Which enzymes digest each food group, and how do we test foods for starch, sugar, protein and fat?
The action of carbohydrase, protease and lipase enzymes, the products of digestion, and the food tests for starch, reducing sugar, protein and fat.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on digestive enzymes and food tests, covering carbohydrase, protease and lipase and their products, and the iodine, Benedict's, biuret and emulsion tests.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
WJEC Double Award Unit 1 wants you to describe the action of carbohydrase, protease and lipase, give the products of digestion, and describe the food tests for starch, reducing sugar, protein and fat.
The digestive enzymes
| Enzyme | Breaks down | Into | Made in |
|---|---|---|---|
| Carbohydrase (e.g. amylase) | Starch | Sugars (e.g. glucose) | Salivary glands, pancreas |
| Protease | Protein | Amino acids | Stomach, pancreas |
| Lipase | Fats (lipids) | Fatty acids + glycerol | Pancreas |
The small soluble products (sugars, amino acids, fatty acids and glycerol) can be absorbed through the wall of the small intestine into the blood, which large insoluble molecules cannot do.
Why enzymes are specific
Each enzyme has a particular shape with an active site that fits only one type of molecule (the substrate), like a lock and key. This is why amylase digests starch but not protein. Enzymes are affected by temperature and pH: they work fastest at body temperature (about 37 degrees Celsius) and at the right pH for where they act (protease in the stomach works best in acid; enzymes in the small intestine work best in slightly alkaline conditions).
The food tests
For all the tests, set up a control with a substance you know does not contain the food, and keep volumes the same, so the colour change is a fair comparison.
Why digested food must be soluble
The whole point of chemical digestion is to turn large insoluble molecules (starch, protein, fat) into small soluble ones (sugars, amino acids, fatty acids and glycerol). Only small soluble molecules can pass through the wall of the small intestine and dissolve in the blood plasma to be carried around the body. This is why the food tests matter: starch gives a negative Benedict's result because it is too large, but after amylase has digested it, the same sample will give a positive Benedict's result, showing sugar has been produced. Examiners often link the enzyme action to a change in food-test result.
How temperature and pH affect enzymes
Each enzyme has an optimum temperature and pH where it works fastest. Below the optimum temperature, reactions are slow because particles move slowly and collide less often. Above the optimum (around 40 degrees Celsius for human enzymes), the enzyme is denatured: its active site changes shape so the substrate no longer fits, and the enzyme stops working. The same happens at the wrong pH. Denaturing is permanent, which is a common exam point, so cooling a denatured enzyme down does not bring it back.
Try this
Q1. State the products when lipase digests a fat. [1 mark]
- Cue. Fatty acids and glycerol.
Q2. Name the reagent used to test for protein and its positive colour. [2 marks]
- Cue. Biuret solution; it turns purple/lilac.
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 enzyme that digests protein, state its product, and describe the food test you would use to confirm protein is present.Show worked answer →
A Unit 1 structured question worth 4 marks. Reward: the enzyme is protease (1), which breaks protein down into amino acids (1). To test for protein you add biuret solution (1); it turns from blue to purple/lilac if protein is present (1). Markers credit the enzyme, the product and the test with its positive colour change. A common error is to give the wrong colour or to confuse biuret with Benedict's.
WJEC style3 marksDescribe how you would test a food sample for a reducing sugar such as glucose, and give the result if sugar is present.Show worked answer →
A Unit 1 food-test question. Add Benedict's solution to the food sample and heat in a water bath (1 mark for reagent, 1 for heating). If a reducing sugar is present the blue solution changes to green, yellow, orange or brick-red depending on how much is present (1 mark). Markers reward the reagent, the heating step and the colour change. A common error is to forget the heating, which is essential for Benedict's test.
Related dot points
- The organs of the human digestive system and their functions, the role of mechanical and chemical digestion, and the action of bile.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on the digestive system, covering the digestive organs and their functions, mechanical and chemical digestion, and the role of bile in emulsifying fats and neutralising stomach acid.
- Absorption of the soluble products of digestion in the small intestine, and how the villi are adapted for efficient absorption.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on absorption, covering how the soluble products of digestion are absorbed in the small intestine and how the villi are adapted for efficient absorption by diffusion and active transport.
- Diffusion, osmosis and active transport as ways substances cross cell membranes, the factors affecting diffusion, the effect of osmosis on cells, and the osmosis required practical.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on movement across membranes, covering diffusion and the factors affecting it, osmosis and its effect on plant and animal cells, active transport and its energy demand, and the osmosis required practical.
- Animal and plant cell structure and the function of each part, the differences between plant and animal cells, and how cells are organised into tissues, organs and organ systems.
A focused answer to the WJEC GCSE Science Double Award Unit 1 topic on cells, covering the parts of animal and plant cells and what each does, the differences between plant and animal cells, and how cells build tissues, organs and organ systems.