The Need For a Transport System in Animals

• All living organisms have the need to exchange substances with their environment:

  • They need to take oxygen and nutrients in
  • They need to release waste products
• The location for exchange within an organism is called an exchange site. For example lungs in humans (gases) and roots in plants (water and minerals).
• Single-cell organisms able to exchange substances directly with their environment

  • This is due to their large surface area: volume ratio
  • The diffusion or transport distance in these organisms is very small so essential nutrients or molecules are able to reach the necessary parts of the cell efficiently
  • Smaller organisms also tend to have lower levels of activity and so smaller metabolic demands
• Larger organisms require specialised mass transport systems for three reasons:

  1. They have large transport distances
  2. They have high levels of activity
  3. They have a low surface area to volume ratio
• Large transport distances:

  In larger, more complex organisms (both plants and animals) the important exchange sites tend to be far away from the other cells within the organism. This large transport distance means that diffusion would not be fast enough to meet the metabolic requirements of cells.
• High levels of activity:

  Larger organisms are not only more physically active but they also contain more cells than smaller organisms. A larger number of cells results in a higher level of metabolic activity. As a result, the demand for oxygen and nutrients is greater and more waste is produced. The increased demand for oxygen and nutrients along with the greater need for the disposal of waste means that diffusion is not an efficient transport mechanism for larger organisms.
• Low surface area to volume ratios:

  Surface area and volume are both very important factors in the exchange of materials. The surface area refers to the total area of the organism that is exposed to the external environment. The volume refers to the total internal volume of the organism. As the surface area and volume of an organism increase (and therefore the overall ‘size’ of the organism increases), the surface area to volume ratio decreases. This is because volume increases much more rapidly than surface area as size increases.
• Single-celled organisms have a high SA : V, 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. In addition, the greater volume results in a longer diffusion distance to the cells and tissues of the organism.