AQA A Level Biology

Revision Notes

3.4.1 Features of Mass Transport

Mass Transport

  • All organisms have the need to exchange substances with their surrounding environment
  • 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
  • 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 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
  • There is still some diffusion involved but only at the exchange sites at the start and end of the route travelled by the substances
  • Exchange sites are connected to mass transport systems, for example:
    • The digestive system in mammals
    • The circulatory system
  • Mass transport is the bulk movement of gases or liquids in one direction, usually via a system of vessels and tubes
  • 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
  • Mass transport systems help to bring substances quickly from one exchange site to another
  • They also help to maintain the diffusion gradients at exchange sites and between cells and their fluid surroundings
  • Mass transport systems ensure effective cell activity by keeping the immediate fluid environment of cells within a suitable metabolic range

The double circulatory system in mammals, IGCSE & GCSE Biology revision notes

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

Exam Tip

Make sure not to forget about the mass flow systems present in plants! The xylem is essential for plants if they are to grow and become large, multicellular organisms.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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