AQA A Level Biology复习笔记3.4.1 Features of 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 such as glucose and oxygen to diffuse 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 are connected to a mass transport system, for example
  • The digestive system is connected to the circulatory system
  • The lungs are connected to 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
  • Mass transport systems 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.