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3.5.2
Respiration
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
30
Total marks
65
Marks / Q
2.2
Accessibility
59.2%
Mastery
29.0%
Student strength
41.3%
01
At a glance
3.5.2 · Respiration
8YRSYNTHESIS
3.5.2 (Respiration) appeared in 8 of the 8 years between 2017 and 2024, contributing 30 questions and 65 marks across Papers 1, 2 and 3. APPLICATION dominates the mark distribution at 67.7% of total marks. The accessibility–mastery gap sits at 30.2 percentage points (59.2% vs 29.0%) — most students reach partial credit, but full marks remain harder to secure. Mastery varied year-to-year, lowest in 2019 (19.3%) and highest in 2022 (37.4%). Calculation marks are a small share (12.3%) but typically sit at the lower end of the mastery distribution.
Access–mastery gap
+30 pp
Lowest mastery
2019 · 19.3%
Highest mastery
2022 · 37.4%
02
Question type split
By marks · compound to dominant
65MARKS
65
marks
Application67.7%44 marks
Knowledge20.0%13 marks
Calculation12.3%8 marks
(by marks; compound rows assigned to dominant type):
03
Credit terms — quick reference
Mark scheme tier-locked
23TERMS
Tier 1 · Always credit
ATPKrebs cycle
Tier 2 · Sometimes credit
aerobic respirationmitochondriaoxidationslow muscle fibresanaerobic respirationoxygenelectron transfer chainglucosepyruvatereduced NADstandard form
Reject · Never credit
production of energy (unqualified)Krebs cycle (given in stem)succinic acid dehydrogenase activity alone'no ATP produced''energy is produced''making energy'NADP (wrong molecule); FAD (wrong molecule); electron transport chain (not relevant here)'results are significant' (never accepted); 'standard error'; 'correlation does not mean causation' unqualified; sample size/age assumptions'correlation does not mean causation' unqualified; sample size; duration of investigationrespiration 'uses energy' or 'produces energy'
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 | 7 | 56.0% | 20.3% |
| 2018 | 4 | 7 | 52.2% | 26.5% |
| 2019 | 3 | 8 | 60.0% | 19.3% |
| 2020 | 4 | 10 | — COVID | — COVID |
| 2021 | 2 | 6 | — COVID | — COVID |
| 2022 | 5 | 10 | 61.6% | 37.4% |
| 2023 | 5 | 9 | 61.4% | 31.4% |
| 2024 | 4 | 8 | 62.5% | 32.0% |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
28TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| ATP | 4 | 2017, 2020, 2021 | |
| Krebs cycle | 3 | 2017, 2019, 2020 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| aerobic respiration | 4 | 2017, 2019 | |
| mitochondria | 2 | 2017 | |
| oxidation | 2 | 2017, 2021 | |
| slow muscle fibres | 2 | 2017, 2020 | |
| anaerobic respiration | 2 | 2017, 2018 | |
| oxygen | 2 | 2018, 2021 | |
| electron transfer chain | 2 | 2020, 2021 | |
| glucose | 2 | 2021, 2023 | |
| pyruvate | 2 | 2021, 2023 | |
| reduced NAD | 2 | 2021 | |
| standard form | 2 | 2022, 2023 |
Commonly rejected language
| Term | Times rejected | Years | Why rejected |
|---|---|---|---|
| production of energy (unqualified) | 1 | 2017 | |
| Krebs cycle (given in stem) | 1 | 2017 | |
| succinic acid dehydrogenase activity alone | 1 | 2017 | |
| 'no ATP produced' | 1 | 2018 | |
| 'energy is produced' | 1 | 2018 | |
| 'making energy' | 1 | 2018 | |
| NADP (wrong molecule); FAD (wrong molecule); electron transport chain (not relevant here) | 1 | 2019 | |
| 'results are significant' (never accepted); 'standard error'; 'correlation does not mean causation' unqualified; sample size/age assumptions | 1 | 2019 | |
| 'correlation does not mean causation' unqualified; sample size; duration of investigation | 1 | 2019 | |
| respiration 'uses energy' or 'produces energy' | 1 | 2020 | |
| produces heat energy | 1 | 2020 | |
| reduced NADP | 1 | 2020 | |
| NADPH2 | 1 | 2020 | |
| vacuum | 1 | 2022 | |
| syringe added (rejects mp2 and mp3) | 1 | 2022 |
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
- Lactate reconversion to pyruvate described as oxidation but the coenzyme direction reversed — in 2019, students who wrote that lactate is converted back to pyruvate by reduction rather than oxidation lost the mark; the conversion of lactate to pyruvate involves the removal of hydrogen (oxidation of lactate), and the hydrogen is accepted by NAD to form reduced NAD; students who wrote that NAD is "released" or that NADH is "broken down" demonstrated confusion about which molecule is oxidised and which is reduced (2019 P2 Q04.3)
- Respirometer modification described by adding syringe rather than specifying KOH removal of CO₂ — in 2022, a common error was suggesting that adding a syringe or a scale would modify the respirometer to measure a different variable; the question required describing how to modify the apparatus to measure CO₂ production rather than O₂ consumption; only 2% of students correctly described removing the KOH so CO₂ is not absorbed, allowing the manometer fluid to move in response to CO₂ volume (2022 P2 Q04.4)
