Explain how signal amplification occurs during the response of a target cell to adrenaline, including the roles of adenylyl cyclase, cyclic AMP, and protein kinase A.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Adrenaline binds to its G‑protein coupled receptor, which activates the Gαs subunit. The Gαs subunit then activates adenylyl cyclase. Each activated adenylyl cyclase converts many ATP molecules into cyclic AMP, so one hormone–receptor event produces a large number of cAMP molecules. cAMP binds to the regulatory subunits of protein kinase A, releasing the catalytic subunits. Each catalytic PKA can phosphorylate many target proteins or enzymes. Thus, at each step the number of active molecules multiplies, giving signal amplification so that a few adrenaline molecules produce a large cellular response.
Examiner tips
- Show the cascade in order: receptor → G‑protein → adenylyl cyclase → cAMP → PKA.
- Mention the multiplication of molecules at each step to justify amplification.
- Use the exact terms ‘adenylyl cyclase’, ‘cAMP’, ‘protein kinase A’ and ‘phosphorylate’.
- Keep the answer concise – 4 marks only.
Common mistakes
- Confusing adenylyl cyclase with phosphodiesterase.
- Saying that only one cAMP molecule activates one PKA, ignoring the multiplicity.
- Using ‘second messenger’ without linking to amplification.
Mark scheme (4 marks)
- Adrenaline binds its receptor, activating a G-protein, which in turn activates adenylyl cyclase
- Each activated adenylyl cyclase converts many ATP molecules into cyclic AMP (cAMP), so one hormone–receptor event produces many second messenger molecules
- cAMP activates protein kinase A (PKA), and each active PKA molecule can phosphorylate many substrate proteins / enzymes
- Amplification arises because each step in the cascade multiplies the number of active molecules, so a very small number of hormone–receptor binding events produces a very large cellular response
Key terms in this question
signal amplification · adenylyl cyclase
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Chemical signalling (HL only) questions
- Explain how down-regulation of receptors allows a target cell to reduce its sens…
- Explain how a lipid-soluble hormone produces a response in a target cell, includ…
- Explain why a water-soluble hormone, such as adrenaline, cannot enter its target…
- Explain how the number of receptors on a target cell determines its sensitivity …