Exchanging substances

• Gas Exchange


  Happens in the Lungs
• Job of the Lungs

  To take in oxygen and remove carbon dioxide from the blood
• Lungs


  • Contain alveoli where gas exchange takes place
• Blood


  • Carries oxygen
  • Carries carbon dioxide
• The alveoli are spongy to allow for a large surface area (about 75 square meters in humans)
• The alveoli have a very thin wall to allow for efficient gas exchange
• Villi


  Tiny projections that cover the inside of the small intestine
• Villi


  • Increase the surface area for absorption of digested food
  • Have a single layer of surface cells
  • Have a good blood supply to assist quick absorption
• Absorption of digested food

  Moves into the blood by diffusion and active transport
• The Structure of Leaves

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

  1. Diffuses into air spaces within the leaf

  2. Diffuses into the cells where photosynthesis happens
• Leaf underside


  • Exchange surface
  • Covered in stomata which carbon dioxide diffuses in through
• Oxygen and water vapour diffusion in leaves

  Diffuse out through the stomata
• Stomata


  Little holes on the underside of the leaf
• Guard cells


  • Control the size of the stomata
  • Close the stomata if the plant is losing water faster than it is being replaced by the roots
• Leaf shape


  • Flattened to increase the area of the exchange surface
• Cells inside the leaf

  • Form another exchange surface
  • Air spaces inside the leaf increase the area of this surface
• Water vapour diffusion in leaves

  1. Evaporates from the cells inside the leaf

  2. Escapes by diffusion because there's a lot of it inside the leaf and less of it in the air outside
• Gills


  • The gas exchange surface in fish
• Gas exchange in fish gills

  1. Water (containing oxygen) enters the fish through its mouth and passes out through the gills

  2. Oxygen diffuses from the water into the blood in the gills

  3. Carbon dioxide diffuses from the blood into the water
• Gill filaments


  • Thin plates that give a big surface area for exchange of gases
• Lamellae


  • Tiny structures covering the gill filaments that increase the surface area even more
  • Have lots of blood capillaries to speed up diffusion
  • Have a thin surface layer of cells to minimise the distance that the gases have to diffuse
• Water and blood flow in gills

  1. Blood flows through the lamellae in one direction

  2. Water flows over in the opposite direction

  3. This maintains a large concentration gradient between the water and the blood
• The concentration of oxygen in the water is always higher than that in the blood, so as much oxygen as possible diffuses from the water into the blood