The Cellular Process in Which a Signal Is Conveyed to Trigger a Change in the Activity or State of a Cell. Signal Transduction Begins with Reception of a Signal (e.g. a Ligand Binding to a Receptor or Receptor Activation by a Stimulus Such As Light) or For Signal Transduction in the Absence of Ligand Signal Withdrawal or the Activity of a Constitutively Active Receptor. Signal Transduction Ends with Regulation of a Downstream Cellular Process E.g. Regulation of Transcription or Regulation of a Metabolic Process. Signal Transduction Covers Signaling From Receptors Located On the Surface of the Cell and Signaling Via Molecules Located Within the Cell. For Signaling Between Cells Signal Transduction Is Restricted to Events At and Within the Receiving Cell.
The Space External to the Outermost Structure of a Cell. For Cells Without External Protective or External Encapsulating Structures This Refers to Space Outside of the Plasma Membrane. This Term Covers the Host Cell Environment Outside an Intracellular Parasite.
The Action Characteristic of a Hormone Any Substance Formed in Very Small Amounts in One Specialized Organ or Group of Cells and Carried (sometimes in the Bloodstream) to Another Organ or Group of Cells in the Same Organism Upon Which It Has a Specific Regulatory Action. the Term Was Originally Applied to Agents with a Stimulatory Physiological Action in Vertebrate Animals (as Opposed to a Chalone Which Has a Depressant Action). Usage Is Now Extended to Regulatory Compounds in Lower Animals and Plants and to Synthetic Substances Having Comparable Effects; All Bind Receptors and Trigger Some Biological Process.