Gut Reaction

Gut Reaction: digestive enzymes and bile

This free game is part of our Pearson Edexcel iGCSE Biology (4BI1) course for exams in EXAM YEARS 2026-2029.

Why we need digestive enzymes

Starch, proteins and lipids are large, insoluble molecules. They are far too big to pass through the wall of the small intestine into the blood. Digestive enzymes break them down into small, soluble molecules that can be absorbed.

Enzymes are biological catalysts: they speed up reactions without being used up, so the same enzyme molecule can work again and again. Each enzyme is specific. Its active site has a shape that only fits one kind of substrate, which is why amylase digests starch but not protein. In Gut Reaction, an enzyme that doesn't fit simply bounces off.

The three groups of digestive enzymes

For the Pearson Edexcel International GCSE (spec point 2.29) you need to know these reactions:

• Carbohydrases: amylase breaks starch into maltose, then maltase breaks maltose into glucose. Amylase is made in the salivary glands and the pancreas. Maltase is made by the small intestine.
• Proteases: break proteins into amino acids. Pepsin is made in the stomach. The pancreas makes proteases such as trypsin, and the small intestine makes more to finish the job.
• Lipases: break lipids (fats and oils) into fatty acids and glycerol. They are made in the pancreas and work in the small intestine.

pH and the role of bile

Each enzyme works best at its own optimum pH. The stomach makes hydrochloric acid, so it is about pH 2. That suits pepsin, and it also kills bacteria in food. But the acid changes the shape of amylase's active site, so amylase from saliva stops working in the stomach: it is denatured.

Enzymes in the small intestine work best in slightly alkaline conditions. That is where bile comes in (spec points 2.30 and 2.31):

• Bile is made in the liver and stored in the gall bladder.
• It is alkaline, so it neutralises stomach acid as food enters the small intestine.
• It emulsifies lipids: it breaks big fat globules into tiny droplets, giving lipase a much larger surface area to work on.
• Bile is not an enzyme. It doesn't break any chemical bonds.

How to play Gut Reaction

Food molecules float through a 3D cutaway of the gut, pushed along by waves of peristalsis. Pick an enzyme at the bottom of the screen (or press 1 to 4), then tap a molecule to fire it. The enzyme flies out of the organ that makes it and, if its active site fits, cuts the molecule into its products.

• The mouth: use salivary amylase on starch. Protein and fat pass on.
• The stomach: at pH 2, pepsin cuts proteins into peptides, and any amylase you fire is denatured.
• The duodenum: acid keeps arriving from the stomach and the pancreatic enzymes stop working. Squirt bile to neutralise it, and to emulsify fat globules before you fire lipase.
• The ileum: maltase finishes off maltose, and the villi absorb the glucose, amino acids, fatty acids and glycerol. Leave the fibre alone: no human enzyme fits it.
• Big Meal Rush: everything at once. Lose three hearts and it's over. Build combos, answer a gut check after every zone and climb from Gut Trainee to Master of Digestion.

Exam tips

• Name the products exactly: starch to maltose (amylase), maltose to glucose (maltase), proteins to amino acids, lipids to fatty acids and glycerol.
• Bile is made in the liver and stored in the gall bladder. Don't mix the two up, and never call bile an enzyme.
• When you explain emulsification, say it increases the surface area for lipase, so fat is digested faster.
• When pH or temperature stops an enzyme working, say the active site changes shape, so the substrate no longer fits and the enzyme is denatured.