As you have seen, there are different kinds of signals and receptors. Not surprisingly, the ways that signals are transduced, and the resulting cellular responses, also vary. Some signal transduction pathways are quite simple and direct, whereas others involve multiple steps. As we mentioned in Key Concept 7.1, signal transduction pathways can involve enzymes and transcription factors. In addition, second messengers can diffuse throughout the cytoplasm and mediate further steps in pathways.
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A signal transduction cascade transmits and amplifies a signal inside a cell.
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Signal transduction is regulated in a cell by various mechanisms.
In many cases, a signal can initiate a cascade (chain) of events, in which proteins interact with other proteins, which interact with still other proteins until the final responses are achieved. Through such a cascade, an initial signal can be both amplified and distributed to cause several different responses in the target cell. In this section we will examine the kinds of molecules that transduce signals and look at several different signal transduction pathways.