Features of Mass Transport (AQA A Level Biology) : Revision Note

Mass Transport

The need for a circulatory system

• The cells of all living organisms need a constant supply of reactants for metabolism, e.g. oxygen and glucose

• Single-celled organisms can gain oxygen and glucose directly from their surroundings, and the molecules can diffuse to all parts of the cell quickly due to short diffusion distances

• Larger organisms, however, are made up of many layers of cells, meaning that the time taken for substances to diffuse from the exchange surface to every cell in the body would be far too long

  • The diffusion distances involved are too great

• To solve this problem their exchange surfaces, i.e. the digestive system and lungs, are connected to a mass transport system, i.e. the circulatory system

• Mass transport is the bulk movement of gases or liquids in one direction, usually via a system of vessels and tubes

  • 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

• 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