A level Biology exam revision resources written by A level Examiners


Living organisms possess sensitivity. This means they can detect and respond to change in a way that increases the chance of the organism's survival. The stages from stimulus to response are often depicted as a flowchart.

Figure 1 : The stages from stimulus to response
Figure 1 : The stages from stimulus to response


  • A stimulus can be defined as a change to a condition, which can be either internal (e.g. blood temperature) or external (e.g. light intensity).
  • Receptors are usually specialised cells (often within a sense organ).
  • The co-ordinator is the Central Nervous System (brain and spinal cord).
  • Effectors are muscles or glands.
  • Responses are as varied as gross movement of the whole organism to changes in metabolism within cells.

The two blue arrows within the organism represent communication between different parts of the organism. Information is passed from the receptors to the co-ordinator and from the co-ordinator to the effectors. This internal communication is an essential feature of the scheme. There are two ways information can be transmitted around an organism, these are:

  • Nervous communication
  • Hormonal communication

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