- Maltose hydrolysis described as "broken down" rather than hydrolysed — in 2023, describing the digestion of maltose using the term "broken down" was penalised; hydrolysis is the specific mechanism (cleavage by water); "broken down" is insufficiently precise and implies a different process (2023 P3 Q02.3)
Vocabulary errors
- "Energy is produced" or "making energy" used instead of "ATP is produced" — energy is not a molecule and cannot be produced; the mark scheme requires students to state that ATP is synthesised; "energy is produced," "energy released," and "making energy" were all rejected across 2018 and 2020 (2018 P2 Q03.1, 2020 P2 Q10.1)
- NADP substituted for NAD in respiration contexts — NADP is the coenzyme in photosynthesis; NAD is the hydrogen carrier in glycolysis, the link reaction, and the Krebs cycle; writing "NADP" in a respiration answer was penalised in 2019 and 2020 (2019 P2 Q04.3, 2020 P2 Q10.1)
- "/day" used instead of "day⁻¹" in rate unit notation — although primarily penalised in 3.5.4, this notation error occurs across quantitative biology questions; solidus notation (/day) was rejected in unit answers where the superscript negative notation (day⁻¹) was required (2018 P2 Q03.2)
Application errors
- Equilibration period omitted from respirometer protocol — in 2018, fewer than 17% included the equilibration step in their description of how to use a respirometer; the apparatus must be allowed to equilibrate at the experimental temperature before readings begin, to allow air pressure to stabilise; students who described setting up the apparatus and immediately recording data missed this procedural requirement (2018 P2 Q03.1)
- Diameter used instead of radius in cross-sectional area calculations — in 2018, a calculation requiring πr² was answered by students who substituted the diameter value directly without halving it; the area was then four times too large; the examiner noted this as a consistent arithmetic error that was not the result of not knowing the formula (2018 P2 Q03.3)
- Control experiment confused with controlled variable — in 2024, students who described a control experiment (a separate experimental group with one variable removed) when the question asked for a controlled variable (a variable kept constant across all groups) lost the mark; these are distinct methodological concepts and the question language specifies which is required (2024 P3 Q02.3)
High-impact failures · examiner narrative
2022 P2 Q04.42 marks2%full marks
Modification to respirometer to measure CO₂ production. Only 2% scored both marks. The question asked how the respirometer apparatus should be modified to change from measuring O₂ consumption to CO₂ production. The required answer was to remove the KOH (which normally absorbs CO₂) so that the CO₂ produced by respiration is not absorbed, allowing the change in gas volume to reflect CO₂ released rather than O₂ consumed. Most students added components rather than removing the KOH, and some described adding a syringe or a different indicator, neither of which was relevant. The examiner flagged this as a question requiring genuine understanding of what the KOH does in the standard apparatus, not just procedural familiarity with the equipment.
2019 P2 Q04.32 marks
Oxidation and reduction in the conversion of lactate to pyruvate. The question required identifying that lactate is oxidised and that this is coupled to the reduction of NAD to reduced NAD. Students who wrote that pyruvate is reduced to produce lactate (the reverse direction) answered the wrong reaction. Students who correctly identified the direction but then wrote that NADH is produced without connecting it to NAD accepting hydrogen from lactate earned partial credit only. The examiner noted that the oxidation/reduction vocabulary was frequently reversed, suggesting that students recognised the terms but had not fixed which compound loses hydrogen and which gains it in anaerobic/aerobic transitions.
08
Difficulty signals
Performance metric synthesis
30PP GAP
Mean accessibility
59.2%
Mean mastery
29.0%
Mean student strength
41.3%
The accessibility–mastery gap of 30.2 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), 3.5.2 ranks 2 of 4 sub-sections by mean mastery (1 = hardest). Mastery trajectory is rising across the cohort window: 20.3% in 2017 → 32.0% in 2024 (+11.7 percentage points). Mean mastery was lowest in 2019 (19.3%) and highest in 2022 (37.4%).