Which enzymes digest each food group, and how do we test a food for starch, sugar, protein and fat?
The action of carbohydrases, proteases and lipases in digestion, the products they form, the effect of temperature and pH on enzymes, and the food tests for starch, reducing sugars, protein and lipids.
A focused answer to the WJEC GCSE Biology section 1.3 topic on digestive enzymes and food tests, covering carbohydrases, proteases and lipases and their products, the effect of temperature and pH on enzymes including denaturing, and the iodine, Benedict's, biuret and emulsion food tests.
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What this dot point is asking
WJEC wants you to name the three main groups of digestive enzyme and the products they make, explain how temperature and pH affect enzymes, and describe the food tests for starch, reducing sugars, protein and lipids.
Digestive enzymes
There are three main groups of digestive enzyme, each working on one food group.
- Carbohydrases (for example amylase) break down carbohydrates. Amylase breaks starch into simple sugars such as maltose and glucose. It is made in the salivary glands, the pancreas and the small intestine.
- Proteases break down proteins into amino acids. They are made in the stomach (as pepsin), the pancreas and the small intestine.
- Lipases break down lipids (fats) into fatty acids and glycerol. They are made in the pancreas and the small intestine.
The effect of temperature and pH
Because enzymes are proteins, their activity depends on temperature and pH.
Temperature. As temperature rises from low values, the enzyme and substrate gain kinetic energy and collide more often, so the rate increases up to the optimum temperature (around 37 degrees Celsius for human enzymes). Above the optimum, the heat breaks bonds holding the enzyme in shape, so the active site changes shape, the substrate no longer fits, and the enzyme is denatured. Denaturing is permanent, and the rate falls sharply.
pH. Each enzyme has an optimum pH. Protease in the stomach works best in acidic conditions (about pH 2), while enzymes in the small intestine work best in slightly alkaline conditions. If the pH is too far from the optimum, the active site changes shape and the enzyme denatures.
The food tests
Each food group has its own test with a clear colour change. Learn the reagent, the method and the positive result for each.
| Food group | Reagent | Method | Positive result |
|---|---|---|---|
| Starch | Iodine solution | Add a few drops | Orange-brown to blue-black |
| Reducing sugar | Benedict's solution | Add and heat in a water bath | Blue to brick-red |
| Protein | Biuret solution | Add to the sample | Blue to purple (lilac) |
| Lipid (fat) | Ethanol then water | Mix with ethanol, add to water | Cloudy white emulsion |
- Starch test
- Add a few drops of orange-brown iodine solution; it turns blue-black if starch is present.
- Reducing sugar test
- Add Benedict's solution and heat in a water bath. A positive result changes from blue through green, yellow and orange to brick-red, and the more sugar there is, the further the colour changes.
- Protein test
- Add biuret solution (or sodium hydroxide followed by copper sulfate). It changes from blue to purple if protein is present.
- Lipid test (emulsion test)
- Mix the sample with ethanol, then pour into water. A cloudy white emulsion forms if lipid is present.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksName the three main types of digestive enzyme and state the products each one produces.Show worked answer →
A 4-mark recall question. One mark per enzyme with its products; the fourth mark for full accuracy across all three.
Carbohydrases (such as amylase) break down carbohydrates (starch) into simple sugars (such as glucose or maltose). Proteases break down proteins into amino acids. Lipases break down lipids (fats) into fatty acids and glycerol.
Markers reward each enzyme paired with the correct products. Common slips are saying amylase makes "sugar" without naming it, or forgetting that lipase produces both fatty acids and glycerol.
WJEC style5 marksDescribe how you would test a food sample for starch and for protein, giving the result for each.Show worked answer →
A 5-mark practical description. Award method and result marks for each test.
For starch: add a few drops of orange-brown iodine solution to the sample. If starch is present, the colour changes to blue-black; if not, it stays orange-brown.
For protein: add biuret solution (or sodium hydroxide then copper sulfate) to the sample. If protein is present, the colour changes from blue to purple (lilac); if not, it stays blue.
Markers reward: the named reagent, the method, and the positive colour change for each test. A frequent error is to swap the reagents, for example using iodine for protein.
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Sources & how we know this
- WJEC GCSE Biology specification (from 2016) — WJEC (2016)