Learning focus
Build precise biological explanations, interpret diagrams and data, and connect structure, process and practical evidence.
Storage and churning
The stomach temporarily stores food. Its muscular wall contracts to churn food and mix it with gastric secretions. This physical digestion forms a semi-liquid mixture and increases contact between protein and protease.
Protein digestion
A stomach protease such as pepsin breaks proteins into smaller peptides and ultimately contributes to amino-acid production. The syllabus summarises protease digestion as protein to amino acids. Different proteases act at different stages and pH values.
Hydrochloric acid
Hydrochloric acid creates a low pH suitable for stomach protease and kills many ingested bacteria. The stomach lining is protected by mucus. Acid does not digest protein by itself; it provides conditions and also helps alter protein structure.
Protease pH differences
Pepsin works best in acidic conditions, whereas trypsin in the duodenum works best in alkaline conditions. Enzyme active sites have different structures and are adapted to their environments. A pH far from the optimum reduces activity and may denature the enzyme.
Leaving the stomach
The stomach releases small amounts of its contents into the duodenum. Pancreatic alkaline secretion helps neutralise the acidic mixture, protecting the intestinal lining and providing suitable conditions for intestinal enzymes.
Practical or data skill
Interpret two enzyme activity curves and identify the likely stomach and intestinal protease. Explain both the optimum and the reduction away from it using active-site shape.
Examination tip
Hydrochloric acid kills many ingested bacteria; it is not an enzyme.
Review questions and suggested answers
Question 1
State one role of hydrochloric acid in the stomach.
Suggested answer
It kills many ingested bacteria and provides acidic conditions for pepsin.
Question 2
Why does trypsin have a different optimum pH from pepsin?
Suggested answer
It functions in the alkaline small intestine rather than the acidic stomach.
Question 3
What physical process occurs in the stomach?
Suggested answer
Muscular churning of food.
