AQA AS Level Biology Paper 2, June 2025: Question 3

9 marks · Medium difficulty · Extended Answer

Identify constituent elements of biological molecules, describe the emulsion test for lipids, identify an unsaturated fatty acid, and evaluate the effectiveness of cholestyramine in reducing blood cholesterol concentration.

Practise this question

Question

Question 03 consists of four parts: 03.1 presents a multiple-choice table matching cellulose, triglyceride, and antibody to their constituent elements. 03.2 asks to describe the biochemical test used to confirm the presence of lipid. 03.3 shows structural formulas for saturated Fatty acid A and unsaturated Fatty acid B, asking to state which is unsaturated and explain why. 03.4 describes a study where 10 patients took cholestyramine for 1 month and shows a bar chart of mean blood cholesterol concentration before and after treatment with ±2 standard deviation error bars (decreasing from ~235 to ~120 mg 100 cm⁻³ without bar overlap), asking to evaluate the drug's effectiveness.
Question text

03 Table 1 shows the names of three biological molecules and different combinations

of elements.

Table 1

Cellulose Triglyceride Antibody

A C, H, O C, H, O C, H, O, N

B C, H, O C, H, O C, H, O, N, S

C C, H, O C, H, O, N C, H, O, N, S

D C, H, O, N C, H, O C, H, O, N, P

03.1 Which row identifies the correct elements in each molecule?

Tick ( ) one box.

[1 mark]

03.2 Describe the biochemical test used to confirm the presence of lipid in a sample.

[2 marks]

03.3 Figure 5 shows two types of fatty acids.

Figure 5

State which fatty acid in Figure 5 is unsaturated and explain your answer.

[1 mark]

Unsaturated fatty acid

Explanation

03.4 Cholesterol is a type of lipid transported in the blood. High blood cholesterol

concentrations can lead to cardiovascular disease.

Scientists studied the effectiveness of the drug cholestyramine (CA) in lowering blood

cholesterol concentration.

They:

• gave 10 patients CA for 1 month

• measured the blood cholesterol concentrations at the start of the study and after

1 month of treatment with CA.

Figure 6 shows the scientists’ results.

The error bars represent ±2 standard deviations from the mean, which includes over

95% of the data.

Figure 6

Use the information provided to evaluate the effectiveness of the drug CA in reducing

blood cholesterol concentration.

[5 marks]

Mark scheme

Show the mark scheme Mark scheme for Question 03: 03.1 requires row B (Cellulose: C, H, O; Triglyceride: C, H, O; Antibody: C, H, O, N, S). 03.2 awards 2 marks for adding ethanol then water and shaking/mixing, followed by a white or milky emulsion. 03.3 awards 1 mark for Fatty acid B due to having a double bond between carbon atoms (C=C). 03.4 awards up to 5 marks for points evaluating effectiveness, including: standard deviation bars do not overlap indicating a significant difference, small sample size (10 people), short duration (only 1 month), unknown side effects, and lack of details on patient age/sex/diet.

Question Marking Guidance Mark Comments

Cellulose Triglyceride Antibody 1

03.1 (1 x

AO1)

B C, H, O C, H, O C, H, O, N, S

1. Add ethanol then water and shake/mix

OR

Add ethanol and shake/mix then add water; 2

03.2 (2 x

2. White/milky emulsion AO1) 2. Ignore cloudy

2. Reject precipitate

OR

Emulsion test turns white;

(Fatty acid B) Accept symbols

C = C

Double bond between carbon (atoms) 1

03.3 (1 x

OR AO2)

Two carbons with only one hydrogen;

1. Allow error bars for

1. S.D. (bars) do not overlap so a significant

SD bars

difference/decrease;

1. Reject ‘results are

2. Unknown side effects of treatment(s);

significant’

3. Time scale/duration too short

1. Accept ‘difference

not due to chance’

OR

5 for significant

03.4 (5 x

Only investigated for 1 month;

AO3) 7

4. Small sample size

OR

Only 10 people (investigated);

5. No details on sex/age/diet;

How to answer it

Biological Molecules, Lipid Tests & Clinical Data Evaluation

📋 What this question tests

This question assesses fundamental AS-level biochemistry and essential scientific evaluation skills:

  • Elemental composition of carbohydrates (cellulose), lipids (triglycerides), and proteins (antibodies).
  • Biochemical testing protocol and expected observations for lipids (emulsion test).
  • Molecular structure of fatty acids (distinguishing saturated vs. unsaturated hydrocarbon chains).
  • Evaluating clinical drug trial data using error bars (±2 SD) to deduce statistical significance, alongside recognizing methodological flaws (sample size, trial length, uncontrolled variables, side effects).
Part 03.1 • 1 Mark

Elemental Composition of Biological Molecules

Identify the correct row in Table 1

✅ Correct Answer

Row B

  • Cellulose: C, H, O
  • Triglyceride: C, H, O
  • Antibody: C, H, O, N, S
Award 1 mark (AO1) for ticking row B only.

