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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the given values.
- S2
GPP = 10,000 kJ ha -1 year -1
- S3
R = 4,000 kJ ha -1 year -1
- S4
Step 2: Use the formula for NPP.
- S5
NPP = GPP – R
- S6
Step 3: Substitute the values into the formula.
- S7
NPP = 10,000 kJ ha -1 year -1 – 4,000 kJ ha -1 year -1
- S8
Step 4: Perform the subtraction.
- S9
NPP = 6,000 kJ ha -1 year -1
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the given values.
- S2
GPP = 10,000 kJ ha -1 year -1
- S3
R = 4,000 kJ ha -1 year -1
- S4
Step 2: Use the formula for NPP.
- S5
NPP = GPP – R
- S6
Step 3: Substitute the values into the formula.
- S7
NPP = 10,000 kJ ha -1 year -1 – 4,000 kJ ha -1 year -1
- S8
Step 4: Perform the subtraction.
- S9
NPP = 6,000 kJ ha -1 year -1
Final answer: 6,000 kJ ha -1 year -1
Common mistakes
- Using the wrong units for biomass measurements. — 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. — 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). — 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). — 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. — 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. — 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
- Full worked solution (all marking points)4 marks