Transport systems in animals

• what is meant by the term 'surface area to volume ratio'?

  • how many times larger or smaller the volume is compared to the surface area
  • can either be expressed as a ratio, or as a number
• how does surface area affect the rate of diffusion?

  as surface area increases the rate of diffusion increases
• how does concentration differences affect the rate of diffusion?

  as the concentration differences increase, the rate of diffusion increases
• how does distance affect rate of diffusion?

  as the distance increases, the rate of diffusion decreases
• how does temperature affect rate of diffusion?

  as the temperature increases, the rate of diffusion increases
• what is Fick's law of diffusion?


  (Surface area * concentration gradient) / distance
• what feature can cells include to make their surface area to volume ratio more favourable for faster rate of diffusion?

  • specialised structures like cilia or root hair extension on root hair cell - which increase the surface area by folding the membrane
• why is diffusion sufficient for single celled organisms?

  because they have a large surface area to volume ratio so substances can diffuse rapidly and efficiently into the cell without a transport system - the diffusion distances are short enough
• why so multi-cellular organisms need exchange surfaces and transport systems (wouldn't be able to diffuse rapidly enough for all cells/tissues in the body)

  • to large a distance for just simple diffusion
• why metabolic demands mean multicellular organisms require transport systems?


  • high metabolic demands (need lots of oxygen and food, and produce lots of waste products) so diffusion over long distances is not enough to supply quantities needed
• surface area to volume ratio gets smaller as multicellular organisms get bigger so not only do diffusion distances get bigger but the amount of surface area available to absorb or remove substances becomes relatively small
• molecules such as hormones or enzymes may be made in one place and needed in another
• food will be digested in one system, but needs to be transported to every cell use in respiration and other aspects of cell metabolism
• waste products of metabolism need to be removed from the cells and transported to excretory organs
• why is suitable transport medium (e.g. water) a key feature of a transport system?

  • water is a good solvent so can dissolve polar and ionic substances
  • water is a liquid over a wide range of temperatures so can be moved around easily
• why is mass flow transport a key feature of a transport system?

  • transport medium is moved around in bulk over large distances - due to cohesion as a result of hydrogen bonding
• why is a closed system a key feature of a transport system?

  • tubular vessels contain the transport medium and form a branching network to distribute it
• why is mechanism to move the transport medium a key feature of transport systems?

  • valves prevent blood travelling backwards
  • high hydrostatic pressure ensures blood can only move in one direction
• why is mechanism to control the rate of flow a key feature of transport systems?

  • need to increase or decrease rate of flow to different organs depending on the activity (e.g. divert blood to gut after a meal, or away from gut when exercising)