In simple terms
A friendly intro before the formal notes — no formulas yet.
Messages With the Right Address
Cells cannot shout across the body, so they send chemical messages instead. A hormone released into the blood reaches almost every cell, yet only the cells carrying the matching receptor can read it and respond — like a letter that anyone can carry but only the addressee can open.
Picture a radio broadcast going out across a whole city. The signal reaches every building, but only the houses that own a receiver tuned to that exact frequency actually pick up the programme. A hormone works the same way: it travels everywhere in the bloodstream, but a cell responds only if it owns a receptor whose shape matches the hormone. Turning that received signal into an action inside the cell — switching on an enzyme or a gene — is the job of signal transduction, the wiring that connects the aerial to the loudspeaker.
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An endocrine gland secretes a hormone (the chemical signal) into the bloodstream, which carries it throughout the body.
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The hormone passes almost every cell, but only a target cell carries a receptor with a shape complementary to the hormone.
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The hormone binds to that specific receptor, and the binding is detected at the cell surface or inside the cell.
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Binding triggers signal transduction — a chain of events, often using a second messenger — that produces the cellular response only in the target cell.
Simulations
Every simulation here runs the real model — try the steps on a card, then check what you see against the notes.
1 simulation
- PhETIB C2.1
Membrane Transport
Insert ligand-gated channels into a cell membrane and release ligands to see when the channels open.
Try this
- On Facilitated Diffusion, put Na+ outside the cell and add a ligand-gated Na+ channel: nothing crosses while it is shut.
- Release the ligands and watch for one binding to the channel.
- Take the ligands away again and watch what the channel does.
Look for A signal molecule acts only on a cell with a receptor whose binding site fits it. Binding changes the receptor's shape, here opening an ion channel as acetylcholine does at a synapse, and the response stops when the ligand leaves.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
Full topic notes
Formal explanation with the rigour you need for the exam.
Cell-to-cell communication by chemical signals
In a multicellular organism, cells must coordinate their activities even when they are far apart. They do this by chemical signalling: a signalling cell releases a chemical (a ligand), and a receiving cell detects it using a receptor protein. Some signals travel only a short distance to neighbouring cells; others are carried across the whole body in the blood. Whatever the distance, the logic is always the same — a signal is released, it reaches a cell, and a response occurs only if that cell can bind the signal. The rest of this topic follows one especially important long-distance route: hormonal signalling through the endocrine system.
Cells communicate by releasing chemical signals (ligands) that other cells detect with receptor proteins.
Signals may act locally on nearby cells or travel long distances in the blood.
A response occurs only where a cell carries a receptor able to bind the signal — communication requires both a signal and a matching receptor.
Chemical signalling lets distant cells coordinate growth, metabolism and responses to change without direct contact.
The endocrine system and hormones
The endocrine system is the body's long-distance chemical messaging network. It is made of endocrine glands — such as the pancreas, adrenal glands and thyroid — that secrete hormones directly into the blood (they are ductless glands). The blood then carries each hormone throughout the body. A hormone is therefore a chemical signal that is secreted into the blood and acts on target cells: cells that carry the specific receptor for that hormone. Because a hormone can be carried anywhere the blood goes, one gland can influence many tissues at once, and its effect can last as long as the hormone remains in circulation.
Endocrine glands secrete hormones directly into the blood, which distributes them around the body.
A hormone acts on target cells — cells that carry the specific receptor for that hormone.
Cells reached by the hormone but lacking its receptor are unaffected.
Because the signal travels in the blood and reaches many tissues, endocrine effects are widespread and can be long-lasting.
Receptor–ligand specificity and complementary shape
The reason a hormone in the blood affects only its target cells lies in the shape of the receptor. A receptor has a binding site whose shape is complementary to the hormone (the ligand), rather like the fit between a substrate and an enzyme's active site, or an antigen and an antibody. Only a molecule with a matching shape can bind. This receptor–ligand specificity means a given receptor responds to one signal (or a small family of related signals) and ignores all the others washing past in the blood. So even though a hormone reaches almost every cell, it produces a response only in cells that display a receptor complementary to it — and different cell types can be given different responsibilities simply by carrying different sets of receptors.
A receptor binds a signal because the two have complementary shapes — a specific fit at the binding site.
This specificity means one receptor responds to one signal (or a narrow group), not to others.
