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

A receptor is a protein with a specific binding site for a signalling chemical; when the chemical binds, the receptor 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 isn’t just a correctly sized hole. Its shape, charge distribution, polarity and possible hydrogen-bonding positions all need to match the ligand. That is why acetylcholine receptors respond to acetylcholine, rather than to every small molecule drifting past the membrane.

Receptors are worth comparing with enzymes, since both depend 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. A single bacterium cannot count its neighbours, but it can detect the concentration of a signalling molecule released by the population.

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

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 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, the high bacterial density switches the genes on, producing visible light.

In a 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. The key syllabus point is not the squid detail; it is that quorum sensing links a chemical signal to coordinated bacterial gene expression.

Animal signalling chemicals are usually grouped by what they do, rather than by what they’re made of. That makes sense: two chemicals with very different structures can both work as hormones, while two chemically similar molecules may play quite different signalling roles.

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’re released, may affect many tissues, and often produce responses lasting 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. Neurotransmitter 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 do not 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, $Ca^{2+}$, used as an intracellular signal when its cytoplasmic concentration changes. In muscle fibres, $Ca^{2+}$ released from the sarcoplasmic reticulum permits contraction. In neurons, $Ca^{2+}$ entering a presynaptic terminal triggers exocytosis of neurotransmitter vesicles.

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

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

Signals don’t have one standard “chemical shape”. Over evolutionary time, many different substances have been used 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 background mix of metabolites, ions and proteins. Its solubility also has to match its route. A blood-borne peptide hormone, for example, must move through aqueous plasma, while a steroid hormone needs enough lipid solubility 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 as well. 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 signalling systems flexibility. Hydrophilic messengers can move through extracellular fluid and bind to 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 in one small area 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 intended target is normally one neighbouring cell or muscle fibre.

A distant effect is a response in target cells far from the secreting cells, because the signalling molecule is transported through the body. Hormones are the standard example. They enter the blood and move through the circulation. Only cells with the appropriate receptor respond, so the blood distributes the signal; it doesn’t decide which cells react.

Be precise with this contrast in exams. Neurotransmitters aren’t “stronger” just because they act fast, and hormones aren’t “less specific” just because they travel everywhere. Specificity depends on receptor presence. Distance and duration depend mainly on 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 embedded through the plasma membrane, with 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 that binds a signalling chemical after it 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.

Amino acid distribution explains where the receptor sits. Transmembrane receptors have hydrophobic amino acids in the membrane-spanning region, where they touch the fatty acid tails of phospholipids. The exposed parts 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 starts the chain.

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 altered receptor can then 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 usually give a more direct pathway in one sense, but the outcome is 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.

Across biological communication, the linking pattern is clear: a source cell releases a signal, a target cell detects it using a receptor, and the receiving system changes behaviour. The same logic appears 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: when a 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, changing this voltage is a signal in its own right.

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

Through the open channel, 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.

This local voltage change may set off further changes, such as initiation of an action potential in a neuron or contraction in a muscle fibre. Keep the cause-and-effect order straight: acetylcholine binds $\to$ channel opens $\to$ positive ions enter $\to$ membrane potential changes $\to$ further response may follow.

G protein-coupled receptors are transmembrane receptors that activate $G$ proteins once a ligand binds. They’re usually shortened to $\text{GPCRs}$. Humans have many different $\text{GPCRs}$, 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 and 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 $\text{GDP}$, guanosine diphosphate.

When a ligand binds to the GPCR, the receptor changes shape. $\text{GDP}$ then leaves the alpha subunit, and $\text{GTP}$, guanosine triphosphate, binds instead. $\text{GTP}$ binding activates the $\text{G}$ protein. The activated subunits move away from the receptor and interact with effector proteins inside the cell.

The GPCR itself doesn’t have to carry the whole message through to the final response. Its role is to link extracellular detection with intracellular activation. That helps explain why the same receptor family can be adapted repeatedly for different signals in the human body.

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

Focus on the sequence here. Epinephrine binds to a transmembrane receptor in the plasma membrane. That receptor changes shape and activates a G protein. The activated G protein then activates 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, producing many cAMP molecules and many enzyme activations. In liver cells, this pathway quickly promotes glycogen breakdown, helping glucose become available in the blood.

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

Some receptors act as enzymes. For this syllabus point, the required example 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 is the addition of a phosphate group to a molecule, often changing the 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 precise: the sequence ends with vesicles containing glucose transporters moving to and fusing with the plasma membrane. These 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 and activates it. Once activated, the receptor–hormone complex enters or acts within the nucleus, where it binds to specific DNA sequences. Particular genes are then transcribed more, so more mRNA is produced and specific proteins may be synthesized.

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 therefore often slower than ion-channel responses: changing gene expression takes time, but the effects can be substantial and long-lasting.

Testosterone is 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 uses a narrow set of examples, so keep them focused.

Oestradiol

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. Once activated, the receptor complex promotes transcription linked to gonadotropin-releasing hormone secretion.

Gonadotropin-releasing hormone (GnRH)

Gonadotropin-releasing hormone (GnRH) is a hypothalamic hormone that stimulates the anterior pituitary to release gonadotropins. Around ovulation, oestradiol can stimulate GnRH secretion. This shows how one hormone can control the release of another through gene-expression effects in target cells.

Progesterone

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

Cell signalling pathways need regulation; without it, a small signal could keep running for too long or never build strongly enough.

Positive feedback

is regulation in which a response increases the original stimulus or promotes further response in the same pathway. It pushes a process forward. One example is $\text{oestradiol}$ stimulation of $\text{GnRH}$ release near ovulation: oestradiol from the developing follicle promotes GnRH release, which supports further reproductive hormone signalling at that stage.

Negative feedback

is regulation in which a response reduces the original stimulus or inhibits an earlier step in the pathway. It keeps systems stable. One example is $\text{testosterone}$ regulation: hypothalamus and pituitary signalling stimulate testosterone production, but increased testosterone inhibits GnRH release from the hypothalamus and reduces LH release from the anterior pituitary. That reduces further testosterone production.

The difference is simple, though exam answers often blur it. Positive feedback amplifies change; negative feedback opposes change. Positive feedback helps drive decisive events. Negative feedback helps maintain variables within limits.

For the linking question about negative feedback across levels, use the same logic each time: a product or response feeds back to reduce its own production. At molecular and cellular levels, it can regulate signalling pathways. At organism level, it helps maintain body conditions. At population level, density-dependent effects can limit population growth. Different scale, same control pattern.

The same communication pattern appears across biology too: a signal is produced, transmitted, detected by a receptor or sensor, transduced into a response, and then regulated. Chemical signalling, neural signalling, endocrine control, quorum sensing and ecological communication all use variations of that pattern.