The ability of a cell to receive, process, and transmit signals with its environment and with itself is known as cell signalling (cell signalling in British English). In both prokaryotes and eukaryotes, cell signalling is a fundamental feature of all cellular life. Extracellular signals are physical agents that originate outside of a cell, such as mechanical pressure, electricity, temperature, light, or chemical signals (e.g., small molecules, peptides, or gas). Hydrophobic and hydrophilic chemical signals exist. Autocrine, juxtacrine, intracrine, paracrine, and endocrine signals can occur across short or long distances, and can thus be classed as autocrine, juxtacrine, intracrine, paracrine, or endocrine. Signaling molecules can be produced through a variety of biosynthetic pathways and released via passive or active transport mechanisms, as well as cell injury.
Receptors are important in cell signalling because they can detect chemical or physical stimuli. Receptors are proteins that are found on the cell surface or within the cell’s interior, including the cytoplasm, organelles, and nucleus. Extracellular signals (or ligands) attach to cell surface receptors, causing a conformational change in the receptor that causes it to activate enzymic activity or open or close ion channel activity. Some receptors are related to enzymes or transporters rather than having enzymatic or channel-like characteristics. Nuclear receptors, for example, have a distinct process that involves altering their DNA binding characteristics and cellular localization to the nucleus.
The transformation (or transduction) of a signal into a chemical signal can either activate an ion channel (ligand-gated ion channel) immediately or start a second messenger system cascade that propagates the signal throughout the cell. Second messenger systems can magnify a signal by activating several secondary messengers in response to the activation of a few receptors, multiplying the originating signal (the first messenger). Additional enzymatic activity such as proteolytic cleavage, phosphorylation, methylation, and ubiquitinylation may occur as a result of these signalling pathways.
The cornerstone of development, tissue repair, immunity, and homeostasis is that each cell is programmed to respond to unique extracellular signal molecules. Cancer, autoimmunity, and diabetes can all be caused by errors in signalling connections.
Quorum sensing allows individuals in numerous microscopic organisms, such as bacteria, to start an action only when the population is large enough. The marine bacteria Aliivibrio fischeri, which produces light when the population is dense enough, was the first to notice this signalling between cells. The technique entails the creation and detection of a signalling molecule, as well as the subsequent regulation of gene transcription. Both gram-positive and gram-negative bacteria use quorum sensing, and it works both within and between species.
Individual cells called amoebae gather together in slime moulds to create fruiting bodies and eventually spores, thanks to a chemical signal called acrasin. Chemotaxis is the process by which individuals are drawn to a chemical gradient. Some species, like Polysphondylium violaceum, employ cyclic AMP as a signal, while others, like Polysphondylium violaceum, use other compounds, in this case N-propionyl-gamma-L-glutamyl-L-ornithine-delta-lactam ethyl ester, also known as glorin.
Signaling between cells in plants and animals occurs either through extracellular release, which is separated into paracrine signalling (over short distances) and endocrine signalling (over long distances), or through direct contact, which is known as juxtacrine signalling (e.g., notch signaling). Autocrine signalling is a type of paracrine signalling in which the secreting cell can respond to the signalling chemical produced. Synaptic signalling is a type of paracrine (for chemical synapses) or juxtacrine (for electrical synapses) transmission that occurs between neurons and target cells.