Even the simplest organisms can detect and respond to events in their ever-changing environment. Similarly, within a multicellular organism, cells are surrounded by an extracellular environment from which signals are received and responded to. Extracellular events are decoded and transmitted to relevant parts of individual cells by way of a series of activation/deactivation steps involving many intracellular molecules. This relay of information along molecular pathways is called signal transduction; it is sometimes also simply referred to as ‘signalling’.

The molecular models shown in this chapter were produced using the Brookhaven protein data base (pdb) files indicated in the figure legends. These files can be downloaded, viewed and manipulated using a suitable molecular viewing programme, such as Viewerlite tm.

After studying this course, you should be able to:

• define and use each of the terms printed in bold in the text

• understand the basic principles of signal transduction mechanisms, in particular the concepts of response specificity, signal amplitude and duration, signal integration and intracellular location

• give examples of different types of extracellular signals and receptors, and explain their functional significance

• describe the mechanisms by which different receptors may be activated by their respective ligands

• describe and give examples of the structure and properties of the major components of signal transduction pathways.