When a person unexpectedly comes face to face with a grizzly bear, his or her body quickly shunts blood away from the skin and digestive system and toward the muscles. The heart also beats faster, and the liver releases glucose molecules that provide emergency fuel for what is called the "fight-or-flight" response.
In the fight-or-flight response, the adrenal glands release the hormone epinephrine, which serves as a signal within the body. Certain cells, including liver cells, can detect the signal, after which they process the signal and respond to it. The entire sequence—from signal reception to cellular response—is referred to as a signal transduction pathway. The following animation depicts a signal transduction pathway in a liver cell.
Signal transduction pathways allow cells to respond to environmental signals. In the majority of signal transduction pathways, a signal is amplified such that most steps produce a larger number of activated components than previous steps. Signal amplification, for example, results in a liver cell releasing many glucose molecules after detecting just a single molecule of epinephrine.
Signal amplification can occur at many points. For example, as long as epinephrine remains bound to a receptor, the receptor can activate a succession of G proteins. In addition, each adenylyl cyclase enzyme can convert numerous ATPs into cyclic AMP molecules. Other activated enzymes in the pathway can also continually catalyze reactions. The G protein, in contrast, activates just a single adenylyl cyclase enzyme and must remain attached to it in order for adenylyl cyclase to remain activated.
Termination of the cellular response is as important as its initiation. In order for a cell to respond only when a signal is present, the many players in the pathway have to be regulated so that they are activated for only a short period of time.
Textbook Reference: Concept 5.6 Signal Transduction Allows the Cell to Respond to Its Environment