Exchanging substances

• Oxygen and carbon dioxide are transferred between cells and the environment during gas exchange
• In humans, urea (a waste product produced from the breakdown of proteins) diffuses from cells into the blood plasma for removal from the body by the kidneys
• Organisms exchange substances with their environment
• Multicellular organisms need exchange surfaces
• In single-celled organisms, gases and dissolved substances can diffuse directly into (or out of) the cell across the cell membrane. It's because they 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. This means they need some sort of exchange surface for efficient diffusion. The exchange surface structures have to allow enough of the necessary substances to pass through
• Exchange surfaces are adapted to maximise effectiveness:

  • They have a thin membrane, so substances only have a short distance to diffuse
  • They have a large surface area so lots of a substance can diffuse at once.
  • Exchange surfaces in animals have lots of blood vessels, to get stuff into and out of the blood quickly
  • Gas exchange surfaces in animals are often ventilated too - air moves in and out
• Gas exchange happens in the lungs
• The job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioxide from it.

  To do this the lungs contain millions of little air sacs called alveoli where gas exchange takes place
• The alveoli are specialised to maximise the diffusion of oxygen and carbon dioxide. They have:

  • An enormous surface area
  • A moist lining for dissolving gases
  • very thin walls
  • a good blood supply
• The villi provide a really big surface area
• The inside of the small intestine is covered in millions and millions of these tiny little projections called villi.

  They increase the surface area in a big way so that digested food is absorbed much more quickly into the blood
• Villi have a single layer of surface cells and a very good blood supply to assist quick absorption