Transport in Cells

• Diffusion


  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 across a partially-permeable membrane
• Substances that move by diffusion in animal cells

  • Oxygen
  • Carbon dioxide
  • Urea
• Factors affecting rate of diffusion

  • Difference in concentrations (concentration gradient)
  • Temperature
  • Surface area of the membrane
• Concentration gradient


  The steeper the concentration gradient, the faster the rate of diffusion
• Temperature


  As temperature increases so does the rate of diffusion. However, above the optimum temperature of the proteins in the membrane, the membrane may be disrupted
• Surface area of the membrane

  The larger the surface area, the more diffusion can take place at any one time, so the faster the rate of diffusion
• Single-celled organism


  • High surface area to volume ratio (SA:V)
• Why large SA:V is suitable for single-celled organism

  Diffusion across its membrane is usually sufficient for it to survive (i.e. no need for an exchange organ and transport system)
• An organism has a surface area of 50m2 and a volume of 20m3. Its surface area to volume ratio is 2.5:1
• Why multicellular organisms tend to need exchange surfaces and transport systems

  Low SA:V, therefore cannot rely on diffusion across cell membranes to meet their needs for survival
• Common features that aid the exchange of substances

  • Large surface area
  • Thin wall to provide a short diffusion path
• Further features that aid the exchange of substances in animals but NOT plants

  • Efficient blood supply
  • Ability to ventilate (to allow air to move in and out of the organism)
• Osmosis


  The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane
• Animal cell loses a lot of water


  It crenates (shrivels) and cellular reactions are slowed or stopped
• Animal cell gains a lot of water

  It can lyse (burst) and die
• Plant cell loses a lot of water


  It becomes plasmolysed (cell membrane shrinks away from the cell wall)
• Plant cell gains a lot of water

  It becomes turgid (cell membrane pushes up against the cell wall)
• Active transport


  The movement of substances from a low concentration to a high concentration (against a concentration gradient)
• Active transport


  • Requires a transport protein (found in the cell membrane)
  • Requires energy
• Where active transport happens within a cell

  The mitochondria
• Examples of active transport


  • Plant root hair cells absorbing mineral ions from the soil
  • Glucose absorption from the small intestine into the blood