diffusion

• what is the definition of diffusion?

  diffusion is the spreading out of particles resulting in a net movement from an area of high concentration to an area of low concentration
• net just means overall
• why do cells need oxygen?

  for respiration which is carried out by mitochondria
• cells are surrounded by a high concentration of oxygen

  oxygen is transported in the bloodstream from the lungs
• what is oxygen used to generate?

  energy in respiration and this produces the waste carbon dioxide
• what is urea?

  a waste product produced inside cells
• how does urea diffuse?

  urea diffuses out of the cells into the blood plasma and is excreted by the kidneys
• what are the 3 factors which affect the rate of diffusion?

  • the difference in concentrations (concentration gradient)
  • the temperature
  • the surface area of the membrane
• how does the concentration gradient affect the rate of diffusion?

  the greater the concentration gradient, the faster diffusion takes place
• how does temperature affect the rate of diffusion?

  the higher the temperature, the greater the rate of diffusion
• why is there a greater rate of diffusion when there is high temperatures?

  because the particle have more kinetic energy and are moving faster
• how does the surface are of the membrane affect the rate of diffusion?

  the larger the surface area of the cell membrane, the greater the rate of diffusion
• a single-celled organism has a relatively large surface area to volume ratio, this allows sufficient transport of molecules into and out of the cell to meet the needs of the organism
• what happens to the surface area : volume ratio as organisms get larger

  the ratio falls sharply, this presents a huge problem for multicellular organisms (organisms with more than one cell)
• can cells on the surface get enough oxygen?

  they can simply get it by diffusion however not enough oxygen can diffuse into the cells in the centre of the organisms as they're too far away from the surface
• how do fish get oxygen?

  • the oxygen-rich water passes into the mouth
  • it then flows over gills, where the oxygen is transported into the bloodstream
• how are gills in fish adapted for exchanging materials?

  • the gills are covered in a very large number of fine filaments, this is where gases pass in and out of the blood
  • de-oxygenated blood passes into the filament
  • oxygen diffuses from the water into the blood
  • oxygenated blood returns to the body
• what are the 3 adaptions of filaments to increase the rate of diffusion

  • the filaments give the gills a massive surface area
  • the filaments also have a thin membrane to provide a short diffusion pathway
  • the filaments have an efficient blood supply to take the oxygenated blood away, this ensures that the concentration gradient is always high