Explain why a water-soluble hormone, such as adrenaline, cannot enter its target cell and must rely on a second messenger system to produce a cellular response.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Water‑soluble hormones are polar and cannot cross the hydrophobic core of the phospholipid bilayer. Therefore they bind to a specific receptor protein on the outer surface of the plasma membrane. Hormone–receptor binding activates a G‑protein or adenylyl cyclase, producing a second messenger such as cyclic AMP inside the cell. The second messenger relays and amplifies the signal, activating protein kinases that phosphorylate target proteins to bring about the cellular response.
Examiner tips
- Mention polarity and membrane impermeability first; link receptor location; describe G‑protein/adenylyl cyclase activation; explain cAMP amplification and protein kinase action. Keep answer concise and use correct terminology.
Common mistakes
- Confusing water‑soluble hormones with lipid‑soluble ones; failing to state that the hormone cannot cross the membrane; omitting the role of the second messenger or protein kinases.
Mark scheme (4 marks)
- Water-soluble hormones are polar / hydrophilic and therefore cannot pass through the hydrophobic core of the phospholipid bilayer.
- The hormone binds to a specific receptor protein located on the outer surface of the plasma membrane (extracellular domain).
- Hormone–receptor binding activates a G-protein or adenylyl cyclase, leading to the production of a second messenger such as cyclic AMP (cAMP) inside the cell.
- The second messenger (cAMP) relays and amplifies the signal within the cell, activating protein kinases that phosphorylate target proteins to bring about the cellular response.
Key terms in this question
water-soluble hormone · second messenger
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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