Explain how the ultrastructure of a parietal cell (oxyntic cell) in the gastric gland is related to its function of secreting hydrochloric acid.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Parietal cells line the gastric glands of the stomach and are responsible for secreting hydrochloric acid (HCl) into the gastric lumen. They are among the most metabolically active cells in the body.
Model answer (4 marks)
Parietal cells contain a very high number of mitochondria, which supply the ATP needed for the H⁺/K⁺‑ATPase pumps that move H⁺ into the lumen against a steep gradient.
They have extensive smooth endoplasmic reticulum and deep membrane invaginations (canaliculi) that increase the apical surface area, allowing many H⁺/K⁺‑ATPase pumps to be inserted and thus a high rate of H⁺ secretion.
Cl⁻ ions enter through apical chloride channels; the secreted H⁺ combines with Cl⁻ to form HCl. Cytoplasmic carbonic anhydrase converts CO₂ and H₂O to H⁺ and HCO₃⁻, providing a continuous supply of protons.
The basolateral membrane contains Na⁺/K⁺‑ATPase pumps and HCO₃⁻/Cl⁻ exchangers that maintain ionic gradients and cell pH, which is essential for sustained acid secretion.
They have extensive smooth endoplasmic reticulum and deep membrane invaginations (canaliculi) that increase the apical surface area, allowing many H⁺/K⁺‑ATPase pumps to be inserted and thus a high rate of H⁺ secretion.
Cl⁻ ions enter through apical chloride channels; the secreted H⁺ combines with Cl⁻ to form HCl. Cytoplasmic carbonic anhydrase converts CO₂ and H₂O to H⁺ and HCO₃⁻, providing a continuous supply of protons.
The basolateral membrane contains Na⁺/K⁺‑ATPase pumps and HCO₃⁻/Cl⁻ exchangers that maintain ionic gradients and cell pH, which is essential for sustained acid secretion.
Examiner tips
- Mention mitochondria for ATP supply
- Explain increased surface area via SER and canaliculi
- Include chloride channels and carbonic anhydrase
- State basolateral pumps maintain gradients
Common mistakes
- Forgetting the role of mitochondria
- Confusing apical with basolateral membrane functions
- Omitting chloride channels or carbonic anhydrase
Mark scheme (4 marks)
- Parietal cells possess an exceptionally large number of mitochondria, providing the ATP required for active transport of H⁺ ions against a steep concentration gradient.
- Extensive smooth endoplasmic reticulum (SER) and intracellular canaliculi (deep membrane invaginations) greatly increase the surface area of the apical membrane, allowing more H⁺/K⁺-ATPase pumps to be inserted and increasing the rate of H⁺ secretion.
- Cl⁻ ions are transported across the apical membrane via chloride channels, combining with the secreted H⁺ in the lumen to form HCl; carbonic anhydrase in the cytoplasm catalyses production of H⁺ and HCO₃⁻ from CO₂ and water, sustaining the supply of protons.
- The basolateral membrane contains Na⁺/K⁺-ATPase pumps and HCO₃⁻/Cl⁻ exchangers, maintaining ionic gradients and cell pH homeostasis, which is necessary for sustained secretory activity.
Key terms in this question
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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