1.3.1 Diffusion

• What is diffusion?

  Diffusion is the spreading out of particles from an area of high concentration to an area of lower concentration.
• Substance transported in and out of cells by diffusion:

  • Oxygen and carbon dioxide in gas exchange.
  • Urea from cells into the blood plasma for excretion in the kidney.
• Factors which affect diffusion rate:

  • Concentration gradient - bigger gradient = faster diffusion rate
  • Temperature - higher temperature = faster diffusion rate
  • Surface area - greater surface area = faster diffusion rate
• A single celled organism has a relatively large surface area to volume ratio which allows sufficient transport of molecules in and out of the cell to meet the needs of the organism.
• The effectiveness of an exchange surface is increased by:

  • Having a large surface area
  • A thin membrane to provide a short diffusion path
  • Having an efficient blood supply
  • Being ventilated
• In multicellular organisms, surfaces and organ systems are specialised for exchanging materials. This is to allow sufficient molecules to be transported in and out of cells for the organism's needs.
• How are the lungs specialised for exchange?

  1. The job of the lungs is to transfer oxygen to the blood and to remove waste carbon dioxide from it.
  2. To do this the lungs contain millions of little air sacs called alveoli where gas exchange takes place.
  3. The alveoli are specialised to maximise the diffusion of O2 and CO2. They have:

  • An enormous surface area (about 75 m^2 in humans).
  • A moist lining for dissolving gases.
  • Very thin walls.
  • A good blood supply.
• How is the small intestine specialised for exchange?

  1. The inside of the small intestine is covered in millions and millions of tiny little projections called villi.
  2. They increase the surface area in a big way so that digested food is absorbed much more quickly into the blood.
  3. Notice they have a single layer of surface cells and a very good blood supply to assist quick absorption.
• How are leaves specialised for exchange?

  1. The underneath of the leaf is an exchange surface for photosynthesis.
  2. It is covered in stomata which carbon dioxide diffuses in through.
  3. Oxygen and water vapour also diffuse out through the stomata.
  4. Guard cells allow the stomata to open and close.
  5. The flattened shape of the leaf increases the surface area for more effective exchange.
  6. The walls of the cells inside the leaf form another exchange surface.
  7. The air spaces inside the leaf increase the area of this surface so carbon dioxide can enter the cells easier.
• How are fish gills specialised for exchange?

  1. Water enters the fish through its mouth and exits through the gills, during which diffusion occurs.
  2. Each gill is made of gill filaments, which give a big surface area.
  3. The gill filaments are covered in many tiny structures called lamellae, which increase the surface area.
  4. The lamellae have lots of blood capillaries for fast diffusion and a thin surface layer of cells to minimise diffusion distance.
  5. The concentration of oxygen in the water is higher than in the blood so lots of oxygen diffuses from the water into the blood.