# Explain how the ultrastructure of a parietal cell (oxyntic cell) in the gastric gland is related to its function of secreting hydrochloric acid.

> IB DP Biology Higher Level (2023 syllabus) — B2.3 Cell specialisation · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. Parietal cells possess an exceptionally large number of mitochondria, providing the ATP required for active transport of H⁺ ions against a steep concentration gradient.
2. 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.
3. 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.
4. 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

- [parietal cell](https://www.gradenine.co.uk/glossary/parietal-cell)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-ultrastructure-of-a-575f7cd3) · Published by Druglandscape Ltd.