Signal transduction

In biology, signal transduction is any process by which a cell converts one kind of signal or stimulus into another. Processes referred to as signal transduction often involve a sequence of biochemical reactions inside the cell, which are carried out by enzymes and linked through second messengers. Such processes take place in as little time as a millisecond or as long as a few seconds. Slower processes are rarely referred to as signal transduction.

In many transduction processes, an increasing number of enzymes and other molecules become engaged in the events that proceed from the initial stimulus. In such cases the chain of steps is referred to as a "signaling cascade" or a "second messenger pathway" and often results in a small stimulus eliciting a large response.

In bacteria and other one-cell organisms, the variety of signal transduction processes of which the cell is capable influences how many ways it can react and respond to its environment. In a less direct way the same is true of animals and plants. Sensing in all forms of life depends, at the cellular level, on signal transduction.

Stimuli

The environment of a cell may impinge on it in many ways: different kinds of molecules may buffet its surface, its body may be heated or cooled, it may be struck by light of various wavelengths, stretched, sheared or electrified (the nerves and muscles, for example). Signal transduction mediates how cells respond to such stimuli.

Most stimuli impinge from the outside and interact with the cell membrane. Several "signaling molecules", such as the neurotransmitters, allow nerve cells to communicate across synapses, bind to receptor proteins in the membrane and open their ion channels.

Responses

Responses triggered by signal transduction include the activation of a gene, the production of metabolic energy and cell locomotion, for example through remodelling of the cell skeleton.

Gene activation leads to further effects, since genes are expressed as proteins, many of which are enzymes, transcription factors or other regulators of metabolic activity. Because transcription factors can activate still more genes in turn, an initial stimulus can trigger via signal transduction the expression of entire suite of genes and a panoply of physiolgical events. Such mass activations are often referred to as "genetic programs," one example being the sequence of events that take place when an egg is fertilized by a sperm.

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