Explain why inorganic ions are essential for the transmission of nerve impulses and for the production of thyroid hormones in the human body.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. Sodium ions (Na⁺) are essential for the transmission of nerve impulses.
2. During the resting state the neurone has a negative membrane potential.
3. When a stimulus reaches the axon hillock Na⁺ channels open, Na⁺ rushes into the cell, causing depolarisation and the generation of an action potential.
4. Potassium ions (K⁺) then leave the cell through K⁺ channels, repolarising the membrane and restoring the resting potential.
5. Iodide ions (I⁻) are required for the synthesis of thyroid hormones (T₃ and T₄), which regulate the body’s metabolic rate.
2. During the resting state the neurone has a negative membrane potential.
3. When a stimulus reaches the axon hillock Na⁺ channels open, Na⁺ rushes into the cell, causing depolarisation and the generation of an action potential.
4. Potassium ions (K⁺) then leave the cell through K⁺ channels, repolarising the membrane and restoring the resting potential.
5. Iodide ions (I⁻) are required for the synthesis of thyroid hormones (T₃ and T₄), which regulate the body’s metabolic rate.
Examiner tips
- Use the exact terms – ‘sodium ions’, ‘potassium ions’, ‘iodide ions’, ‘depolarisation’, ‘repolarisation’, ‘action potential’, ‘thyroid hormones’.
- Show the sequence: Na⁺ influx → depolarisation → action potential → K⁺ efflux → repolarisation.
- Mention the role of thyroid hormones in metabolic regulation to link the second part of the question.
Common mistakes
- Confusing Na⁺ with K⁺ as the ion that depolarises; K⁺ actually repolarises.
- Omitting the role of iodide in thyroid hormone synthesis.
- Using vague terms like ‘electrolytes’ instead of specifying Na⁺, K⁺, I⁻.
Mark scheme (5 marks)
- Sodium ions (Na⁺) are involved in nerve impulse transmission
- Sodium ions move into the neurone / axon causing depolarisation / a change in membrane potential / generating an action potential
- Potassium ions (K⁺) also contribute to nerve impulse transmission / repolarisation of the membrane
- Iodine ions / iodide ions are needed to produce thyroid hormones
- Thyroid hormones regulate / control metabolic rate
Key terms in this question
inorganic ions · nerve impulse · thyroid hormones
Related
- All AQA A-Level Biology (7402) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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