C2.1 Chemical signalling

Cells in a large organism rarely act alone — they constantly send and receive chemical messages. Chemical signalling is how one cell changes the behaviour of another by releasing a signalling molecule that binds to a specific receptor. For C2.1 (an HL topic) the central question is one of specificity: how can a hormone travel everywhere in the blood yet affect only certain target cells, and how does a message stopped at the cell surface end up changing what happens deep inside the cell? Answering those two questions — specificity and signal transduction — is what this topic is about.

Signalling molecules, receptors and specificity

A signalling molecule (a ligand) only affects a cell that carries a matching receptor. The fit between ligand and receptor is highly specific, like a key in a lock, so a cell’s response depends entirely on which receptors it possesses. This explains the puzzle of hormones: a hormone in the blood reaches every tissue, but only the target cells — those bearing the right receptor — respond. Cells without that receptor simply ignore the signal.

The syllabus groups chemical signals by how far they travel. Hormones are secreted into the blood and act on distant target cells (endocrine signalling). Neurotransmitters act across the tiny gap of a synapse. Other signals act very locally on neighbouring cells (paracrine) or even on the cell that released them (autocrine). The same logic of ligand-and-receptor underlies all of them.

Receptors on the surface and inside the cell

Where a receptor sits depends on the chemical nature of its ligand.

• Hydrophilic signals (such as peptide hormones and most neurotransmitters) cannot cross the hydrophobic core of the plasma membrane. Their receptors are transmembrane proteins on the cell surface; the message is received outside and relayed inside.
• Hydrophobic (lipophilic) signals (such as steroid hormones and thyroxine) pass straight through the membrane and bind to intracellular receptors in the cytoplasm or nucleus.

This distinction matters because it determines the kind of response. A steroid hormone bound to its intracellular receptor often acts directly as a transcription factor, switching specific genes on or off and so changing which proteins the cell makes — a slower but longer-lasting effect. Surface receptors, by contrast, tend to trigger faster responses through signal transduction.

Signal transduction and second messengers

When a hydrophilic signal binds a surface receptor, the message must be converted into a form that works inside the cell. This conversion is signal transduction. Binding changes the shape of the receptor, which sets off a chain of events inside the cell, frequently involving a second messenger — a small intracellular molecule, such as cyclic AMP (cAMP), that spreads the signal and amplifies it.

A widely studied example involves G-protein-coupled receptors (GPCRs). Ligand binding activates a G protein, which in turn activates an enzyme that produces many molecules of cAMP; cAMP then activates further enzymes that change the cell’s activity. The key idea is amplification: one signalling molecule at the surface can produce a large internal response because each step activates many molecules in the next. This is how a tiny hormone concentration can cause a substantial change in a target cell.

Specificity, sensitivity and the wider picture

Two features of chemical signalling are worth holding together. Specificity comes from receptor distribution: only cells with the receptor respond, and the same hormone can even cause different effects in different tissues if their receptors are linked to different internal pathways. Sensitivity comes from transduction and amplification: very low concentrations of a signal can produce a strong, coordinated response.

Because receptors are so central, they are common drug targets. Many medicines work as agonists (mimicking a natural ligand to switch a receptor on) or antagonists (blocking a receptor so the natural ligand cannot bind). Understanding chemical signalling therefore links cell biology directly to pharmacology, and it provides the foundation for the neural and hormonal coordination explored elsewhere in Theme C.