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3.5.3
Energy and Ecosystems
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
22
Total marks
46
Marks / Q
2.1
Accessibility
68.2%
Mastery
31.7%
Student strength
49.2%
01
At a glance
3.5.3 · Energy and Ecosystems
8YRSYNTHESIS
3.5.3 (Energy and Ecosystems) appeared in 7 of the 8 years between 2017 and 2024, contributing 22 questions and 46 marks across Papers 1, 2 and 3. APPLICATION dominates the mark distribution at 37.0% of total marks. The accessibility–mastery gap sits at 36.5 percentage points (68.2% vs 31.7%) — most students reach partial credit, but full marks remain harder to secure. The largest single question observed is worth 5 marks, signalling that AQA expects complete hierarchical accounts in this sub-section. Mastery varied year-to-year, lowest in 2023 (5.0%) and highest in 2022 (40.0%).
Access–mastery gap
+37 pp
Lowest mastery
2023 · 5.0%
Highest mastery
2022 · 40.0%
02
Question type split
By marks · compound to dominant
46MARKS
46
marks
Application37.0%17 marks
Calculation32.6%15 marks
Knowledge30.4%14 marks
(by marks; compound rows assigned to dominant type):
03
Credit terms — quick reference
Mark scheme tier-locked
17TERMS
Tier 1 · Always credit
respirationNPPGPP
Tier 2 · Sometimes credit
dry massshoot biomassexcretionfaeces
Reject · Never credit
no photosynthesis (reject for mark 4)15.5% (wrong sig figs without standard form); 0.0155% (power of 10 error)vacuumnone ('light is not absorbed' alone insufficient)colorimeterenergy used in respirationenergy producedenergy generatedkg⁻¹m⁻² h⁻¹ kg
04
Question patterns
Recurring formats & tariff structure
0PARAGRAPHS
05
Year-over-year presence
P1 + P3 · 2017–2024
8YEARS
| Year | Questions | Total marks | Mean accessibility | Mean mastery |
|---|---|---|---|---|
| 2017 | 3 | 8 | 71.7% | 23.3% |
| 2018 | 1 | 2 | 72.0% | 24.0% |
| 2019 | 2 | 3 | 42.5% | 36.5% |
| 2020 | 6 | 14 | — COVID | — COVID |
| 2021 | 3 | 6 | — COVID | — COVID |
| 2022 | 6 | 10 | 72.3% | 40.0% |
| 2023 | 1 | 3 | 80.0% | 5.0% |
| 2024 | 0 | 0 | — COVID | — COVID |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
19TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| respiration | 5 | 2018, 2021, 2022, 2023 | |
| NPP | 4 | 2017, 2018, 2021, 2022 | |
| GPP | 3 | 2017, 2018, 2021 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| dry mass | 2 | 2017, 2022 | |
| shoot biomass | 2 | 2017 | |
| excretion | 2 | 2022, 2023 | |
| faeces | 2 | 2022, 2023 |
Commonly rejected language
| Term | Times rejected | Years | Why rejected |
|---|---|---|---|
| no photosynthesis (reject for mark 4) | 1 | 2018 | |
| 15.5% (wrong sig figs without standard form); 0.0155% (power of 10 error) | 1 | 2019 | |
| vacuum | 1 | 2020 | |
| none ('light is not absorbed' alone insufficient) | 1 | 2020 | |
| colorimeter | 1 | 2022 | |
| energy used in respiration | 1 | 2022 | |
| energy produced | 1 | 2022 | |
| energy generated | 1 | 2022 | |
| kg⁻¹ | 1 | 2022 | |
| m⁻² h⁻¹ kg | 1 | 2022 | |
| h⁻¹ m⁻² kg | 1 | 2022 | |
| photosynthesis; decomposition; 'energy lost in waste' (vague) | 1 | 2023 |
Marks in this sub-section are typically awarded for precise terminology and correct application of biological principles. Sequential mark schemes — where each mark requires building on the previous one — are common in multi-mark questions; stating the first step without progression rarely earns more than one mark. Calculation marks are typically split between method (correct setup and value extraction) and answer (accurate numerical result), allowing partial credit when arithmetic errors occur.
