Which molecules are produced when enzymes break down proteins?
- Sugars
- Amino acids
- Fatty acids
- Glycerol
Show mark scheme
Answer: B B amino acids (1)
Enzymes are biological catalysts. (a) Explain what is meant by a biological catalyst. (2) (b) Name the smaller molecules that enzymes join together to make a protein. (1)
Show mark scheme
- (a) speeds up a reaction (1)
- without being used up / without being changed (1)
- (b) amino acids (1)
Describe the chemical tests you would use to show that a food contains protein and starch. Include the positive result for each test.
Show mark scheme
- protein: add biuret reagent / solution (1) allow add sodium hydroxide solution then copper sulfate solution
- positive result for protein: blue to purple / lilac (1)
- starch: add iodine solution, which turns from orange-brown to blue-black (1)
Describe how to test a food sample for (a) reducing sugars and (b) fats. Include the positive result for each test.
Show mark scheme
- (a) add Benedict's solution and heat in a hot water bath (1)
- colour changes from blue to green / yellow / orange / brick-red (1)
- (b) shake the sample with ethanol and then add / pour into water (1)
- cloudy white emulsion forms (1)
A student burned 0.8 g of a food to heat 25 g of water. The temperature of the water rose by 16 °C. Use the equation: energy released (J) = mass of water (g) × 4.2 × temperature change (°C) Calculate the energy released per gram of food.
Show mark scheme
- 25 × 4.2 × 16 (1)
- 1680 J (1)
- 1680 ÷ 0.8 (1)
- 2100 J/g (1) correct answer with no working gains full marks
The energy per gram the student calculated was lower than the value printed on the food packet. Suggest two reasons for this.
Show mark scheme
- heat / energy was lost to the surroundings / air (1)
- the food did not burn completely / incomplete combustion (1)
- some energy heated the boiling tube / not all the energy transferred to the water (1)
- Max 2