Describe the role of hydrochloric acid in the stomach and explain why the enzyme lipase is secreted into the duodenum rather than acting in the stomach.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Hydrochloric acid in the stomach
1. Kills harmful microorganisms and bacteria that enter with food.
2. Creates a very low (acidic) pH, which is the optimum environment for the protease pepsin to hydrolyse proteins.
Lipase secretion into the duodenum
1. The stomach’s pH is too low for lipase to work; the enzyme is inactivated in such an acidic environment.
2. Bile released into the duodenum neutralises the stomach acid, raising the pH to neutral or slightly alkaline, which is the optimum for lipase activity.
1. Kills harmful microorganisms and bacteria that enter with food.
2. Creates a very low (acidic) pH, which is the optimum environment for the protease pepsin to hydrolyse proteins.
Lipase secretion into the duodenum
1. The stomach’s pH is too low for lipase to work; the enzyme is inactivated in such an acidic environment.
2. Bile released into the duodenum neutralises the stomach acid, raising the pH to neutral or slightly alkaline, which is the optimum for lipase activity.
Examiner tips
- Use the exact terms ‘hydrochloric acid’, ‘pepsin’, ‘low pH’, ‘lipase’, ‘duodenum’, ‘bile’, ‘neutralise’
- Show the cause–effect link: acid kills microbes and activates pepsin; acid inhibits lipase, bile neutralises acid to allow lipase
Common mistakes
- Confusing pepsin with trypsin or other proteases
- Saying lipase works in the stomach instead of the duodenum
- Using ‘high pH’ instead of ‘low pH’ for the stomach
Mark scheme (4 marks)
- Hydrochloric acid kills harmful microorganisms / bacteria in food
- Hydrochloric acid provides an optimum (low / acidic) pH for protease / pepsin activity
- The stomach is too acidic / has too low a pH for lipase to function effectively
- Bile neutralises stomach acid in the duodenum, providing a neutral / slightly alkaline pH that is the optimum for lipase activity
Key terms in this question
hydrochloric acid · lipase · duodenum
Related
- All Cambridge International IGCSE Biology (0610) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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