diffusion

• Substances may move into and out of cells across the cell membranes via diffusion.
• what is 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
• What does a high surface area to volume (SA:V) ratio allow small organisms to do?


  It allows easier exchange of gases, nutrients, and waste.
• Why do larger organisms have a lower SA:V ratio?


  Because they have less surface area relative to their volume.
• Why do multicellular organisms need exchange surfaces?


  • To get enough oxygen and nutrients into cells
  • To remove waste efficiently
  • Simple diffusion is insufficient for larger sizes
• What are the adaptations of the small intestine for exchange surfaces?

  • Villi and microvilli increase surface area
  • Thin membrane for faster diffusion
  • Good blood supply for quick nutrient transport
• What adaptations do lungs have for effective gas exchange?

  • Alveoli increase surface area
  • Thin walls for efficient diffusion
  • Good blood supply
  • Ventilation maintains concentration gradients
• How do fish gills facilitate oxygen exchange?


  • Thin gill filaments
  • Large surface area for efficient oxygen transfer
• What adaptations do roots and leaves have for exchange surfaces in plants?

  • Roots: Root hairs increase surface area for water and mineral absorption
  • Leaves: Large, flat structure for gas exchange; stomata allow gas movement
• How is the effectiveness of exchange surfaces increased?


  • Large surface area for more molecule passage
  • Thin membrane for shorter diffusion distance
  • Efficient blood supply for quick transport
  • Ventilation to maintain concentration gradients
• What substance must diffuse into cells for aerobic respiration?

  Oxygen
• What substance must diffuse into photosynthesizing plant cells?

  Carbon dioxide
• Why must urea diffuse out of liver cells?

  It is a toxic waste product
• What must diffuse out of cells due to aerobic respiration?

  Carbon dioxide
• What drives the movement of substances by diffusion into and out of cells?

  • A concentration gradient
  • Difference in concentration between cell and environment
• What happens to the movement of a substance by diffusion if the concentration gradient changes?

  The movement direction will also change