A hormone acts only where cells carry the matching receptor, so the receptor pattern — not the delivery — decides which cells respond.
Changing which receptors a cell displays changes which signals it can respond to.
Where the receptor sits: peptide vs steroid hormones
Whether a receptor lies on the cell surface or inside the cell depends on the chemistry of the hormone. Peptide hormones (made of amino acids, e.g. insulin and glucagon) are water-soluble and cannot cross the hydrophobic phospholipid bilayer, so their receptors must sit on the cell surface, spanning the membrane. Steroid hormones (derived from cholesterol, e.g. oestrogen and testosterone) are lipid-soluble and diffuse straight through the membrane, so their receptors are intracellular — in the cytoplasm or nucleus. The consequences differ: a peptide hormone must relay its message inward through signal transduction, whereas a steroid hormone–receptor complex commonly acts in the nucleus to alter gene expression, switching particular genes on or off.
Peptide hormones are water-soluble, cannot cross the membrane, and bind cell-surface (transmembrane) receptors; the message is relayed inward by signal transduction.
Steroid hormones are lipid-soluble, diffuse through the membrane, and bind intracellular receptors in the cytoplasm or nucleus.
A steroid hormone–receptor complex typically alters gene expression, changing which proteins the cell makes.
The hormone's solubility, not the cell type, decides whether its receptor is on the surface or inside.
Signal transduction and second messengers
Binding a hormone to its receptor is only the first step; the cell still has to turn that event into an action. Signal transduction is the process that converts the signal at the receptor into a cellular response. For a cell-surface receptor, binding changes the receptor's shape and sets off a relay of events inside the cell. A key part of many of these pathways is a second messenger — a small intracellular molecule such as cyclic AMP (cAMP) — produced in response to the binding. The second messenger spreads the signal through the cytoplasm and amplifies it, so that one hormone molecule at the surface can trigger a large response inside. The eventual response depends on the cell: it might activate an enzyme, change how much of a substance the cell takes up, or alter gene expression. You are expected to understand this as an overview — a signal at the receptor triggers, via transduction and (for surface receptors) a second messenger, a response inside the cell.
Signal transduction converts binding at the receptor into a cellular response.
For a cell-surface receptor, binding changes the receptor and starts an intracellular relay.
A second messenger (e.g. cAMP) carries the signal into the cytoplasm and amplifies it.
The final response varies by cell type — activating an enzyme, changing uptake, or altering gene expression.
For a peptide-hormone question, do not write that the hormone 'enters the cell and acts directly' — it cannot cross the membrane. The creditable idea is that binding at the SURFACE triggers signal transduction and a SECOND MESSENGER carries the signal inside. Naming the second messenger step is often where the transduction mark is won or lost.
Hormonal signalling versus neural signalling
The body has a second way to send messages: the nervous system. Comparing the two shows why it needs both. Hormonal signalling releases a chemical into the blood; the message spreads to any tissue with the right receptor, travels at the speed of the circulation, and can persist while the hormone remains — so it is slow, widespread and long-lasting. Neural signalling sends an electrical impulse along a specific neuron and passes it to the next cell using a neurotransmitter at a synapse; the message goes only where the neuron reaches, arrives in milliseconds, and stops quickly once the impulse ends — so it is fast, localised and short-lived. Fast, precise control (a reflex, a muscle twitch) suits neural signalling; slow, body-wide, sustained control (metabolism, growth, blood glucose over hours) suits hormonal signalling.
Hormonal: chemical signal in the blood — slow, widespread, long-lasting.
Neural: electrical impulse along neurons + neurotransmitter at synapses — fast, localised, short-lived.
Neural signalling suits rapid, precisely targeted responses; hormonal signalling suits sustained, body-wide coordination.
The two systems complement each other, covering different speeds, reaches and durations of control.
Negative feedback control: blood glucose
Hormones are the tools the body uses to hold internal conditions steady, and it does so mainly through negative feedback: a change is detected, and a response is triggered that opposes the change, returning the variable towards its set point. Blood glucose is the classic example, controlled by two antagonistic peptide hormones from the pancreas. When blood glucose rises after a meal, the pancreas secretes insulin; insulin causes body cells to take up glucose and the liver to store it as glycogen, so glucose falls back towards normal. When blood glucose falls — during fasting or exercise — the pancreas secretes glucagon; glucagon causes the liver to break glycogen back down and release glucose, so glucose rises towards normal. Each response removes the stimulus that caused it (rising glucose triggers insulin, which lowers glucose), which is exactly what makes it negative feedback and keeps blood glucose oscillating narrowly around its set point.
