Chemical signalling begins with selectivity. A cell can be surrounded by many signalling chemicals, but it responds only to those that match a receptor it has.

A receptor is a protein with a specific binding site for a signalling chemical; when that chemical binds, the protein changes its activity or shape. A ligand is a molecule or ion that binds selectively to a specific site on another molecule. In this topic, the ligand is the signalling chemical.

The binding site is not just a hole with the right size. It also has to match the ligand in shape, charge distribution, polarity and possible hydrogen-bonding positions. That is why acetylcholine receptors respond to acetylcholine, but not to every small molecule drifting past the membrane.

Receptors are worth comparing with enzymes, since both rely on molecular specificity. The difference is what happens next. An enzyme binds a substrate and catalyses its conversion into product. A receptor binds a ligand and passes on information; the ligand is usually released unchanged. So don’t write that receptors “digest” or “break down” their signalling chemical — they detect it.

Quorum sensing is cell signalling in bacteria that changes gene expression when the local population density rises above a threshold. One bacterium can’t count the cells around it, but it can sense how much signalling molecule the population has released.

Each bacterial cell releases a small amount of signalling chemical. At low density, the chemical diffuses away, so too few receptors bind it to switch on the response. At high density, the chemical builds up. Many receptors are occupied, and the cells change gene expression together. It’s a clear example of interaction and interdependence: the signal moves between cells, and the behaviour only pays off when many cells do it at once.

In the marine bacterium Vibrio fischeri, quorum sensing controls bioluminescence, which is light production by living cells due to an enzyme-catalysed chemical reaction. V. fischeri cells release an autoinducer, a signalling molecule that is made by the responding cells and increases its own population-level effect as cell density rises.

Once the autoinducer concentration is high enough, it binds to a receptor protein inside V. fischeri. The autoinducer–receptor complex then binds DNA and promotes transcription of the genes needed to make luciferase. Luciferase is an enzyme that catalyses a light-producing oxidation reaction. Free-living bacteria are too spread out to produce strong light; inside a host light organ, high bacterial density switches the genes on, so visible light is produced.

In its mutualistic relationship with bobtail squid, the squid supplies nutrients and a protected light organ, while the bacteria produce light that helps reduce the squid’s shadow in moonlit water. For the syllabus, the key point is not the squid detail; it is that quorum sensing connects a chemical signal with coordinated bacterial gene expression.

Animal signalling chemicals are often grouped by their role rather than their chemical structure. That makes sense: two chemicals with very different structures can both act as hormones, while two chemically similar molecules may carry out quite different signalling jobs.

A hormone is a signalling chemical secreted by endocrine cells and carried in the blood to target cells with matching receptors. Hormones can act far from where they are made, may affect many tissues, and often cause responses that last seconds to hours or longer. Insulin, thyroxine and testosterone are typical examples.

A neurotransmitter is a signalling chemical released by a neuron that diffuses across a synapse and binds to receptors on a postsynaptic cell. This kind of signalling is local, fast and short-lived. The message is usually cleared quickly by breakdown, reuptake or diffusion away, so one synapse can be controlled without automatically activating the whole nervous system.

A cytokine is a small protein signalling molecule secreted by cells, especially in immune and developmental contexts, that usually acts on the secreting cell or nearby cells. Cytokines bind to receptors in the plasma membrane because they cannot cross the lipid bilayer. Their effects often involve changes in gene expression, such as during inflammation or immune coordination.

A calcium ion is a positively charged ion, Ca²⁺, used as an intracellular signal when its cytoplasmic concentration changes. In muscle fibres, Ca²⁺ released from the sarcoplasmic reticulum allows contraction. In neurons, Ca²⁺ entering a presynaptic terminal triggers exocytosis of neurotransmitter vesicles.

Comparison of animal signalling categories by source, movement, range, timing, receptor location and roles.

The main contrasts are distance, speed and typical target range. Hormones travel in the blood and can have widespread effects. Neurotransmitters cross tiny synaptic gaps, so their action stays highly localized. Cytokines often work locally in tissue signalling, especially between immune cells. Calcium ions commonly act inside cells as a rapid internal signal.

There isn’t one standard “chemical shape” for a signal. Evolution has used many different substances as messengers, as long as cells can make them, transport them, detect them specifically with receptors and then remove or regulate them after use.

Specificity is the key requirement. A signalling chemical needs distinctive chemical properties, so its receptor can pick it out from the surrounding mix of metabolites, ions and proteins. Its solubility also has to suit the route it takes: a blood-borne peptide hormone must move through aqueous plasma, while a steroid hormone must be lipid-soluble enough to cross membranes.

Hormones include several chemical groups:

• Amines are small signalling molecules derived from amino acids; epinephrine and thyroxine are examples.
• Protein or peptide hormones are chains of amino acids used as signalling chemicals; insulin and glucagon are examples.
• Steroid hormones are lipid-soluble signalling molecules derived from cholesterol; oestradiol, progesterone and testosterone are examples.

Neurotransmitters vary chemically too. A neurotransmitter class may include amino acids such as glutamate, peptides, amines such as dopamine, and gaseous signalling molecules such as nitrous oxide as named in the syllabus. Acetylcholine is another important neurotransmitter you meet again in membrane-potential signalling.

