The need for animal transport

• Exchange site


  The location within an organism where the exchange of substances with the surrounding environment occurs
• Exchange sites


  • Lungs in humans (gases)
  • Roots in plants (water and minerals)
• Substances are said to not have entered or left an organism until it crosses the cell surface membrane
• Small organisms like Chlamydomonas


  • They are able to exchange substances directly with the environment due to their large surface area: volume ratio
  • The diffusion or transport distance in these organisms are very small so essential nutrients or molecules are able to reach the necessary parts of the cell efficiently
  • They tend to have lower levels of activity and so smaller metabolic demands
• Larger organisms


  • They require specialised mass transport systems due to increasing transport distances
  • Decreasing surface area: volume ratio
  • Increasing levels of activity
• Surface area


  The total area of the organism that is exposed to the external environment
• Volume


  The total internal volume of the organism (total amount of space inside the organism)
• As the surface area and volume of an organism increase

  The surface area: volume ratio decreases
• Single-celled organisms


  • They have a high SA: V ratio which allows for the exchange of substances to occur via simple diffusion
  • The large surface area allows for maximum absorption of nutrients and gases and secretion of waste products
  • The small volume means the diffusion distance to all organelles is short
• As organisms increase in size

  • Their SA: V ratio decreases
  • There is less surface area for the absorption of nutrients and gases and secretion of waste products
  • The greater volume results in a longer diffusion distance to the cells and tissues of the organism
• Metabolic activity


  The level of chemical processes that occur within a living organism to maintain life
• Larger organisms


  They are not only more physically active but they also contain more cells, resulting in a higher level of metabolic activity
• Mass transport systems


  • Specialised systems that enable the efficient transport of nutrients and waste in larger organisms
  • They involve the bulk movement of materials (mass flow) and still involve some diffusion at specific exchange sites
• Mass transport systems


  • They bring substances quickly from one exchange site to another
  • They maintain the diffusion gradients at exchange sites and between cells and their fluid surroundings
  • They ensure effective cell activity by keeping the immediate fluid environment of cells within a suitable metabolic range
• Mass transport system


  • The circulatory system in mammals