General

• Diffusion

  The net movement of particles from an area of higher concentration to an area of lower concentration
• Factors affecting rate of diffusion

  • Concentration gradient - larger gradient, faster diffusion
  • Temperature - higher temperature, faster diffusion
  • Surface area - larger surface area, faster diffusion
• Substances transported by diffusion

  • Lungs: oxygen diffuses into the blood from the lungs and carbon dioxide diffuses into the lungs from the blood
  • Kidney: urea diffuses from cells into blood plasma so it can be excreted in urine
• Adaptations of single-celled organisms for diffusion

  • They have a large surface area to volume ratio - maximises the rate of diffusion of molecules to meet the organism's needs
• Factors increasing effectiveness of gas exchange surface

  • Large surface area
  • Thin membrane (short diffusion path)
  • Efficient blood supply (animals)
  • Ventilation (animals)
• Osmosis


  The movement of water from a dilute solution to a concentrated solution through a partially permeable membrane
• Isotonic solution


  The concentrations of the external and internal (inside cell) solutions are the same
• Hypertonic solution

  The concentration of the external solution is higher than that of the internal solution (inside cell)
• Hypotonic solution

  The concentration of the external solution is lower than that of internal solution (inside cell)
• Animal cell in very hypotonic solution

  Water moves into the cell, causing it to burst
• Animal cell in very hypertonic solution

  Water moves out of the cell, causing it to shrivel up
• Turgor pressure

  Water moves in by osmosis, causing the vacuole to swell and the cytoplasm to press against the cell wall
• Plant cell in very hypertonic solution


  Water moves out of the cell by osmosis and the vacuole and cytoplasm decrease in size. The cell membrane may pull away from the cell wall, causing the cell to become plasmolysed
• Active transport


  The movement of molecules from a more dilute solution to a more concentrated solution against a concentration gradient, using energy from respiration
• Use of active transport by plant root hair cells


  • Root hair cells use active transport to take up mineral ions from a more dilute solution in soils. Ions such as magnesium and nitrates are required for healthy growth
• Use of active transport to absorb products of digestion


  • Active transport is used to transport glucose from a lower concentration in the gut to a higher concentration in the blood. Glucose is then transported to the tissues where it can be used in respiration