07
Where students lose marks
Examiner-anchored error patterns
2CASE STUDIES
Conceptual errors
- NPP defined without including faeces as a loss, or using biomass rather than dry mass — in 2017, only 5% scored both marks on the NPP definition question; the two marks required stating that NPP equals GPP minus respiratory losses, and that NPP represents energy available for growth and transfer to the next trophic level; students who omitted the faeces component of losses, or who described NPP in terms of fresh biomass rather than dry mass, earned one mark at most (2017 P2 Q05.3)
- NPP assumed to increase with succession on sand dunes — in 2018, the most common error on a question about energy flow in sand dune succession was stating that NPP increases as the vegetation becomes more complex; NPP does not necessarily increase with succession; in mature climax communities, GPP and respiratory losses may balance such that NPP is relatively low; students who conflated increasing biomass (standing crop) with increasing NPP demonstrated a misunderstanding of the relationship between the two measures (2018 P2 Q04.4)
- Faeces described as excretion rather than egestion — in 2023, only 5% scored both marks on a question about energy loss from the ecosystem; the mark required identifying faeces as a material that is egested (not fully digested and absorbed) rather than excreted (metabolic waste removed from body fluids); "excretion" in the context of faeces was rejected, and students who used it lost the mark for that loss pathway (2023 P2 Q07.3)
Vocabulary errors
- "Energy used in respiration," "energy produced," and "energy generated" rejected as descriptions of respiratory losses — in 2022, all three phrasings were explicitly rejected; the mark required stating that energy is lost as heat during respiration, or that energy is used in metabolic processes and not available for transfer; vague energy-language without specifying the fate of the energy was penalised (2022 P2 Q02.1)
- NADPH written instead of reduced NADP in energy transfer contexts — in 2022, NADPH appeared in answers about photosynthesis energy transfer when the question context required reduced NADP; while NADPH is chemically equivalent, AQA mark schemes specify "reduced NADP" and NADPH was rejected as a non-specification abbreviation (2022 P2 Q01.1)
- Unit errors in productivity calculations — in 2019, 15.5% was penalised for wrong significant figures without standard form notation, and 0.0155% was penalised for a power-of-ten error; efficiency calculations require attention to both the numerical result and the precision and notation of the answer (2019 P2 Q01.2)
Application errors
- Percentages-of-percentages calculation errors — in 2019, questions requiring a percentage efficiency calculation were answered by students who treated one percentage as an absolute value and applied another percentage to it; the mark required identifying the correct values from the data and performing the division correctly; stacking percentage operations without returning to raw values produced systematically wrong answers (2019 P2 Q01.2)
- "No photosynthesis" given as the reason light is not fully absorbed by chlorophyll — in 2018, a mark requiring the reason why not all light energy is converted to chemical energy in photosynthesis was answered with "no photosynthesis occurs"; the mark required stating that not all wavelengths are absorbed by chlorophyll, or that light energy is reflected or transmitted; "no photosynthesis" was explicitly rejected (2018 P2 Q04.4)
- "Energy lost in waste" used as a vague catch-all — in 2023, describing energy as "lost in waste products" without specifying whether this means faeces (egestion), urine (excretion), or heat (respiration) was penalised; the mark required the specific pathway, not a general description of loss (2023 P2 Q07.3)
High-impact failures · examiner narrative
2017 P2 Q05.32 marks5%full marks
Definition and calculation of NPP. Only 5% scored both marks. The two marks required: NPP = GPP − respiratory losses (mp1), and the identification that faeces and dead matter are additional routes through which energy leaves the organism and does not enter the next trophic level (mp2). Most students knew the GPP − respiration formula but omitted faeces as a separate energy loss category, conflating all non-trophic energy losses into respiratory waste. The examiner noted that students who could calculate NPP from a table often could not state what the formula represents conceptually, particularly regarding the fate of energy in faeces.
2023 P2 Q07.33 marks
Energy losses from a trophic level in an ecosystem. Mastery 5.0% — the lowest of any year for this sub-section. The question required identifying three routes through which energy leaves a consumer: respiration (heat loss), faeces (egestion), and excretion (urea, metabolic waste). Students conflated egestion and excretion throughout, describing faeces as excreted rather than egested; describing decomposition as a loss mechanism when the question asked about losses from a living organism rather than from the trophic level as a whole; and writing "photosynthesis" or "decomposition" as energy loss routes, which are not direct losses from the consumer. The examiner reported this as an answer where most students knew one or two pathways but could not cleanly distinguish between them.
08
Difficulty signals
Performance metric synthesis
37PP GAP
Mean accessibility
68.2%
Mean mastery
31.7%
Mean student strength
49.2%
The accessibility–mastery gap of 36.5 percentage points characterises this sub-section's difficulty profile. Most students reach partial credit; full marks remain harder to achieve. Within 3.5 (Energy transfers in and between organisms), 3.5.3 ranks 3 of 4 sub-sections by mean mastery (1 = hardest). Mastery trajectory is falling across the cohort window: 23.3% in 2017 → 5.0% in 2023 (-18.3 percentage points). Mean mastery was lowest in 2023 (5.0%) and highest in 2022 (40.0%).