A-Level · Biology · AQA · Mark scheme decoded
AQA A-Level Biology: Energy Transfers in Ecosystems: Biomass, Production, and Efficiency — mark scheme explained
The short answer
In any ecosystem, plants play a crucial role by synthesizing organic compounds from atmospheric or aquatic carbon dioxide through the process of photosynthesis. This synthesis is fundamental to the energy flow within ecosystems.
The question
A field has a gross primary production (GPP) of 10,000 kJ ha -1 year -1 . If the respiratory losses (R) are 4,000 kJ ha -1 year -1 , calculate the net primary production (NPP).
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For calculation questions, marks are typically awarded for correct substitution of values into formulas, accurate arithmetic, and providing the final answer with appropriate units. For conceptual questions, marks are given for clear and concise explanations that demonstrate a deep understanding of the topic.
What the command words demand
- Calculate
- Perform a numerical calculation using given data and appropriate formulas.
- Explain
- Provide a detailed description of the concept, including relevant principles and relationships.
- Determine
- Find or derive a specific value or quantity based on given information.
- Analyze
- Examine and interpret data or graphs to draw conclusions about the behavior of the system.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate approximately 5-7 minutes per question to ensure you have enough time to carefully read the problem, perform calculations, and check your work.
- Identify the given values.0 marksGPP = 10,000 kJ ha -1 year -1R = 4,000 kJ ha -1 year -1
- Use the formula for NPP.1 markNPP = GPP – R
- Substitute the values into the formula.1 markNPP = 10,000 kJ ha -1 year -1 – 4,000 kJ ha -1 year -1
- Perform the subtraction.2 marksNPP = 6,000 kJ ha -1 year -1
Final answer: 6,000 kJ ha -1 year -1
Work through every step correctly and you earn all 4 marks.
Another worked example
A farmer measures the chemical energy store in ingested food (I) for a herd of cattle as 50,000 kJ. The chemical energy lost to the environment in faeces and urine (F) is 15,000 kJ, and the respiratory losses (R) are 10,000 kJ. Calculate the net production of the cattle (N).
- Identify the given values.0 marksI = 50,000 kJF = 15,000 kJR = 10,000 kJ
- Use the formula for N.1 markN = I – (F + R)
- Substitute the values into the formula.1 markN = 50,000 kJ – (15,000 kJ + 10,000 kJ)
- Perform the addition inside the parentheses.1 markN = 50,000 kJ – 25,000 kJ
- Perform the subtraction.1 markN = 25,000 kJ
Final answer: 25,000 kJ
Work through every step correctly and you earn all 4 marks.
Common mistakes
Using the wrong units for biomass measurements.
Why it happens: Students often forget to convert units to standard form (e.g., kJ ha -1 year -1 ) before using them in calculations.
Fix: Always ensure that biomass measurements are in the appropriate units when substituting into formulas for GPP, NPP, and consumer production.
Forgetting to account for respiratory losses (R) when calculating NPP.
Why it happens: The importance of subtracting respiratory losses can be overlooked, leading to incorrect calculations of NPP.
Fix: Always use the formula NPP = GPP – R and ensure that you include the value of R in your calculations.
Confusing gross primary production (GPP) with net primary production (NPP).
Why it happens: Students sometimes mix up the definitions of GPP and NPP, leading to incorrect answers in problems involving energy flow.
Fix: Remember that GPP is the total photosynthetic output before respiratory losses, while NPP is the remaining energy available for plant growth and reproduction.
Misinterpreting the formula for net production of consumers (N).
Why it happens: Students may not fully understand how to apply the formula N = I – (F + R), leading to incorrect calculations.
Fix: Practice using the formula step-by-step. Identify the values of I, F, and R, and substitute them into the formula accurately.
Incorrectly calculating primary or secondary productivity rates.
Why it happens: Students may use the wrong units or make arithmetic errors when calculating productivity rates.
Fix: Always ensure that you are using the correct units (e.g., kJ ha -1 year -1 ) and double-check your calculations for accuracy.
Failing to explain the impact of farming practices on energy transfer efficiency clearly.
Why it happens: Students may struggle to articulate how specific farming practices affect energy transfer, leading to vague or incorrect explanations.
Fix: Practice explaining the impact of simplifying food webs and reducing respiratory losses in clear, concise terms. Use examples to illustrate your points.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| Calculate NPP | Calculate net primary production from GPP and respiratory losses using the NPP formula. | 4 |
| Calculate Net Production | Use the energy formula N = I − (F + R) to find net production with units. | 4 |
| Calculate Respiratory Losses | Rearrange the NPP equation to find respiratory losses from given GPP and NPP values. | 4 |
| Calculate GPP | Rearrange the NPP formula to find gross primary production from NPP and respiratory losses. | 4 |
| Total across these question types | 16 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.