← Back to Intelligence
// TOPIC PROFILE
3.3.4
Mass Transport
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
50
Total marks
113
Marks / Q
2.3
Accessibility
57.5%
Mastery
23.6%
Student strength
37.3%
01
At a glance
3.3.4 · Mass Transport
8YRSYNTHESIS
3.3.4 (Mass Transport) appeared in 9 of the 8 years between 2017 and 2024, contributing 50 questions and 113 marks across Papers 1, 2 and 3. APPLICATION dominates the mark distribution at 55.8% of total marks. The accessibility–mastery gap sits at 33.9 percentage points (57.5% vs 23.6%) — 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 2023 (7.5%) and highest in 2022 (34.1%).
Access–mastery gap
+34 pp
Lowest mastery
2023 · 7.5%
Highest mastery
2022 · 34.1%
02
Question type split
By marks · compound to dominant
113MARKS
113
marks
Application55.8%63 marks
Knowledge26.5%30 marks
Calculation17.7%20 marks
(by marks; compound rows assigned to dominant type):
03
Credit terms — quick reference
Mark scheme tier-locked
32TERMS
Tier 1 · Always credit
water potentialosmosishydrostatic pressure
Tier 2 · Sometimes credit
haemoglobintissue fluidmass flowactive transportxylem transportlower affinityleft ventricleatriumventricleaortic valveexerciseaerobic respirationtranspirationwater potential gradientaortapulmonary arterypulmonary veinvena cavalow pO2
Reject · Never credit
high pressure at arteriole end (must specify higher than normal)diameter instead of radiusnot multiplied by 4mass/volume of water lostone side of leaf onlyair bubblesmass flowphotosynthesisradioactive carbon described without photosynthesis linkdescription of data only
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 | 4 | 8 | 41.0% | 22.8% |
| 2018 | 7 | 15 | 52.0% | 17.6% |
| 2019 | 8 | 16 | 52.9% | 30.6% |
| 2020 | 10 | 19 | — COVID | — COVID |
| 2021 | 2 | 3 | — COVID | — COVID |
| 2022 | 7 | 18 | 74.9% | 34.1% |
| 2023 | 4 | 11 | 53.8% | 7.5% |
| 2024 | 3 | 8 | 63.3% | 21.7% |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
33TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| water potential | 3 | 2017, 2022, 2023 | |
| osmosis | 3 | 2017, 2022, 2023 | |
| hydrostatic pressure | 3 | 2017, 2019, 2022 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| haemoglobin | 3 | 2019, 2022 | |
| tissue fluid | 2 | 2017, 2023 | |
| mass flow | 2 | 2018, 2022 | |
| active transport | 2 | 2018, 2022 | |
| xylem transport | 2 | 2018 | |
| lower affinity | 2 | 2019, 2022 | |
| left ventricle | 2 | 2019, 2025 | |
| atrium | 2 | 2019, 2022 | |
| ventricle | 2 | 2019, 2022 | |
| aortic valve | 2 | 2020, 2025 | |
| exercise | 2 | 2020, 2022 | |
| aerobic respiration | 2 | 2020, 2022 | |
| transpiration | 2 | 2022, 2023 | |
| water potential gradient | 2 | 2022, 2023 | |
| aorta | 2 | 2022, 2025 |
Commonly rejected language
| Term | Times rejected | Years | Why rejected |
|---|---|---|---|
| high pressure at arteriole end (must specify higher than normal) | 1 | 2017 | |
| diameter instead of radius | 1 | 2017 | |
| not multiplied by 4 | 1 | 2017 | |
| mass/volume of water lost | 1 | 2017 | |
| one side of leaf only | 1 | 2017 | |
| air bubbles | 1 | 2017 | |
| mass flow | 1 | 2017 | |
| photosynthesis | 1 | 2017 | |
| radioactive carbon described without photosynthesis link | 1 | 2018 | |
| description of data only | 1 | 2018 | |
| 'very similar' or 'not very different' (too vague) | 1 | 2018 | |
| difference noted without SD overlap | 1 | 2018 | |
| heat treatment applied halfway up plant (incorrect context for this question) | 1 | 2018 | |
| active site | 1 | 2018 | |
| 'positive cooperativity' unqualified (must explain mechanism) | 1 | 2018 |
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
4CASE STUDIES
Conceptual errors
- "Tissue fluid reabsorbed" written instead of "water returned by osmosis" at the venule end — the process is osmosis returning water specifically into the capillary; stating that tissue fluid is reabsorbed describes the outcome at the wrong level of precision and was penalised in 2017; blood proteins reduce water potential of plasma, drawing water in by osmosis (2017 P1 Q03.3, 2023 P1 Q09.6)
- "Positive cooperativity" named without explaining the conformational mechanism — only 11% of students scored both marks on the 2018 haemoglobin question; naming the phenomenon without describing the oxygen-induced tertiary/quaternary structure change that exposes a second binding site produced zero marks; the examiner explicitly noted that naming a process is not explaining it (2018 P3 Q04.1)
- Phloem mass flow confused with xylem transport and vice versa — in 2023, translocation source/sink language was used to describe cohesion-tension in xylem; in 2022, mass flow in phloem was described as diffusion along a pressure gradient; these cross-system errors are the most consequential failure mode in multi-mark questions on either transport pathway (2022 P1 Q09.1, 2023 P1 Q02.3)
- Muscles in arteries described as pumping blood — muscle action in artery walls produces vasoconstriction, not pumping; pumping is the function of the cardiac muscle; vasoconstriction itself applies to arterioles, not the aorta; the elastic tissue of the aorta, not muscle, maintains pressure between heartbeats (2024 P1 Q08.1)
Vocabulary errors
- "Similar" used instead of "no significant difference" for statistical conclusions — insufficiently precise for AQA; standard deviation data must be used to conclude that groups are not significantly different, not merely similar (2018 P1 Q08.2)