Negative feedback: a detected change triggers a response that opposes the change, restoring the set point.
High blood glucose → pancreas releases insulin → cells take up glucose, liver stores it as glycogen → glucose falls.
Low blood glucose → pancreas releases glucagon → liver breaks down glycogen and releases glucose → glucose rises.
Insulin and glucagon are antagonistic, so glucose can be corrected in either direction and held near its set point.
Common mistakes examiners penalise
Saying the hormone 'only travels to' its target cells — the hormone in the blood reaches nearly ALL cells; the difference is that only target cells carry the complementary RECEPTOR. State the receptor idea explicitly.
Forgetting to mention complementary shape / specificity — 'the cell responds because it is a target cell' is circular. The creditable reason is that the receptor's shape is complementary to the hormone.
Swapping the receptor locations — steroid (lipid-soluble) hormones use INTRACELLULAR receptors; peptide (water-soluble) hormones use CELL-SURFACE receptors. Reversing these is a common lost mark.
Claiming a peptide hormone enters the cell to act — it cannot cross the membrane; it binds at the surface and its message is relayed inward by signal transduction / a second messenger.
Mixing up insulin and glucagon — insulin LOWERS blood glucose (released when it is high); glucagon RAISES it (released when it is low). Getting the direction wrong reverses the feedback.
Describing feedback as 'positive' or leaving out the set point — glucose control is NEGATIVE feedback: the response opposes the change and returns the variable towards its set point.
Blurring hormonal and neural signalling — pair the properties correctly: hormonal is slow/widespread/long-lasting, neural is fast/localised/short-lived. Do not attribute one system's properties to the other.
Model answer — marked the way our engine marks it
Most C2.1 marks are explanation marks, and our engine awards them analytically: each distinct valid biological point is worth one mark, up to the number of marks available. Answer marks (A) credit a correct step in the reasoning, and error-carried-forward (ECF) means a slip early on does not automatically cost the later marks, provided the reasoning that follows is sound. Study how each mark below is tied to a specific, named idea — 'complementary receptor', 'binds', 'signal transduction' — rather than to loose phrasing, because that is exactly what the Practice button rewards.
Where this leads
The logic of C2.1 — a signal, a complementary receptor, a transduced response — is the template for cell communication everywhere in biology. The same receptor–ligand specificity underlies how neurotransmitters act at synapses, how the immune system recognises antigens, and how many medicines work by mimicking or blocking a natural ligand at its receptor. Negative feedback, met here in blood glucose, is the pattern behind homeostasis of temperature, water balance and hormone levels throughout the course. Master the idea that a response depends on both a signal and a matching receptor, and you have a framework that explains coordination from a single synapse to the whole endocrine system.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
A newly identified hormone is a short chain of amino acids that slows the division of certain skin cells. Predict where its receptor is located and outline how it produces a response in a target cell. Explain your reasoning. [4]
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Model answer. Because the hormone is made of amino acids, it is a peptide hormone and is water-soluble, so it cannot cross the hydrophobic cell membrane [1]. Its receptor must therefore be on the cell surface, spanning the membrane, and the hormone binds to this receptor because their shapes are complementary [1]. Binding triggers signal transduction inside the cell — often producing a second messenger — which relays and amplifies the signal without the hormone itself entering the cell [1]. This intracellular pathway leads to the cellular response, here slowing cell division, and only skin cells carrying the specific receptor respond [1].
After a carbohydrate-rich meal a person's blood glucose concentration rises sharply, then returns to its normal level over the next two hours. Using the idea of negative feedback, explain how the hormone insulin brings the blood glucose concentration back down. [4]
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Model answer. The rise in blood glucose above the set point is detected by the pancreas, which secretes insulin into the blood [1]. Insulin is carried in the blood and binds to specific receptors on its target cells, such as liver and muscle cells [1]. This stimulates these cells to take up glucose from the blood and the liver to convert glucose into glycogen for storage [1]. As glucose is removed from the blood, its concentration falls back towards the set point; because the response opposes the original rise, this is negative feedback, and the falling glucose reduces further insulin secretion [1].
