BIOL10008 · Foundations Of Biology: Life's Machinery
Cell Signalling
A cell is sealed inside a membrane yet must know what is happening outside. Cell signalling is the system that converts an outside message into an inside action, and every pathway runs the same three stages: reception → transduction → response. This chapter covers how a ligand docks on a receptor (surface receptors for large/charged ligands; intracellular receptors for small non-polar ones), how the signal classes are defined by reach (autocrine, paracrine, endocrine, direct contact), and how specificity works — a cell responds only if it carries the matching receptor. It explains transduction as a relay that amplifies the signal through a second messenger (classically cAMP), so one ligand can change thousands of molecules inside, and the signature exam point: the same second messenger produces different responses in different cells because each cell carries different target proteins. It closes with signalling as the machinery of homeostasis — the molecular sense-and-respond loop closed by negative feedback.
What this chapter covers
- 01Reception → transduction → response: the universal three-step
- 02Signals classed by reach: autocrine, paracrine, endocrine, direct contact
- 03Two receptor locations and the membrane rule
- 04Specificity — a cell hears a signal only with the matching receptor
- 05Transduction, second messengers (cAMP) and signal amplification
- 06Why signalling matters: homeostasis and negative feedback
Worked example: one hormone, one second messenger, different responses
- +1(a) Different responses: cAMP is the same molecule in every cell — it does not carry the instruction. The response is set by the cell's own target proteins waiting downstream: liver target proteins release glucose, heart target proteins speed contraction.
- +1(a) Same first and second messenger: same adrenaline (first messenger), same cAMP (second messenger), different downstream proteins → different outcomes.
- +1(b) Why one cell ignores it: a cell with no matching receptor cannot receive the signal, no matter how much adrenaline is present — this is specificity: no receptor, no response.
- +1(c) The wrong answer: 'the cAMP is different in different cells' — this is the planted trap; cAMP is identical everywhere.
- +1(c) The right answer: the specificity of the response lives in the cell's different downstream target proteins, not in the messenger.
Key terms
- Reception → transduction → response
- The universal three-step of cell signalling: a ligand binds a receptor (reception), an intracellular relay carries and amplifies the message (transduction), and a target protein carries out the action (response).
- Ligand & receptor
- The ligand is the signal molecule; the receptor is the shape-matched protein that binds it. Surface receptors serve large or charged ligands that cannot cross the membrane; intracellular receptors serve small non-polar ligands (e.g. steroids) that can.
- Specificity
- A cell responds to a signal only if it carries the matching receptor, and the response is set by the cell's own target proteins. This is why the same hormone affects only some tissues, and why the same messenger can give different responses.
- Second messenger (cAMP)
- A small, diffusible intracellular molecule (classically cyclic AMP, made from ATP) that relays the signal inside the cell and spreads it quickly to many targets, amplifying it as it goes.
- Signal amplification
- Because each active molecule in a transduction cascade switches on many of the next, one ligand at the surface can change thousands of molecules inside — the cascade is a chemical amplifier, which is why hormones work at tiny concentrations.
Cell Signalling FAQ
What are the three stages of cell signalling?
Reception → transduction → response. In reception, a ligand binds a receptor (usually on the cell surface), which changes the receptor's shape on the inside. In transduction, that change triggers an intracellular relay — often via a second messenger like cAMP — that carries and amplifies the message. In response, the final activated target protein carries out the cellular action: switching a gene on, opening a channel, or driving metabolism.
Why does a cell respond to some signals and ignore others?
Specificity — a cell hears a signal only if it owns the matching receptor. Adrenaline floods every tissue in the blood, but a cell with no adrenaline receptor simply ignores it. It is a shape-matched lock (receptor) and key (ligand): no receptor means no response, no matter how much ligand is present. Cells choose which signals to hear by controlling which receptors they make.
Why does the same second messenger cause different effects in different cells?
Because the messenger does not carry the instruction — the cell's downstream target proteins do. cAMP is identical in a liver cell and a heart cell; in the liver it activates proteins that release glucose, in the heart it activates proteins that speed contraction. This is the signature exam answer: if asked why one hormone has different effects in different tissues, say 'different downstream target proteins,' never 'different cAMP.'
What is the point of a 'second' messenger and of amplification?
The ligand (the first messenger) is stuck outside the cell, so a small, soluble intracellular molecule (the second messenger, e.g. cAMP) carries the news inside and spreads it quickly. Amplification then multiplies the signal: because each active enzyme in the cascade is a catalyst that switches on many of the next, one hormone molecule can trigger thousands of downstream molecules. That is how hormones act effectively at vanishingly small concentrations.
Exam move
These are reasoning questions, not recall. Lock the three-step (reception → transduction → response) and be able to explain two things in plain words: specificity (a cell responds only with the matching receptor; no receptor, no response) and the signature point that the same second messenger gives different responses because of different downstream target proteins. Keep the doorbell/radio-tuner analogies for the mechanism, and remember the planted trap: never say 'different cAMP' — cAMP is identical everywhere; the response lives in the target proteins. Finally, connect signalling to homeostasis as a negative-feedback sense-and-respond loop.
