= Signal transduction
{wiki}
<Signal transduction> converts a detected signal into a change inside a <biological cell>. For chemical signals, distinguish reception by a <receptor>, intracellular <signal transduction>, and the response: altered <gene expression>, <enzyme> activity or <ion channel> opening, for example. A <cell-surface receptor> can relay a <ligand>'s presence across a <cell membrane> without the <ligand> entering the <cytoplasm>. Thus a <hydrophilic> signal need not cross the membrane to influence the interior. See https://openstax.org/books/biology-2e/pages/9-2-propagation-of-the-signal[Clark, Douglas and Choi, Biology 2e, 9.2: intracellular signal propagation].
\Image[/cell-signal-transduction-cascade.png]
{title=Reception, intracellular <signal transduction> and a cellular response}
{description=Original cartoon of a <ligand> outside a <biological cell>, a <cell-surface receptor> spanning its <cell membrane>, and intracellular relays branching toward <gene expression> or another response. Arrows denote causal steps, not transport of the <ligand> through the membrane. Shapes, distances and relay counts are illustrative.}
For a computed example of possible <signal amplification>, suppose one activated <receptor> activates five relay <proteins>, and each relay activates twenty target <proteins> during a chosen interval. If the targets are distinct and available, $1\times5\times20=100$ targets are activated. Those assumed counts illustrate multiplication, not a measured pathway or a universal gain. Actual <signal transduction> can branch, saturate or inhibit activity. Changes in <gene expression> and changes in existing <enzyme> activity are different responses. See https://openstax.org/books/biology-2e/pages/9-3-response-to-the-signal[Clark, Douglas and Choi, Biology 2e, 9.3: cellular responses].
<Signal amplification>, <signaling feedback> and <signaling adaptation> are possible properties of <signal transduction>, rather than requirements of every cascade. Intracellular relays can use <proteins> or <second messengers>; different <biological cells> can interpret the same <ligand> differently because they contain different <receptors> and relay machinery. The useful and striking idea is that recognition and response are separable: an extracellular chemical message can control many internal tasks without itself performing those tasks. See <Molecular Biology of the Cell, 4th edition by Bruce Alberts and others/General Principles of Cell Communication>.
An original experimental route to this separation appears in T. W. Rall and Earl W. Sutherland's 1958 work on <cAMP>. They described a heat-stable factor formed from <ATP> in separated <biological cell> preparations; it affected <enzyme> activity in other preparations where the initiating chemical signals had no direct effect. This helped distinguish an initial signal from a molecule carrying its effect to another target. https://doi.org/10.1016/S0021-9258(19)77422-5[Rall and Sutherland, Formation of a cyclic adenine ribonucleotide by tissue particles, Journal of Biological Chemistry 232 (1958), pp. 1065–1076, introduction]{ref}
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