Cell Transport

• Diffusion


  The movement of particles from an area of high concentration to an area of low concentration. Particles move down a concentration gradient.
• Why can we smell things which are far away?
• Diffusion


  • Occurs in liquids and gases where molecules are constantly moving and bumping into each other, allowing them to spread out
  • Does not occur in solids where particles are tightly packed
• As temperature increases


  Kinetic energy of particles increases, so they move around faster and diffusion is more rapid
• Concentration gradient


  The difference in concentration between two areas
• The larger the concentration gradient, the higher the rate of diffusion
• Where diffusion occurs in living organisms

  • Animal cells
  • Plant cells
  • Unicellular organisms
• Substances that diffuse in and out of an animal cell

  • Glucose (from food)
  • Oxygen (from lungs)
  • Carbon dioxide (excreted via lungs)
  • Urea from liver cells (excreted via kidneys)
• Words that describe diffusion

  • Occurs in Solids
  • Movement of molecules from an area of lower to higher concentration
  • Passive process (doesn't require extra input of energy)
  • Movement is Random
  • Occurs in Fluids
  • Active process (requires extra input of energy)
  • Movement of molecules from an area of higher to lower concentration
• Diffusion


  The movement of particles from an area of high concentration to an area of low concentration
• As temperature increases


  Particles have higher kinetic energy
• The higher the concentration gradient

  The higher the rate of diffusion
• Every organism, whatever its size, needs to exchange materials with the environment
• Surface area to volume ratio (SA:V ratio)


  How easy the exchange of substances is, depends on the organism's surface area to volume ratio
• An ant has a much larger surface area to volume ratio than an elephant
• As the size of an organism gets bigger


  The surface area to volume ratio (SA:V) gets smaller
• Large organisms, like humans, cannot absorb enough oxygen through our skin surface, so we need efficient gas exchange surfaces such as lungs
• Features of a good gas exchange system

  • Folded shape - large surface area
  • Thin - to provide a short diffusion pathway
  • Efficient blood supply - to maintain a high concentration gradient
  • Ventilation - ensuring there is always lots of oxygen to maintain a high concentration gradient
• A good blood supply in the lungs and small intestine

  Maintains a high concentration gradient, which increases the rate of diffusion
• Features of a good gas exchange system

  • Non-flat shape - increases surface area
  • Thin - reduces diffusion distance
  • Good blood supply - maintains high concentration gradient
  • Ventilation - maintains high concentration gradient
• Soil has a lower concentration of mineral ions than root hair cells
• Root hair cells need to use active transport to move mineral ions from an area of lower to higher concentration
• Active transport


  Using energy to move molecules against a concentration gradient with the help of a transport protein
• Organisms use active transport when molecules need to move from low to high concentration