← Back to Intelligence
// TOPIC PROFILE
3.6.3
Skeletal Muscles as Effectors
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
16
Total marks
42
Marks / Q
2.6
Accessibility
67.3%
Mastery
27.6%
Student strength
56.2%
01
At a glance
3.6.3 · Skeletal Muscles as Effectors
8YRSYNTHESIS
3.6.3 (Skeletal Muscles as Effectors) appeared in 6 of the 8 years between 2017 and 2024, contributing 16 questions and 42 marks across Papers 1, 2 and 3. APPLICATION dominates the mark distribution at 50.0% of total marks. The accessibility–mastery gap sits at 39.7 percentage points (67.3% vs 27.6%) — 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 2019 (15.5%) and highest in 2024 (34.6%). Calculation marks are a small share (11.9%) but typically sit at the lower end of the mastery distribution.
Access–mastery gap
+40 pp
Lowest mastery
2019 · 15.5%
Highest mastery
2024 · 34.6%
02
Question type split
By marks · compound to dominant
42MARKS
42
marks
Application50.0%21 marks
Knowledge38.1%16 marks
Calculation11.9%5 marks
(by marks; compound rows assigned to dominant type):
03
Credit terms — quick reference
Mark scheme tier-locked
24TERMS
Tier 1 · Always credit
Tier 2 · Sometimes credit
ATPglucosediametermyosin headactinpowerstroketropomyosinactinomyosin bridgescalcium ionssarcoplasmic reticulumglycogenglycogenolysisrespirationsynaptic cleft
Reject · Never credit
meanaverage (for mode comparison)shortening of sarcomerepulls actin alongactive site (penalised once); ATP synthasenone (ignore references to A band)active site; myofibril for myosinPCr abbreviation; aerobic/anaerobic exercise types (not required); starch as storageglucose enters mitochondria; 'respiration provides energy' (too vague); fatty acids converted to glucosesynapse for D
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 | 6 | 58.3% | 29.0% |
| 2018 | 0 | 0 | — COVID | — COVID |
| 2019 | 2 | 7 | 69.0% | 15.5% |
| 2020 | 1 | 3 | — COVID | — COVID |
| 2021 | 1 | 2 | — COVID | — COVID |
| 2022 | 0 | 0 | — COVID | — COVID |
| 2023 | 4 | 11 | 60.5% | 23.8% |
| 2024 | 5 | 13 | 77.4% | 34.6% |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
29TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| ATP | 3 | 2023, 2024 | |
| glucose | 3 | 2023, 2024 | |
| diameter | 2 | 2017 | |
| myosin head | 2 | 2017, 2019 | |
| actin | 2 | 2017, 2021 | |
| powerstroke | 2 | 2017, 2023 | |
| tropomyosin | 2 | 2019, 2021 | |
| actinomyosin bridges | 2 | 2019, 2023 | |
| calcium ions | 2 | 2019, 2023 | |
| sarcoplasmic reticulum | 2 | 2023, 2024 | |
| glycogen | 2 | 2023, 2024 | |
| glycogenolysis | 2 | 2023, 2024 | |
| respiration | 2 | 2023 | |
| synaptic cleft | 2 | 2024 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| mean | 1 | 2017 | |
| average (for mode comparison) | 1 | 2017 | |
| shortening of sarcomere | 1 | 2017 | |
| pulls actin along | 1 | 2017 | |
| active site (penalised once); ATP synthase | 1 | 2019 | |
| none (ignore references to A band) | 1 | 2020 | |
| active site; myofibril for myosin | 1 | 2023 | |
| PCr abbreviation; aerobic/anaerobic exercise types (not required); starch as storage | 1 | 2023 | |
| glucose enters mitochondria; 'respiration provides energy' (too vague); fatty acids converted to glucose | 1 | 2023 | |
| synapse for D | 1 | 2024 | |
| neuromuscular junction for D | 1 | 2024 | |
| acetylcholine for C | 1 | 2024 | |
| ribosome for C | 1 | 2024 | |
| muscle for sarcolemma | 1 | 2024 | |
| signals/messages | 1 | 2024 |
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
3CASE STUDIES
Conceptual errors
- Sarcomere shortening described as the mechanism instead of myosin moving past actin — in 2017, a novel application question showed myosin moving past actin to pull a mitochondrion along; students who described "shortening of the sarcomere" applied rote recall rather than engaging with the diagram, earning no marks; the best answers explicitly described myosin moving past actin in the direction shown in the figure, not contracting toward a Z-line (2017 P3 Q01.2)
- "Active site" used instead of "binding site" for the actin–myosin interaction — in 2019, some students referred to myosin's "active site" on actin; active sites are features of enzymes; the structure on actin where myosin attaches is its binding site; this error was also noted in 2023, where it was penalised once (2019 P2 Q04.3, 2023 P2 Q10.1)
- Glycogen confused with glucagon — in 2023, a small but notable proportion of students wrote "glucagon" when describing the intramuscular carbohydrate store; glycogen is the polysaccharide stored in muscle; glucagon is the pancreatic hormone that raises blood glucose; these are phonetically similar but mechanistically entirely different (2023 P2 Q10.3)
