Exchanging Substances

• A single-celled organism has a relatively large surface area to volume ratio which allows sufficient 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 meaning that they need exchange surfaces for efficient diffusion so they can supply substances to their entire volume
• Exchange surfaces are adapted to diffuse by having a thin membrane so substances only have a short distance to diffuse
• Exchange surfaces are adapted to diffuse by having a large surface area so large amounts of substance can diffuse at once
• Exchange surfaces in animals are adapted to diffuse by having lots of blood vessels to transfer substances in and out of blood quickly
• Gas exchange surfaces in animals are adapted to diffuse by being ventilated so air can move in and out
• Lungs transfer oxygen to blood and remove waste carbon dioxide from blood in the alveoli
• The alveoli are specialised to maximise diffusion of oxygen and carbon dioxide by having

  • Having a large surface area
  • Moist lining for dissolving gasses
  • Very thin walls
  • A good blood supply
• The small intestine is covered in villi to absorb digested food into the blood
• The villi are effective by having

  • A single layer of surface cells
  • A very high blood supply
  • A high surface area
• The leaf's structure is adapted to allow carbon dioxide to diffuse into the air spaces so photosynthesis can take place and to release water vapour through the stomata
• The size of the stomata are controlled by guard cells which close the stomata if the plant is loosing too much water
• The flattened surface of the leaf increases the area of the gas exchange so it is more effective
• Gills are the gas exchange surface for fish
• Water enters the fish through its mouth and passes through the gills where oxygen diffuses from the water to the blood and carbon dioxide diffuses from the blood to the water
• Gills are efficient at gas exchange because

  • They have a large surface area
  • the lamellae have lots of blood capillaries
  • The gill filaments have a thin surface layer
  • Blood flow through that lamellae in one direction and water flows in the other creating a large concentration gradient