IGCSE · Practice questions · Chemical digestion

The right enzyme, at the right pH.

Six original Cambridge-style questions on chemical digestion: enzymes, substrates and products, sites of secretion and action, stomach acid, pH data and (Extended) pepsin, trypsin, maltase and bile.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Enzyme, substrate, product, place.

Amylase: starch to simple reducing sugars (maltose); salivary glands and pancreas.
Proteases: protein to amino acids; stomach and pancreas. Lipase: fats to fatty acids and glycerol; pancreas.
HCl: kills microorganisms and gives the acidic optimum pH for the stomach protease.
Extended: pepsin in acid, trypsin in alkali; maltase on the epithelium; alkaline bile neutralises acid.

Recall

[3 marks]

Describe what is meant by chemical digestion and state why it is needed.

The breakdown of large insoluble molecules into small soluble molecules. ✓
By enzymes. ✓
So that the small soluble molecules can be absorbed through the wall of the small intestine into the blood. ✓

Recall

[6 marks]

Copy and complete a table for amylase, protease and lipase, giving for each the substrate, the products, one place where it is secreted and one place where it acts.

Amylase: starch to simple reducing sugars (maltose). ✓
Amylase: secreted by the salivary glands (or pancreas); acts in the mouth (or duodenum). ✓
Protease: protein to amino acids. ✓
Protease: secreted by the stomach (or pancreas); acts in the stomach (or small intestine). ✓
Lipase: fats and oils to fatty acids and glycerol. ✓
Lipase: secreted by the pancreas; acts in the small intestine (duodenum). ✓

Explain

[2 marks]

Describe the functions of hydrochloric acid in gastric juice.

It kills harmful microorganisms in the food. ✓
It provides an acidic pH, the optimum for the protease (pepsin) in the stomach. ✓

Data

[4 marks]

The activity of two proteases was measured at different pH values.

pH	1	2	4	6	8	10
Protease X activity / %	70	100	45	5	0	0
Protease Y activity / %	0	0	10	55	100	60

Extended. (a) State the optimum pH of each enzyme. (b) Identify which enzyme is pepsin and which is trypsin, and explain your choice.

(a) X: pH 2. Y: pH 8. ✓
(b) X is pepsin: its optimum is acidic, matching the stomach, where hydrochloric acid gives about pH 2. ✓
Y is trypsin: its optimum is alkaline, matching the small intestine. ✓
There the stomach acid has been neutralised by alkaline bile (and pancreatic juice). ✓

Explain

[3 marks]

Extended. Describe the complete digestion of starch to glucose in the alimentary canal.

Amylase from the salivary glands (in the mouth) and from the pancreas (in the duodenum) breaks starch down to maltose. ✓
Maltase breaks maltose down to glucose. ✓
Maltase acts on the membranes of the epithelium lining the small intestine. ✓

Analysis

[3 marks]

Extended. A person has their gall bladder removed, so bile is released into the duodenum only in small amounts. Suggest and explain how this affects digestion in the small intestine.

Less bile to neutralise the acidic mixture arriving from the stomach, so the pH in the duodenum is less suitable and the enzymes there (trypsin, amylase, lipase) work more slowly. ✓
Fats are less well emulsified, so there is a smaller surface area for lipase. ✓
So fat digestion in particular is slower and less complete (fatty meals may cause discomfort). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.