Diffusion

• Diffusion is the movement of particles rom an area of high concentration to an area of low concentration.
• Diffusion doesn't occur in solids because the particles are unable to move.
• Particles move down the concentration gradient during diffusion.
• No energy is required for diffusion so it is a passive process.
• Examples of diffusion in humans include nutrients in the small intestine which diffuse into the blood in capillaries through the villi, oxygen diffusing from the air in the alveoli into the blood in capillaries and urea diffusing out of cells into the blood for excretion (by the kidney).
• In fish, oxygen from the water diffuses into the gills and carbon dioxide diffuses out.
• In plants, carbon dioxide used for photosynthesis diffuses into leaves through the stomata and oxygen diffuses out of the stomata.
• Alveoli are adapted for diffusion. They are surrounded by many capillaries to ensure there is always a gradient for gases to diffuse across. Alveoli have a large surface area so gases can diffuse as well as walls that are one cell thick. They also have moist linings so gases can dissolve in fluids more easily.
• The small intestine is adapted for diffusion as all inside linings contain villi and microvilli to increase surface area and allow for more diffusion. They are one cell thick and have a good blood supply to further help the process.
• Leaves contain many holes on the underside called stomata which create an entry and exit point for gases to diffuse. They are also flat to increase surface area and contain air spaces to allow easier diffusion. Spongey cells also increase the surface area of individual cells in a leaf.
• Each gill in a fish is made of lots of thin plates (filaments) which are covered in lamellae. Each lamellae is thin and has lots of blood capillaries so diffusion is easier. Blood also flows in the opposite direction to water.
• Difference in concentration affects the rate of diffusion because a steeper concentration gradient means a faster rate of diffusion.
• Temperature affects the rate of diffusion because higher temperatures leads to a faster rate of diffusion.
• Surface area affects diffusion because the larger the surface area, the faster the rate of diffusion.
• Smaller objects have a better surface area to volume ratio.
• Substances may move into and out of cells across the cell membranes via diffusion.
• Diffusion is the spreading out of the particles of any substance in solution, or particles of a gas, resulting in a net movement from an area of higher concentration to an area of lower concentration.
• Some of the substances transported in and out of cells by diffusion are oxygen and carbon dioxide in gas exchange, and of the waste product urea from cells into the blood plasma for excretion in the kidney.
• Factors which affect the rate of diffusion are:

  • the difference in concentrations (concentration gradient)

  • the temperature

  • the surface area of the membrane
• 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.
• In multicellular organisms, surfaces and organ systems are specialised for exchanging materials. This is to allow sufficient molecules to be transported into and out of cells for the organism’s needs. The effectiveness of an exchange surface is increased by:

  • having a large surface area

  • a membrane that is thin, to provide a short diffusion path

  • (in animals) having an efficient blood supply

  • (in animals, for gaseous exchange) being ventilated