AQA AS Level Biology Paper 2, June 2022: Question 2
8 marks · Medium difficulty · Practical Techniques & Data Analysis
Answer questions on carbohydrates, reducing sugar tests using a colorimeter, controlled variables, mass calculation, and water potential changes in stored apples.
Practise this questionQuestion
Question text
02.1 What term is used to describe the different structures of α-glucose and β-glucose?
[1 mark]
02.2 A student investigated the difference in the reducing sugar content of two fruit juices.
He performed a biochemical test on each fruit juice using Benedict’s solution. He
then used a colorimeter with each test result.
Describe how the results from the colorimeter can identify the fruit juice containing the
higher sugar content.
[1 mark]
02.3 The student controlled variables in the test using Benedict’s solution.
Give two variables the student controlled.
[2 marks]
02.4 Apples consist of flesh tissue which surrounds core tissue where the seeds are
located.
A student has an apple with a mass of 180 g
The ratio of flesh tissue to core tissue in this apple is 5:1
8% of the whole apple is sugar.
Calculate the mass of sugar in the flesh tissue.
Show your working.
[2 marks]
*04* Answer g
02.5 Iodine solution stains fresh apple tissue black. When iodine solution is added to
apples stored for a week, the stain is less black.
The water potential of apple juice decreases when apples are stored.
Suggest why the water potential of apple juice decreases when apples are stored.
[2 marks]
Mark scheme
Show the mark scheme
Question Marking Guidance Mark Comments
Isomer(ism); Accept phonetic
02.1 spelling
(AO1)
Ignore structural
High(er) absorbance (has more sugar) Accept a description
1 of absorbance or
02.2 OR transmission
(AO2)
Low(er) transmission (has more sugar);
1. Benedict’s (solution) volume; Accept examples of
volumes and
2. Benedict’s (solution) concentration;
2 max concentrations and
02.3 3. (Fruit) juice volume; temperatures
(2 x AO2)
4. (Water bath/water/solution) temperature;
5. Duration of heating (in water bath);
Correct answer for 2 marks = 12;;
Accept for 1 mark,
30 (correct mass of apple core)
OR
150 (correct mass of apple flesh)
02.4 OR
(2 x AO2)
0.08 / × incorrect mass calculated using
the ratio
OR
14.4 (correct mass in whole apple);
1. Starch hydrolysed;
6 2. Maltose is soluble, (so reduces Ψ) 2 2. Accept glucose for
02.5 maltose
OR (2 x AO2)
2. Ignore sugar
Starch is insoluble;
How to answer it
Carbohydrates, Biochemistry, and Water Potential
What this question tests
This question assesses your understanding of carbohydrate chemistry (isomers of glucose), practical biochemistry (using Benedict's solution and colorimeters), variable control in biological experiments, multi-step proportional calculations, and the physical principles of water potential governing cell contents.
Carbohydrate Isomers
What term is used to describe the different structures of alpha-glucose and beta-glucose? [1 mark]
✅ Correct Answer
Isomer (or isomerism). Phonetic spellings are accepted, but structural terms like "structural isomer" are not credited here since alpha and beta glucose are stereoisomers.
💡 Key Knowledge
Alpha-glucose and beta-glucose have the same molecular formula (C₆H₁₂O₆) but a different arrangement of atoms in space (specifically the hydroxyl group on carbon-1 pointing down in alpha and up in beta).
Colorimetry and Reducing Sugars
Describe how the results from the colorimeter can identify the fruit juice containing the higher sugar content. [1 mark]
✅ Correct Answer
The juice with the higher sugar content produces more brick-red precipitate with Benedict's solution, which leaves less blue copper sulfate solution in the supernatant after centrifugation/settling. This results in a higher absorbance reading or a lower transmission reading.
🧠 Exam Technique
Be precise with colorimeter terminology. Always link the reading (absorbance/transmission) directly back to the concentration of the resulting solution or precipitate.
Controlling Variables
Give two variables the student controlled in the test using Benedict's solution. [2 marks]
✅ Correct Answer
Any two of the following:
- Volume of Benedict's solution
- Concentration of Benedict's solution
- Volume of fruit juice
- Temperature of the water bath
- Duration of heating
❌ Common Errors
Students often lose marks by writing vague statements like "amount of solution" instead of specifying volume or concentration. "Amount" is penalized by examiners.
Ratio and Mass Calculation
Calculate the mass of sugar in the flesh tissue. Show your working. [2 marks]
📐 Step-by-Step Calculation
- Find the total parts in the ratio: Flesh : Core = 5 : 1. Total parts = 5 + 1 = 6 parts.
- Calculate mass of the flesh tissue: (5 / 6) × 180 g = 150 g .
- Calculate mass of sugar in the flesh: 8% of 150 g = 0.08 × 150 = 12 g .
Final Answer: 12 g
❌ Common Calculation Traps
A frequent error is calculating 8% of the whole apple (180 g × 0.08 = 14.4 g) without accounting for the flesh-to-core ratio. This alternative path only earns 1 mark out of 2.
Water Potential and Starch Breakdown
Suggest why the water potential of apple juice decreases when apples are stored. [2 marks]
✅ Correct Answer
- Starch is hydrolysed (broken down) into maltose / glucose.
- Maltose or glucose is soluble, whereas starch is insoluble, increasing the number of free solute molecules and therefore lowering the water potential.
💡 Key Knowledge
Insoluble storage polysaccharides like starch exert no osmotic effect because they do not dissolve. When enzymes break them down into soluble sugars, solute concentration rises sharply, causing water potential to become more negative (decrease).
Topics
Biology · Practical skills · 3.1 Biological molecules · Experimental design · Data analysis
Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.