- "The mouse has lower affinity for oxygen" — "haemoglobin" must be specified; the affinity belongs to haemoglobin, not to the organism; this phrasing was penalised in 2019 and the missing word cost the mark (2019 P1 Q09.3)
- Counter-current flow cited as a difference between fish and mammal circulatory systems — counter-current flow is a feature of gill gas exchange, not of the circulatory system structure; the circulatory difference is single versus double circuit with different oxygenation states of blood in the heart (2022 P1 Q08.4)
- "Tissue fluid returns" written instead of "water returns" — in 2023 Q09.6, this precision error cost one mark from otherwise well-structured accounts; proteins reduce water potential of capillary blood, so water moves back by osmosis, not tissue fluid as a whole (2023 P1 Q09.6)
Application errors
- Phloem tracer data described rather than explained — in 2018 Q08.1, nearly 60% failed to progress; stating "radioactive carbon dioxide was not found throughout the plant" described the data without applying knowledge; the chain required was: CO₂ fixed in photosynthesis → sugar produced in leaf → sugar transported in phloem → phloem damage prevents transport (2018 P1 Q08.1)
- COHb affinity reversed — in 2023 Q07.1, almost as many students said COHb has lower oxygen affinity as said higher, despite the graph clearly showing a left-shifted curve; the Bohr shift and pH changes were also given extensively despite being irrelevant to the question (2023 P1 Q07.1)
- Percentage change in stroke volume calculated using wrong denominator — using the diseased heart's stroke volume rather than the healthy heart's stroke volume in the denominator produces −55% rather than the correct −35%; this was the primary arithmetic error in 2024 Q08.3 (2024 P1 Q08.3)
- Sigmoid oxyhaemoglobin dissociation curves drawn as straight lines — in 2019 Q09.1, three-quarters of students plotted the data points correctly but did not draw sigmoid curves, and did not extend the axes beyond the provided data to complete the curve shape (2019 P1 Q09.1)
High-impact failures · examiner narrative
2018 P3 Q04.12 marks11%full marks
Tested the mechanism of cooperative oxygen binding in haemoglobin. Only 11% scored both marks. The examiner's observation was precise: stating "this is positive cooperativity" scored zero; explaining that oxygen binding causes a change in tertiary/quaternary structure, exposing a second binding site, was required for the marks. Students who wrote about shape changes without specifying tertiary or quaternary structure failed at the vocabulary level. Those who used "active site" — haemoglobin is not an enzyme — were also penalised. The question is a clear case where knowing the correct term for a phenomenon is not the same as understanding the mechanism.
2023 P1 Q02.33 marks
Tested the cohesion-tension mechanism of xylem transport. Fewer than 5% achieved all three marks. About half of students mentioned a "water column" and some correctly described cohesion via hydrogen bonds, but very few explained how tension is generated — the evaporation of water from mesophyll cells, the resulting low water potential in leaves pulling the water column up. A common misconception was applying translocation terminology (source, sink, mass flow of sucrose) to the xylem, producing a mechanistically inverted account. The examiner did not flag the error as occasional — it appeared as a systematic confusion between the two transport pathways.
2023 P1 Q07.13 marks
Tested interpretation of an oxyhaemoglobin dissociation curve for carboxyhaemoglobin (COHb). 40% scored zero. The dominant failure was applying general knowledge about haemoglobin rather than reading the graph: students wrote that COHb would load more readily at low pO₂ (true for normal Hb, false for COHb in this context) without checking the graph, which showed COHb unloads far less oxygen at tissue pO₂. Almost equal numbers of students stated COHb had lower affinity as stated higher — the correct answer was available from the figure throughout. Bohr shift and pH effects were offered by many students in an attempt to explain what was shown, despite being irrelevant to the question.
2022 P1 Q09.15 marks
Tested phloem mass flow from source to sink. A third of students achieved 4 or 5 marks, meaning a substantial fraction could not access the upper marks. The specific losses: sucrose was not named as the transported carbohydrate (only carbohydrate was credited if correctly named); co-transport of sucrose into phloem companion cells was poorly described, with H⁺ movement directions frequently inverted; mass flow was described as diffusion along a pressure gradient; and sucrose unloading at the sink was described as simple diffusion rather than active transport or facilitated diffusion. The question discriminated between students who understood the mechanism and those who had memorised a loose summary.
08
Difficulty signals
Performance metric synthesis
34PP GAP
Mean accessibility
57.5%
Mean mastery
23.6%
Mean student strength
37.3%
The accessibility–mastery gap of 33.9 percentage points characterises this sub-section's difficulty profile. Most students reach partial credit; full marks remain harder to achieve. Within 3.3 (Organisms exchange substances with their environment), 3.3.4 ranks 2 of 4 sub-sections by mean mastery (1 = hardest). Mastery trajectory is broadly flat across the cohort window: 22.8% in 2017 → 21.0% in 2025 (-1.8 percentage points). Mean mastery was lowest in 2023 (7.5%) and highest in 2022 (34.1%).