AQA A-Level Biology Paper 2, June 2025: Question 3

6 marks · Medium difficulty · Short Answer

Describe how to determine dry mass using an oven and balance, calculate energy released in kJ g⁻¹ from calorimetry data, and explain how above-optimum temperatures reduce biomass using net primary production.

Practise this question

Question

Question 03 containing three parts: 03.1 asks to describe how to use an oven and a weighing balance to determine the dry mass of lettuce leaves (2 marks). 03.2 provides experimental values for calorimeter combustion of 1.50 g dry mass lettuce leaves (150 cm³ of water, temperature increase of 18.2 °C, and 4.18 J required to raise 1 cm³ water by 1 °C) and asks to calculate heat energy released in kJ g⁻¹ (2 marks). 03.3 asks to explain why growing crop plants at temperatures higher than their optimum can decrease biomass, using knowledge of net primary production (2 marks).
Question text

03.1 A student determined the dry mass of lettuce leaves. The equipment used was an

oven and a weighing balance.

Describe how the student could use this equipment to determine the dry mass of

lettuce leaves.

[2 marks]

03.2 The student then used a calorimeter to determine the chemical energy stored in a

sample of lettuce leaves with a dry mass of 1.50 g

The 1.50 g sample was fully combusted in a calorimeter.

The volume of water in the calorimeter was 150 cm3

The increase in temperature of the water was 18.2 °C

4.18 J of heat energy are required to increase the temperature of 1 cm3 of water

by 1 °C

Use this information to calculate the heat energy released, in kJ g–1, of dry mass of

lettuce leaves.

Show your working.

[2 marks]

Answer7 kJ g–1

03.3 Biomass can be measured as the dry mass of tissue per given area. Some crop

plants are grown in large greenhouses (glasshouses) where optimum growing

conditions can be maintained.

Growing crop plants at higher temperatures than their optimum temperature can

decrease their biomass.

*06Use your knowledge of net primary production to explain why.*

[2 marks]

Mark scheme

Show the mark scheme Mark scheme for question 03. For 03.1: 1. Heat and then weigh; 2. Until mass is constant (2 marks). For 03.2: 7.6 / 7.61 / 7.608 / 7.6076 kJ g⁻¹ (or 8) scores 2 marks, with 1 mark for common calculation or unit conversion errors. For 03.3: Mark 1 for Net primary production = gross primary production minus respiratory losses (NPP = GPP - R); Mark 2 for noting that at higher temperature photosynthesis/GPP decreases, respiration increases, or enzymes for photosynthesis denature (2 marks).

Question Marking Guidance Mark Comments

1. Heat and (then) weigh; Accept ‘heat until

sample remains the

2. (Until) mass is constant; same mass’ for 2

marks

1. Accept any

2 specified temperature

03.1 (2 x for ‘heat’

AO3) 1. Ignore length of

time heated

1. Reject ‘burn’

2. Accept weight for

mass

7.6 / 7.61 / 7.608 / 7.6076 (kJ g−1) = 2 marks;; Accept 8 for 2 marks.

Incorrect, but answer shows 76 (ignore any

subsequent numbers, position of decimal point and

preceding/subsequent zeros i.e. incorrect

conversion) = 1 mark

OR

Incorrect answer of 11.4 (kJ g−1) (ignore any

subsequent numbers i.e. no division by 1.5.) = 1

mark

OR

Incorrect answer of 0.05 (kJ g−1) (ignore any

subsequent numbers i.e. not multiplied by 150) = 1

mark 2

03.2 OR (2 x

AO2)

Incorrect answer of 1.8 (kJ g−1) (ignore any

subsequent numbers i.e. not multiplied by 4.18) = 1

mark

OR

Incorrect answer of 0.418 / 0.42 (kJ g−1) (ignore

any subsequent numbers i.e. not multiplied by

18.2) = 1 mark

OR

Incorrect answer of 17 (kJ g−1) (ignore any

subsequent numbers i.e. multiplied rather than

divided by 1.5) = 1 mark;

– A-LEVEL BIOLOGY – 7402/2 –

1. Accept ‘is’ for =

1. Net primary production = gross primary

sign

production minus respiratory losses

1. Accept RL

(respiratory losses) for

OR

R

NPP = GPP – R;

1. Accept respiration

2. (At higher temperature) photosynthesis/GPP for ‘respiratory losses’

decreases

(1 x

03.3 2. Accept ‘respiratory

AO1, 1

OR losses’ for respiration

x AO2)

(At higher temperature) respiration increases 2. Reject enzymes for

photosynthesis and

OR respiration denature

(At higher temperature) enzymes for 2.Reject ‘more energy

photosynthesis denature; used in respiration’

How to answer it

Plant Biomass, Calorimetry & Net Primary Production

AQA A-Level Biology • Organisms Exchange Substances / Energy Transfers

What this question tests

This question assesses fundamental practical and theoretical skills related to ecosystem energetics and biomass measurement:

  • Practical methodology: Standard laboratory procedure for measuring dry mass without combusting biological tissue.
  • Mathematical skills (calorimetry): Using the heat energy equation q = m × c × ΔT to calculate energy content per gram of dry biomass ( kJ g⁻¹ ).
  • Ecological concepts: Explaining changes in plant biomass using the net primary productivity equation ( NPP = GPP - R ) and enzyme kinetics at supra-optimal temperatures.
Question 03.1 • 2 Marks

Determining the Dry Mass of Biological Tissue

AO3 (Practical Experimental Design)

✅ Mark Scheme Requirements

  1. Heat the leaves and (then) weigh; [1 mark]
  2. Repeat heating and weighing until the mass is constant; [1 mark]
Alternative: "Heat until the sample remains the same mass" achieves both marks simultaneously.

