Biological catalysts, food tests and energy in food — Edexcel GCSE Biology
The importance of enzymes in making and breaking down carbohydrates, proteins and lipids; food tests and calorimetry (Triple only).
Revision notes
3 short notes, in the order of the specification. Each one in short:
Enzymes are biological catalysts that speed up reactions in cells, which would otherwise be far too slow to keep us alive. They are needed to build and break down large molecules: carbohydrates are made from and broken into sugars, proteins into amino acids, and lipids into fatty acids and glycerol. Different enzymes catalyse each reaction, because enzymes are specific.
Food tests use reagents to identify biological molecules by a colour change. Iodine solution turns from orange-brown to blue-black with starch. Benedict's solution, heated in a water bath, turns from blue to green, yellow, orange or brick-red with reducing sugars. Biuret reagent turns from blue to purple with protein, and ethanol forms a cloudy white emulsion with fats.
Measuring the energy in food
Triple onlyCalorimetry measures the energy in food by burning it to heat a known mass of water and recording the temperature rise. Energy released (J) = mass of water (g) × 4.2 × temperature change (°C), and dividing by the mass of food gives the energy per gram. The result is usually too low, because heat is lost to the surroundings.
Exam questions
6 exam-style questions (17 marks), each with its mark scheme.
Answer the questionsFlashcards
8 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of key concepts in biology on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheet
Method, variables, risks and a results table, with questions to answer.
- Core practical 3: Food testsTriple only
Print it
Free PDFs to print or save.
Quick check questions
What are proteins broken down into?
Show answer
Amino acids.
What does a catalyst do?
Show answer
It speeds up a reaction and is not used up.
Which reagent turns purple when protein is present?
Show answer
Biuret reagent (Triple only).
How can the energy in food be measured?
Show answer
By burning it to heat water and using the temperature change (calorimetry) (Triple only).
Frequently asked questions
Why are enzymes important in the body?
Enzymes are important because chemical reactions in living things would be far too slow to keep us alive without them. As biological catalysts, they speed up reactions in cells. They are needed both to break down large molecules such as proteins, carbohydrates and lipids, and to build them up from smaller molecules.
What are carbohydrates, proteins and lipids broken down into?
Carbohydrates are broken down into sugars, proteins into amino acids, and lipids into fatty acids and glycerol. Each reaction is catalysed by a different enzyme, because enzymes are specific. The same small molecules are also used to synthesise the large carbohydrates, proteins and lipids that the body needs.
How do you test for protein in food?
Triple only To test for protein, add biuret reagent to a liquid sample of the food and mix. If protein is present, the colour changes from blue to purple (lilac). If there is no protein, it stays blue. Solid food is first crushed with a little water. Always wear eye protection during food tests.
What is the test for starch?
Triple only The test for starch is to add a few drops of iodine solution to the sample. If starch is present, the iodine turns from orange-brown to blue-black. If there is no starch, it stays orange-brown. Iodine solution is also used in the pH core practical to show when amylase has broken down all the starch.
Why does calorimetry give a lower value for the energy in food?
Triple only Calorimetry usually gives a lower value than the real energy in the food because heat is lost to the surroundings and the food may not burn completely. The results can be improved by putting a draught shield around the boiling tube, holding the burning food close to the tube and stirring the water.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026