes by diffusion
• Gills


  • Gas exchange surface in fish
• Gas exchange in gills

  1. Water containing oxygen enters mouth

  2. Oxygen diffuses from water into blood

  3. Carbon dioxide diffuses from blood into water

  4. Water exits through gills
• Gill structure


  • Made of thin gill filaments
  • Covered in tiny gill lamellae
  • Contain blood capillaries
• Gill lamellae


  • Increase surface area for gas exchange
  • Have thin cell layer to minimise diffusion distance
• Water flow and blood flow in gills

  Flow in opposite directions to maintain concentration gradient
• Oxygen concentration in water is always higher than in blood