Shortened revision notes on the Communication A2 Biology Topic for the OCR exam board.

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Key Definitions to remember.

You must be able to define:

  • Homeostasis
  • Cell Signalling
  • Negative Feedback
  • Positive Feedback
  • Receptors
  • Effectors
  • Stimulus
  • Response
  • Physiological Response
  • Behavioural Response

Please find your own definitions for these terms, as different books describe them differently :)

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Communication Systems in Multicellular Organisms

  • Multicellular organisms are complex, and need a communication pathway in order to coordinate responses within their body. They need to be able to respond to internal and external changes in order to maintain a constant internal environment ideal for metabolic processes (Homeostasis)


  • Cell signalling is required in order to coordinate the activities of different organs and tissues.
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Neuronal and Hormonal Systems

  • The Neuronal system is a form of cell signalling where impulses (action potentials) are sent through nerve cells in order to cause a change in a target cell.


  • The Hormonal system is a form of cell signalling in which chemical messengers are transmitted into the blood stream to target cells by endocrine cells.
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  • Homeostasis is the maintenance of a constant internal environment.
  • This is vital for cells to function normally and to prevent damage from occuring. (e.g. enzyme activity requires a constant specific temperature)


  • Homeostatic systems detect a change and respond through negative feedback. Receptors detect when a change is too high or too low and this is communicated through the neuronal or hormonal system to effectors which then respond to counteract that change returning the level to normal.
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Physiological + Behavioural responses


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How the body detects temperature change

  • The Hypothalamus receives information about temperature from thermoreceptors. Thermoreceptors in the hypothalamus detect internal (core) temperature changes whereas thermoreceptors in the skin (peripheral thermoreceptors) detect external temperature changes.


  • These then send impulses along sensory neurones to the hypothalamus that sends impulses along motor neurones to effectors which respond by restoring the body temperature to normal.
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