Chemical diversity of hormones and neurotransmitters with examples named in the section.

This diversity gives cells flexibility. Different chemical properties fit different roles: hydrophilic messengers can travel in extracellular fluid and bind membrane receptors; hydrophobic messengers can pass through membranes and bind intracellular receptors; gases can diffuse rapidly through tissues.

The same basic idea — a ligand binding to a receptor — can handle communication right next door or across the whole body.

A localized effect is a response limited to cells near the site where the signal is released, because the signalling molecule moves only a short distance or gets removed quickly. Neurotransmitters are the usual comparison: they diffuse across the synaptic gap from a presynaptic cell to a postsynaptic cell, so the target is usually one nearby cell or muscle fibre.

A distant effect is a response in target cells far from the secreting cells, because the signalling molecule is carried through the body. Hormones are the standard example. They enter the blood and travel around the circulation. Only cells with the appropriate receptor respond, so the blood works as a distribution system, not the decision-maker.

Be precise with this contrast in exams. Neurotransmitters are not “stronger” just because they act quickly, and hormones are not “less specific” because they travel everywhere. Specificity comes from receptor presence. Distance and duration depend mainly on the transport route, diffusion distance and how quickly the signal is removed.

Receptor location depends on whether the signalling chemical can cross the plasma membrane.

A transmembrane receptor is a receptor protein that spans the plasma membrane. It has an extracellular ligand-binding region and an intracellular region that triggers the response inside the cell. Cells use these receptors for signalling chemicals that stay outside the cell, especially hydrophilic molecules and large molecules such as many protein hormones.

An intracellular receptor is a receptor protein in the cytoplasm or nucleus. It binds a signalling chemical only after that chemical has entered the cell. Small hydrophobic signalling chemicals, especially steroid hormones, use these receptors because they can dissolve through the hydrophobic core of the phospholipid bilayer.

The amino acid distribution explains where each receptor sits. Transmembrane receptors contain hydrophobic amino acids in the membrane-spanning region, where they touch the fatty acid tails of phospholipids. The exposed regions contain more hydrophilic amino acids, since they contact extracellular fluid or cytosol. Intracellular receptors don’t need a membrane-spanning hydrophobic band; their surfaces suit the aqueous cytoplasm or nucleoplasm.

So the practical rule is simple: if the signal cannot penetrate the membrane, the receptor must face outward in the plasma membrane; if the signal penetrates the membrane, the receptor can be inside the cell.

A receptor isn’t the final response. It’s where the chain begins.

A signal transduction pathway is a sequence of molecular interactions inside a cell that converts receptor activation into a cellular response. “Transduction” means the information changes form: ligand binding outside the cell becomes activity inside the cell.

With transmembrane receptors, ligand binding changes the receptor’s conformation. The changed receptor may activate a protein, open a channel, phosphorylate another molecule or produce a small intracellular messenger. A second messenger is a small intracellular signalling molecule produced or released after receptor activation that relays and often amplifies the signal inside the cell.

Intracellular receptors work more directly in one sense, but the outcome is usually slower. The signalling chemical enters the cell, binds its receptor, and the activated ligand–receptor complex affects transcription by binding particular DNA sequences. The response then depends on mRNA production and protein synthesis.

The pattern linking biological communication is pretty consistent: a source cell releases a signal, a target cell detects it with a receptor, and the receiving system changes behaviour. The same logic shows up in quorum sensing, synaptic signalling, endocrine control and gene regulation — different scale, same communication pattern.

Some transmembrane receptors also work as ion channels. It’s a neat design: once the ligand binds, ion movement changes directly.

Membrane potential is the voltage difference across a plasma membrane caused by unequal distribution of charged ions on the two sides. In neurons and muscle fibres, a change in this voltage acts as a signal itself.

The acetylcholine receptor is the required example. Acetylcholine is a neurotransmitter that binds to receptor proteins in the postsynaptic membrane at many synapses, including neuromuscular junctions. When acetylcholine binds, the receptor changes shape and opens an ion channel in the receptor.

The open channel lets positively charged ions, especially sodium ions, diffuse into the cell down their electrochemical gradient. The inside of the membrane becomes less negative than it was before. Depolarization is a reduction in the voltage difference across a membrane caused by the inside becoming less negative relative to the outside.

That local voltage change may then trigger other changes, such as initiation of an action potential in a neuron or contraction in a muscle fibre. Keep the sequence clear: acetylcholine binds → channel opens → positive ions enter → membrane potential changes → further response may follow.

G protein-coupled receptors are transmembrane receptors that activate G proteins when a ligand binds. They’re often shortened to GPCRs. Humans have many different GPCRs, and they respond to a wide range of signals, including hormones, neurotransmitters, odour molecules and light-related signals.

A G protein is a membrane-associated protein that binds guanine nucleotides, then passes signals from an activated receptor to intracellular effectors. In the common model, it has three subunits: alpha, beta and gamma. When inactive, the alpha subunit holds GDP, guanosine diphosphate.