Vocabulary errors
- "Myofibril" used instead of "myosin" — in 2023, this substitution negated the mark for describing ATP's role in breaking actinomyosin bridges; myofibril is the contractile organelle containing many filaments; myosin is the specific protein that binds actin and undergoes the power stroke; these cannot be substituted (2023 P2 Q10.1)
- PCr abbreviation used instead of phosphocreatine — in 2023, PCr was listed as a rejected abbreviation; phosphocreatine must be written in full; students who abbreviated it lost the mark for that point (2023 P2 Q10.3)
- "Respiration provides energy" without specifying ATP — in 2023, this vague phrasing was penalised; the mark required stating that respiration produces ATP; "energy" is not a molecule and cannot be transferred to actinomyosin bridge cycling; the specific product, ATP, must be named (2023 P2 Q10.4)
Application errors
- Cross-sectional area calculated using diameter rather than radius — in 2017, over 50% of students scored zero on the muscle fibre diameter calculation; the most common incorrect answer (83.3) arose from converting the field of view area to micrometres and then dividing by 15 without first halving the diameter to get the radius for πr²; using diameter directly where radius is required quadruples the area (2017 P2 Q03.1)
- y-axis of pH/temperature experiment misread as showing absolute values — in 2019, students who did not recognise that the y-axis represented force as a percentage of the control at 100% concluded that no controls existed and that the conclusion could not be evaluated; this reading failure led to the correct data being rejected as insufficient evidence (2019 P2 Q04.1)
- Fibre type matched to the wrong exercise type — in 2023, a significant number of students correctly named fast and slow muscle fibres but then assigned them to the wrong exercise categories; the mark required linking slow oxidative fibres to sustained aerobic exercise and fast glycolytic fibres to short maximal-effort exercise; reversing these reduced the maximum attainable mark (2023 P2 Q10.3)
High-impact failures · examiner narrative
2017 P2 Q03.12 marks
Calculation of muscle fibre diameter from a photomicrograph. Over 50% scored zero. The common error was converting area rather than diameter to micrometres, then dividing by 15 to obtain 83.3 — which applies the correct logic to the wrong measurement. The mark scheme required: measuring the fibre diameter in the image, applying the magnification factor, and expressing the result in micrometres. Students who correctly identified that magnification was involved but then used area instead of a linear dimension produced an answer with plausible units but a fundamentally wrong calculation.
2019 P2 Q04.14 marks
Evaluation of conclusion about pH and temperature on muscle force. Only 6% scored maximum marks. The question required students to use the y-axis (force as percentage of control at 100%) to compare conditions against the control, rather than against each other. Students who did not recognise the y-axis meaning could not evaluate whether force was greater or lesser than the normal pH condition — leading them to conclude that no controls existed, making evaluation impossible. At higher temperatures, force exceeded the control value, a nuance that required axis interpretation unavailable to students who misread the scale.
2024 P2 Q02.34 marks
Comparison of fast and slow muscle fibres. Only 5% scored all four marks; 83% scored at least one. The comparison mark required describing both fibre types together rather than each in isolation; students who described fast fibres and slow fibres separately without explicitly comparing them failed to earn the comparison mark point. Writing "working anaerobically" rather than "fast fibres use anaerobic respiration" or "slow fibres use aerobic respiration" was also rejected, as the mark required the fibre type to be named alongside the respiration type.
08
Difficulty signals
Performance metric synthesis
40PP GAP
Mean accessibility
67.3%
Mean mastery
27.6%
Mean student strength
56.2%
The accessibility–mastery gap of 39.7 percentage points characterises this sub-section's difficulty profile. Most students reach partial credit; full marks remain harder to achieve. Within 3.6 (Organisms respond to changes in their environments), 3.6.3 ranks 3 of 4 sub-sections by mean mastery (1 = hardest). Mastery trajectory is broadly flat across the cohort window: 29.0% in 2017 → 34.6% in 2024 (+5.6 percentage points). Mean mastery was lowest in 2019 (15.5%) and highest in 2024 (34.6%).