Explain how a hormone produces a response in its specific target cells but not in other cells. [4]
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Model answer. The hormone is secreted into the blood, which carries it throughout the body so that it reaches essentially all cells. Only the target cells, however, carry a receptor whose shape is complementary (specific) to the hormone. The hormone binds to this specific receptor on (or in) the target cell. Binding triggers signal transduction — a chain of events inside the cell, often involving a second messenger — that produces the cellular response. Because non-target cells lack the specific receptor, the hormone cannot bind them and no response occurs, so the response happens only in the target cells.
How it all connects
The big idea sits in the middle — tap a linked idea to explore the link.
Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.
Glossary
Key terms for this topic — skim now; the Check step will test them.
- Chemical signalling
Communication between cells in which one cell releases a chemical signal (ligand) that is detected by another cell carrying a matching receptor, producing a coordinated response — often over distances too great for direct contact.
- Hormone
A chemical signal secreted by an endocrine gland directly into the blood, which transports it throughout the body to act on target cells that possess the specific receptor for it.
- Endocrine system
The system of ductless glands (e.g. pancreas, adrenal glands, thyroid) that secrete hormones INTO THE BLOOD. Because the signal is carried in the blood, endocrine signalling is slow, widespread and long-lasting.
- Target cell
A cell that can respond to a particular hormone BECAUSE it carries the specific receptor for that hormone. Cells lacking the receptor cannot detect or respond to the hormone, even though the hormone reaches them.
- Receptor–ligand specificity
A receptor binds only ligands whose shape is COMPLEMENTARY to its binding site. The precise fit means one receptor responds to one signal (or a narrow group), which is why a hormone acts only on cells with the matching receptor.
- Signal transduction
The process by which binding of a signal to its receptor is CONVERTED into a cellular response — a relay of intracellular events, often amplifying the signal, that ends in an action such as activating an enzyme or altering gene expression.
- Second messenger
A small intracellular molecule (e.g. cyclic AMP, cAMP) produced when a signal binds a cell-surface receptor. It relays and amplifies the signal inside the cell — the hormone itself never enters the cell.
- Peptide hormone
A water-soluble (hydrophilic) hormone made of amino acids, e.g. insulin, glucagon. It CANNOT cross the membrane, so it binds a CELL-SURFACE receptor and works through signal transduction / a second messenger.
- Steroid hormone
A lipid-soluble (hydrophobic) hormone derived from cholesterol, e.g. oestrogen, testosterone. It DIFFUSES THROUGH the membrane and binds an INTRACELLULAR receptor, and the complex usually alters gene expression.
- Hormonal vs neural signalling
Hormonal: chemical in the blood, SLOW, WIDESPREAD, LONG-LASTING. Neural: electrical impulse along neurons plus neurotransmitter at synapses, FAST, LOCALISED, SHORT-LIVED. Different speeds and reach suit different jobs.
- Negative feedback
A control mechanism in which a change is DETECTED and a response is triggered that OPPOSES the change, returning the variable towards its set point. Blood glucose control by insulin and glucagon is the classic example.
- Insulin vs glucagon
Both are peptide hormones from the pancreas. Insulin is released when blood glucose is HIGH and LOWERS it (uptake and storage as glycogen). Glucagon is released when glucose is LOW and RAISES it (glycogen breakdown). They are antagonistic.
Name it
Read the meaning, then pick which of this lesson’s terms it describes. Miss one and you see what your choice really means.
A water-soluble (hydrophilic) hormone made of amino acids, e.g. insulin, glucagon. It CANNOT cross the membrane, so it binds a CELL-SURFACE receptor and works through signal transduction / a second messenger.
Quick check
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Teach it back
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Teach it back
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Revision flashcards
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Key takeaways
Review these before you close the topic — retrieval beats re-reading.
Cells communicate by releasing chemical signals (ligands) that other cells detect with receptor proteins.
Signals may act locally on nearby cells or travel long distances in the blood.
A response occurs only where a cell carries a receptor able to bind the signal — communication requires both a signal and a matching receptor.
Chemical signalling lets distant cells coordinate growth, metabolism and responses to change without direct contact.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Get a Paper 2 question marked: explain hormone specificity, signal transduction, or negative feedback control of blood glucose with full reasoning
Get a Paper 2 question marked: explain hormone specificity, signal transduction, or negative feedback control of blood glucose with full reasoning
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