💡 Key Knowledge

  • Carbohydrates (Cellulose): Composed solely of carbon, hydrogen, and oxygen.
  • Lipids (Triglycerides): Formed from 1 glycerol and 3 fatty acids via ester bonds; contain only C, H, and O (unlike phospholipids, which also contain P).
  • Antibodies: Immunoglobulin proteins composed of amino acids containing C, H, O, and N. Crucially, antibodies contain disulfide bridges formed by sulfur-containing cysteine residues, so they contain S.
Part 03.2 • 2 Marks

The Emulsion Test for Lipids

Describing the biochemical test for lipids

✅ Correct Answer

  1. Add ethanol, then add water and shake / mix.
    (OR: Add ethanol and shake/mix, then add water) [1 mark]
  2. A white / milky emulsion forms. [1 mark]
Max 2 marks (2 × AO1)

🧠 Exam Technique

Examiners look for three precise keywords in order:

  • Ethanol (to dissolve the non-polar lipid)
  • Water (because lipids are insoluble in water, causing droplets to disperse)
  • Emulsion (the exact biological term for droplets of oil suspended in water)

❌ Common Errors

  • Reject: Stating that a "precipitate" forms. Lipids form a milky liquid suspension (an emulsion), not a solid precipitate!
  • Ignore: Describing the result simply as "cloudy" without stating white, milky, or emulsion.
  • Forgetting to state mix or shake.
Part 03.3 • 1 Mark

Saturated vs. Unsaturated Fatty Acids

Identifying unsaturated fatty acids from structural diagrams

✅ Correct Answer

Unsaturated fatty acid: Fatty acid B

Explanation: Contains a double bond between carbon atoms (or C=C / two carbons with only one hydrogen each).

Award 1 mark (AO2) for both naming B and providing the valid structural explanation.

💡 Key Knowledge

  • Saturated fatty acids (Fatty acid A) contain only single C–C bonds along their hydrocarbon chain; all carbon atoms are saturated with hydrogen.
  • Unsaturated fatty acids (Fatty acid B) contain at least one double bond between carbon atoms ( C=C ), creating a kink or bend in the hydrocarbon chain.

❌ Common Errors

  • Writing just "it has a double bond". This loses the mark because both molecules have a C=O double bond in their carboxyl group (-COOH). You must specify a double bond between carbon atoms ( C=C ).
Part 03.4 • 5 Marks

Evaluating Clinical Trial Data

Evaluating the effectiveness of cholestyramine (CA) in lowering cholesterol

✅ Mark Scheme Breakdown (Any 5 points)

  1. Error bars / standard deviation (SD) bars do not overlap: indicates a significant difference / decrease in blood cholesterol (difference is not due to chance).
  2. Unknown side effects of the treatment / drug safety not evaluated.
  3. Duration too short: only investigated for 1 month / long-term effects unknown.
  4. Small sample size: only 10 patients tested.
  5. Confounding variables not controlled: no details given on patient age, sex, diet, exercise, or health status.
Max 5 marks (5 × AO3)

🧠 Top-Level Evaluation Strategy

To score high marks on "Evaluate" questions involving a graph with error bars:

  • Look at the error bars first: The bars represent ±2 SD (over 95% of data). Before treatment: ~210 to 260 mg 100 cm⁻³. After treatment: ~105 to 135 mg 100 cm⁻³. There is a clear gap between them (no overlap).
  • State statistical meaning: Non-overlapping SD bars mean the difference is significant or not due to chance.
  • Balance the argument: Give points supporting the drug (marked drop in mean cholesterol, statistically significant) followed by limitations in the experimental design (sample size, trial length, side effects, uncontrolled variables).

❌ Critical Phrasing Trap

  • NEVER write: "The results are significant." Examiners reject this phrase!
  • ALWAYS write: "The decrease in blood cholesterol is significant" OR "The difference between the means is significant" OR "The difference is not due to chance."

Topics

Biology · Practical skills · 3.1 Biological molecules · Data analysis · Experimental design

Question and mark scheme from the AQA AS Level Biology examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.