7.1 Principles and Modes of Cell Signalling

Signalling molecules are often called ligands because they bind selectively to receptor proteins. Complementary shape matters, but binding specificity also depends on charge distribution, hydrogen-bonding opportunities and hydrophobic regions. A chemical can circulate past thousands of cells while only cells expressing a compatible receptor respond. Target-cell identity is therefore a property of receptor expression, not merely proximity to the signal source.

Ligand binding changes receptor conformation or association with other proteins. That molecular event can open an ion channel, activate an enzyme, recruit a G protein or expose a DNA-binding function. Transduction converts the extracellular information into intracellular activity. Cascades can amplify a weak signal because one active receptor can activate several relay molecules, each enzyme can modify many substrates, and each modified target can affect many downstream molecules.

In contact-dependent signalling, membrane-bound molecules on touching cells bind one another, or small substances pass through junctions such as plant plasmodesmata. Paracrine signals diffuse locally and affect nearby cells; they are important in inflammation, development and tissue repair. Autocrine signals act on the same cell that released them and can reinforce differentiation or, when regulation fails, support tumour-cell survival.

Synaptic signalling is spatially precise because a neuron releases neurotransmitter into a narrow cleft beside its target. The axon may be long, but diffusion is local. Endocrine signalling uses hormones carried in blood, so the source and target can be far apart. Distance does not by itself determine speed: synaptic transmission is rapid because the route is electrically directed and the diffusion distance tiny, whereas endocrine responses often include circulation and slower intracellular steps.

Different messengers suit different routes. Sodium and potassium ions carry current across neuronal membranes. Neurotransmitters cross a synaptic cleft. Hormones travel through extracellular fluid and blood. Cytokines, including chemokines and interferons, are secreted signalling proteins that coordinate immune-cell movement, growth and activation through surface receptors. The same molecule can also have different effects in different cells because the cells contain different receptors or downstream proteins.

Many bacteria secrete small signal molecules while also detecting them. At low density, the molecules diffuse away and their local concentration stays below the response threshold. As the population grows, production exceeds dispersal, the signal accumulates and receptor activation changes gene expression across many cells. The signal does not literally count bacteria; its concentration supplies an imperfect proxy influenced by density, diffusion, flow and degradation.

In Vibrio fischeri, high autoinducer concentration activates genes associated with bioluminescence, allowing a dense population in the bobtail squid's light organ to emit light together. Other bacteria coordinate extracellular enzymes, virulence factors or biofilm formation. A biofilm's matrix and close packing can restrict antimicrobial penetration and create chemically distinct microenvironments, so collective behaviour can be harder to eliminate than dispersed cells.

In multicellular organisms, signalling links specialized cells into tissues, organs and organ systems. Lung ventilation, for example, depends on sensory information, brainstem integration, motor output, muscle contraction and feedback from blood chemistry. None of these parts alone ventilates the body. A systems explanation specifies flows of information and matter between parts; a reductionist explanation identifies the molecular mechanisms within each part. Biology needs both.