CIE A Level Biology (9700) 2019-2021

Revision Notes

4.1.4 Cell Signalling

Cell Signalling

  • Cell signalling is the process by which messages are sent to cells
  • Cell signalling is very important as it allows multicellular organisms to control / coordinate their bodies and respond to their environments
  • Cell signalling pathways coordinate the activities of cells, even if they are large distances apart within the organism
  • The basic stages of a cell signalling pathway are:
    • A stimulus or signal is received by a receptor
    • The signal is converted to a ‘message’ that can be passed on – this process is known as transduction
    • The ‘message’ is transmitted to a target (effector)
    • An appropriate response is made

Basic stages of cell signalling pathway, downloadable AS & A Level Biology revision notes

The basic stages of a cell signalling pathway

  • Transmission of messages in cell signalling pathways requires crossing barriers such as cell surface membranes
  • Cell surface membranes are therefore very important in signalling pathways as the membrane controls which molecules (including cell signalling molecules) can move between the internal and external environments of the cell
  • Signalling molecules are usually very small for easy transport across cell membranes
  • Typically in cell signalling pathways, signalling molecules need to cross or interact with cell membranes


  • Signalling molecules are often called ligands
  • Ligands are involved in the following stages of a cell signalling pathway:
    • Ligands are secreted from a cell (the sending cell) into the extracellular space
    • The ligands are then transported through the extracellular space to the target cell
    • The ligands bind to surface receptors (specific to that ligand) on the target cell
      • These receptors are formed from glycolipids and glycoproteins
    • The message carried by the ligand is relayed through a chain of chemical messengers inside the cell, triggering a response

Ligands in cell signalling, downloadable AS & A Level Biology revision notes

The role of ligands in a cell signalling pathway


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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