BIOSCI107 · Biology for Biomedical Science: Cellular Processes and Development
Cell Communication & Signalling
Topic 2 continues with how cells talk to each other: the three universal steps (reception → transduction → response), the main receptor classes (G-protein-coupled receptors and ligand-gated ion channels), phosphorylation cascades, and the second messengers cAMP and Ca²⁺/IP₃. The recurring exam idea is amplification — one hormone triggering millions of downstream product molecules. Assessed in the 30% mid-semester test (paper Teleform MCQ), often as items on the GPCR cycle, second messengers, and where a given ligand's receptor sits.
What this chapter covers
- 01Signalling range: paracrine (local), synaptic (neurotransmitters), autocrine (on self), endocrine (hormones via blood to distant targets)
- 02The three steps: reception (ligand binds, shape change) → transduction (cascade + second messengers) → response
- 03Receptor location by ligand solubility: water-soluble → membrane receptors; lipid-soluble (steroids) → cytoplasmic/nuclear receptors
- 04G-protein-coupled receptors (7-transmembrane) and the G-protein switch: GDP = off, GTP = on; GTPase resets
- 05Ligand-gated (ionotropic) ion channels — fast, used heavily by the nervous system
- 06Phosphorylation cascades: kinases add phosphate (activate), phosphatases remove it (reset); amplification and multiple control points
- 07Second messengers: cAMP (adenylyl cyclase → cAMP → PKA; broken down by phosphodiesterase) and Ca²⁺/IP₃ (PLC → DAG + IP₃ → ER Ca²⁺ release)
- 08Amplification in action (adrenaline → cAMP → glycogen breakdown) and cholera toxin as a signalling disruptor
Why one hormone molecule releases millions of glucose units
- +1Reception + the G-protein switch. Adrenaline binds the GPCR, changing its shape; the activated receptor causes GTP to displace GDP on the associated G protein, switching it on. The active G protein then activates the enzyme adenylyl cyclase. [+1]
- +1First amplification — the second messenger. One active adenylyl cyclase converts many ATP molecules into many cAMP molecules. So one receptor event already produces a large pool of cAMP. [+1]
- +1Second amplification — the kinase cascade. cAMP activates protein kinase A (PKA); each active PKA phosphorylates and activates many phosphorylase-kinase molecules, and each of those activates many glycogen-phosphorylase molecules. Each enzyme in the chain acts on many substrates, so numbers multiply at every tier. [+1]
- +1The output. Each active glycogen phosphorylase cleaves many glucose-1-phosphate units off glycogen. Multiplying the amplification at each catalytic step (enzyme → many products, repeated down the cascade) turns one adrenaline molecule into roughly 10⁸ glucose-1-phosphate molecules. [+1]
Key terms
- G-protein-coupled receptor (GPCR)
- A 7-transmembrane receptor coupled to a G protein that acts as a molecular switch (GDP-bound = off, GTP-bound = on). Ligand binding lets GTP displace GDP, activating the G protein, which activates an enzyme such as adenylyl cyclase; intrinsic GTPase activity then hydrolyses GTP to reset. The target of roughly a third of modern drugs.
- Second messenger
- A small intracellular molecule that spreads and amplifies a signal after receptor activation. The two taught examples are cAMP (made from ATP by adenylyl cyclase, activates PKA, degraded by phosphodiesterase) and Ca²⁺/IP₃ (phospholipase C cleaves PIP₂ into DAG + IP₃; IP₃ opens ER Ca²⁺ channels).
- Phosphorylation cascade
- A relay of protein kinases, each transferring a phosphate from ATP to activate the next protein (often on Ser/Thr residues), with phosphatases resetting them. Its advantages are amplification, multiple regulatory checkpoints and specificity.
- Signal transduction
- The middle of the three signalling steps: converting the received signal into an intracellular response, typically via a phosphorylation cascade and/or second messengers, before the final cellular response.
- Phosphodiesterase (PDE)
- The enzyme that breaks down cyclic nucleotides such as cAMP, switching the signal off. Caffeine inhibits PDE (prolonging cAMP signalling); sildenafil (Viagra) inhibits a cGMP-specific PDE.
- Ligand-gated ion channel
- An ionotropic receptor whose gate opens when a ligand binds, letting ions (Na⁺, K⁺, Ca²⁺ or Cl⁻) flow. Fast and direct, with no amplifying cascade — the nervous system relies on these for rapid signalling.
Cell Communication & Signalling FAQ
What is the difference between a GPCR and a ligand-gated ion channel?
Both are membrane receptors for water-soluble ligands, but they act on very different timescales. A ligand-gated ion channel opens a pore directly when the ligand binds — fast, ionic and with no amplification, ideal for nerve and muscle signalling. A GPCR does not itself carry ions: it activates a G protein, which activates an enzyme, which generates a second messenger and a phosphorylation cascade — slower but massively amplified, and with several points to regulate. Learn which ligands use which (steroids, being lipid-soluble, use neither — they cross the membrane to cytoplasmic/nuclear receptors).
How does a cell switch a signal off?
Every 'on' step has an 'off' counterpart. The G protein's own GTPase activity hydrolyses GTP back to GDP, resetting the switch. Phosphatases remove the phosphates that kinases added, resetting the cascade. Phosphodiesterase degrades cAMP. This is why signalling is transient and controllable — and why a toxin that blocks an 'off' step (cholera toxin locks the cAMP pathway on) is so damaging.
Why is amplification a favourite exam idea?
Because it explains how a vanishingly small amount of hormone produces a large cellular response, and it tests whether you understand that enzymes are catalysts. Each enzyme in a cascade turns over many substrate molecules, so the numbers multiply at every tier — one adrenaline can yield about 10⁸ glucose-1-phosphate. Ligand-gated channels, by contrast, give a fast but non-amplified response.
Can AI help me with cell signalling in BIOSCI 107?
Yes, for study. Sia can walk the GPCR cycle, trace a second-messenger pathway, and quiz you on which receptor a given ligand uses. Use it to prepare for the mid-semester test — it does not sit the test for you, and the test is an AI-free lane under the course's academic-integrity policy. Confirm the rules on Canvas.
Exam move
Learn signalling as a pipeline you can draw: reception → transduction → response, then hang the details on it. Sort receptors by ligand solubility first (water-soluble → membrane GPCR/ligand-gated; lipid-soluble steroids → intracellular receptors), then contrast the two fast-vs-slow membrane routes. Draw the GPCR cycle with the GDP/GTP switch and the GTPase reset, and both second-messenger pathways (cAMP via adenylyl cyclase → PKA; Ca²⁺/IP₃ via phospholipase C). Make amplification concrete with the adrenaline → glycogen example and be able to say where the numbers multiply (every enzymatic tier). Keep a short list of the 'off' switches (GTPase, phosphatase, phosphodiesterase) and the toxin/drug hooks (cholera locks cAMP on, caffeine inhibits PDE). This is test-only material (Topics 1–3); drill it before the mid-semester test and confirm the format on Canvas.
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