The Need for Transport Systems in Animals (OCR A Level Biology)

• All living organisms have the need to exchange substances with their surrounding environment
  • They need to take oxygen and nutrients in
  • Waste products generated need to be released 

• The location within an organism where this occurs is described as an exchange site
  • E.g. lungs in humans (gases) and 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 the single-celled Chlamydomonas are able to exchange substances directly with the environment
  • This is 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
  • Smaller organisms also tend to have lower levels of activity and so smaller metabolic demands

• Larger organisms require specialised mass transport systems for a number of reasons:
  • Increasing transport distances
  • Surface area: volume ratio
  • Increasing levels of activity

Increasing 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 makes simple diffusion a non-viable method for transporting substances all the way from the exchange site to the rest of the organism
  • Diffusion wouldn’t be fast enough to meet the metabolic requirements of cells

As size increases, the surface area : volume ratio decreases

The surface area:volume ratio calculation differs for different shapes (these shapes can reflect different cells or organisms)

Increasing 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

Metabolic rate increases with body mass

Mass transport systems

• Larger organisms have evolved specialised mass flow transport systems that enable the efficient transport of nutrients and waste
  • Mass flow is the bulk movement of materials. It is directed movement so involves some sort of force

• In mass transport systems there is still some diffusion involved but only at specific exchange sites at the start and end of the route travelled by the substances
  • The lungs are the exchange site of the gas exchange system

• Mass transport systems help to:
  • Bring substances quickly from one exchange site to another
  • Maintain the diffusion gradients at exchange sites and between cells and their fluid surroundings
  • Ensure effective cell activity by keeping the immediate fluid environment of cells within a suitable metabolic range

• The circulatory system in mammals is a well-studied example of a mass transport system. The one-way flow of blood within the blood vessels carries essential nutrients and gases to all the cells of the body

Image showing the circulatory system in mammals. The sites of exchange are the lungs and the capillaries.