💡 Key Knowledge

Why measure dry mass rather than fresh mass?

  • Fresh plant tissue contains a large and highly fluctuating volume of water.
  • Water contains no chemical potential energy usable by trophic consumers. Dry mass represents true organic matter (carbohydrates, lipids, proteins).
  • Drying is typically carried out in an oven set at a temperature below 100 °C (e.g., 60–80 °C) to evaporate all moisture without burning organic molecules.

🧠 Exam Technique

  • The "Magic Phrase": Whenever AQA asks how to obtain dry mass, always write: "Weigh, heat in an oven, cool, reweigh, and repeat until constant mass is recorded."
  • This phrase guarantees both marks across both AS and A2 papers.

❌ Common Errors to Avoid

  • Saying "burn": The mark scheme explicitly states: Reject 'burn'. Burning oxidises organic carbon to CO₂, destroying the biomass instead of drying it.
  • Single heating: Stating "leave in an oven for 24 hours" without explaining that it must be weighed until mass stops changing gains only 1 mark maximum.
Question 03.2 • 2 Marks

Calorimetry Calculation

AO2 (Mathematical Application)

📐 Step-by-Step Calculation

  1. Calculate total heat energy absorbed by the water:
    Energy (J) = volume of water × specific heat capacity × temperature change
    Energy (J) = 150 cm³ × 4.18 J cm⁻³ °C⁻¹ × 18.2 °C = 11,411.4 J
  2. Convert Joules (J) to Kilojoules (kJ):
    The question specifies units of kJ g⁻¹.
    11,411.4 J ÷ 1,000 = 11.4114 kJ
  3. Calculate energy released per gram of dry mass:
    Divide by the dry mass combusted ( 1.50 g ):
    11.4114 kJ ÷ 1.50 g = 7.6076 kJ g⁻¹
  4. Final Answer:
    7.6 or 7.61 kJ g⁻¹ (Acceptable range: 7.6 to 7.608, or 8 rounded to 1 s.f.)

✅ Acceptable Answers (Full 2 Marks)

  • 7.6 kJ g⁻¹
  • 7.61 kJ g⁻¹
  • 7.608 kJ g⁻¹
  • 7.6076 kJ g⁻¹
  • 8 kJ g⁻¹

❌ Calculation Traps (Partial Credit: 1 Mark)

  • Unit Error (76): Forgot to divide by 1000 properly when converting J to kJ.
  • Omitted Sample Mass (11.4): Calculated total kJ in the calorimeter but forgot to divide by 1.50 g .
  • Multiplied by Mass (17): Multiplied total energy by 1.5 instead of dividing.
  • Missed Values: Forgetting to multiply by 150 (gives 0.05) or forgetting 4.18 (gives 1.8).
Question 03.3 • 2 Marks

Temperature & Net Primary Production (NPP)

AO1 & AO2 (Ecological Energetics & Physiology)

✅ Mark Scheme Requirements

Mark Point 1: Core Formula

  • NPP = GPP - R OR Net primary production = gross primary production minus respiratory losses. [1 mark]

Mark Point 2: Effect of Supra-optimal Temperature (Any one)

  • Photosynthesis / Gross Primary Production (GPP) decreases; [1 mark]
  • OR Respiration increases; [1 mark]
  • OR Enzymes involved in photosynthesis denature (e.g. Rubisco). [1 mark]

💡 Key Knowledge

Understanding the physiological balance:

  • NPP represents the plant biomass available for growth and reproduction.
  • Photosynthetic enzymes (such as Rubisco in the Calvin cycle) have a lower optimum temperature than respiratory enzymes. Above optimum, active sites denature, rapidly reducing GPP.
  • Concurrently, plant respiration (R) continues to increase with higher thermal kinetic energy.
  • Since NPP = GPP - R , a fall in GPP combined with a rise in R causes NPP (biomass accumulation) to plummet.

🧠 Exam Technique

  • Always state the relationship NPP = GPP - R in words or symbols whenever an ecology question asks you to explain plant biomass changes.
  • Clearly identify which variable in the equation is changing and in which direction (e.g., "GPP decreases" or "R increases").

❌ Common Misconceptions & Examiner Rejections

  • "Both enzymes denature": Do not claim that both photosynthetic and respiratory enzymes denature simultaneously. Respiration often continues or increases while photosynthetic enzymes are already denaturing.
  • Vague phrasing: Saying "more energy is used in respiration" is rejected. Use precise biological terms: "respiratory losses increase" or "rate of respiration increases".

Topics

Biology · Practical skills · 3.5 Energy transfers in and between organisms (A-level only) · Experimental design · Data analysis

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