Once the ligand binds the GPCR, the receptor changes shape. GDP leaves the alpha subunit, and GTP, guanosine triphosphate, binds in its place. Binding GTP activates the G protein. The activated subunits then separate from the receptor and interact with effector proteins inside the cell.

The GPCR does not need to carry the whole message through to the final response. It couples detection outside the cell to activation inside the cell. That’s why the same receptor family can be adapted again and again for different signals in the human body.

Epinephrine and adrenaline are two names for the same hormone. Epinephrine is an amine hormone released during stress responses, and it binds to specific GPCRs on target cells such as liver cells.

What matters here is the sequence. Epinephrine binds to a transmembrane receptor in the plasma membrane. The receptor changes shape, then activates a G protein. The activated G protein switches on adenylyl cyclase, an enzyme in the membrane. Adenylyl cyclase converts ATP into cyclic AMP.

Cyclic AMP (cAMP) is a second messenger made from ATP that relays signals from some activated receptors to intracellular enzymes. In epinephrine signalling, cAMP activates protein kinase enzymes. Protein kinase is an enzyme that transfers phosphate groups to specific proteins, changing their activity.

The cascade amplifies the signal. One hormone molecule can activate one receptor; that receptor can activate several G proteins, leading to many cAMP molecules and many enzyme activations. In liver cells, the pathway quickly promotes glycogen breakdown, helping glucose become available in the blood.

The naming point works well as a nature-of-science example. “Adrenaline” comes from Latin roots meaning near the kidney, while “epinephrine” comes from Greek roots meaning above the kidney; both refer to the adrenal glands. The fact that both names still exist shows how scientific naming depends on international cooperation, history and practical agreement, not just chemistry.

Some receptors are enzymes in their own right. The required example here is the insulin receptor.

A tyrosine kinase receptor is a transmembrane receptor with intracellular enzyme activity that phosphorylates tyrosine amino acids in proteins after ligand binding. Phosphorylation means adding a phosphate group to a molecule, often changing that molecule’s shape or activity.

Insulin is a protein hormone, so it can’t cross the plasma membrane. Instead, it binds to the extracellular part of the insulin receptor. That binding changes the receptor’s shape and switches on tyrosine kinase activity on the cytoplasmic side. Tyrosine residues inside the cell are then phosphorylated, which starts a sequence of intracellular reactions.

For this syllabus point, keep the end of the pathway specific: the sequence ends with vesicles containing glucose transporters moving to and fusing with the plasma membrane. Those added transporters increase the cell’s ability to take up glucose by facilitated diffusion. Don’t turn this into a full blood-glucose homeostasis essay; the receptor mechanism is the focus.

This also covers part of the linking question about communication patterns: increased blood glucose can lead to insulin secretion, and insulin changes intracellular signalling in target cells by activating receptor tyrosine kinase pathways that alter membrane transporter abundance.

Steroid hormone signalling works differently because the receptor sits inside the cell, and the response often involves transcription.

A steroid hormone is a lipid-soluble hormone derived from cholesterol that can diffuse through the phospholipid bilayer and bind an intracellular receptor. Oestradiol, progesterone and testosterone are the syllabus examples.

The signalling chemical binds to a specific site on the receptor, which activates it. The activated receptor–hormone complex enters the nucleus, or acts within it, and binds to specific DNA sequences. That promotes transcription of particular genes, so the cell produces more mRNA and may synthesize specific proteins.

A transcription factor is a protein that binds DNA and regulates the transcription of genes. Activated steroid hormone receptors act as transcription factors. Steroid responses are often slower than ion-channel responses for this reason: changing gene expression takes time, but the effects can be substantial and long-lasting.

Testosterone gives a clear example of the mechanism. It binds an intracellular androgen receptor; the activated complex binds DNA and increases transcription of target genes in responsive cells. You don’t need the details of every target gene here — the core idea is ligand activation of an intracellular receptor followed by DNA binding and increased transcription.

Oestradiol and progesterone are both steroid hormones, so their effects on target cells follow the intracellular receptor pattern from the previous section. The syllabus only needs a narrow set of examples, so keep it tight.

Oestradiol is a steroid hormone involved in reproductive regulation. It can act on hypothalamic cells that secrete gonadotropin-releasing hormone. In these hypothalamic target cells, oestradiol diffuses through the plasma membrane and binds an intracellular receptor. The activated receptor complex then promotes transcription linked to gonadotropin-releasing hormone secretion.

Gonadotropin-releasing hormone (GnRH) is a hypothalamic hormone that stimulates the anterior pituitary to release gonadotropins. Around ovulation, oestradiol can stimulate GnRH secretion. So, one hormone can regulate the release of another by changing gene expression in target cells.

Progesterone is a steroid hormone involved in maintaining the endometrium. The endometrium is the inner lining of the uterus that thickens and becomes prepared to support early pregnancy. Progesterone enters endometrial cells, binds intracellular receptors, and the activated receptor complex alters transcription of genes needed to maintain the uterine lining.

A clean comparison: the oestradiol target cells here are hypothalamic cells involved in GnRH secretion; the progesterone target cells here are endometrial cells involved in maintaining the uterine lining.