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3.2.3
Transport Across Cell Membranes
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
38
Total marks
99
Marks / Q
2.6
Accessibility
66.0%
Mastery
26.9%
Student strength
43.8%
01
At a glance
3.2.3 · Transport Across Cell Membranes
8YRSYNTHESIS
3.2.3 (Transport Across Cell Membranes) appeared in 9 of the 8 years between 2017 and 2024, contributing 38 questions and 99 marks across Papers 1, 2 and 3. APPLICATION dominates the mark distribution at 56.6% of total marks. The accessibility–mastery gap sits at 39.1 percentage points (66.0% vs 26.9%) — 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 2024 (18.0%) and highest in 2019 (40.0%). Calculation marks are a small share (14.1%) but typically sit at the lower end of the mastery distribution.
Access–mastery gap
+39 pp
Lowest mastery
2024 · 18.0%
Highest mastery
2019 · 40.0%
02
Question type split
By marks · compound to dominant
99MARKS
99
marks
Application56.6%56 marks
Knowledge29.3%29 marks
Calculation14.1%14 marks
(by marks; compound rows assigned to dominant type):
03
Credit terms — quick reference
Mark scheme tier-locked
31TERMS
Tier 1 · Always credit
osmosiswater potentialco-transportactive transportconcentration gradientmicrovillifacilitated diffusionATP hydrolysisATP
Tier 2 · Sometimes credit
surface areatemperaturechannel proteinaquaporinwater moves inphospholipid bilayerchannel proteinswater potential gradientcarrier proteinfatty acidsbilayerH+ into vacuole
Reject · Never credit
villithin membranesincorrect chemical symbols for Na⁺ and H⁺results being significant/due to chance (reject once only)probability is 0.05% or 5amount (for mineral ionslightwaterCO₂)nutrients (not acceptable for mineral ions)
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 | 8 | 22 | 67.9% | 30.4% |
| 2018 | 4 | 10 | 58.5% | 20.0% |
| 2019 | 3 | 6 | 66.7% | 40.0% |
| 2020 | 9 | 21 | — COVID | — COVID |
| 2021 | 1 | 5 | — COVID | — COVID |
| 2022 | 4 | 10 | 65.5% | 34.5% |
| 2023 | 1 | 3 | 65.0% | 25.0% |
| 2024 | 4 | 11 | 63.8% | 18.0% |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
36TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| osmosis | 7 | 2017, 2018, 2020, 2021, 2022, 2023, 2025 | |
| water potential | 6 | 2017, 2018, 2022, 2023, 2024, 2025 | |
| co-transport | 5 | 2017, 2020, 2021, 2025 | |
| active transport | 4 | 2017, 2020, 2021, 2025 | |
| concentration gradient | 4 | 2017, 2020, 2021, 2024 | |
| microvilli | 3 | 2017, 2020, 2023 | |
| facilitated diffusion | 3 | 2017, 2020, 2021 | |
| ATP hydrolysis | 3 | 2017, 2020, 2025 | |
| ATP | 3 | 2020, 2021, 2024 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| surface area | 2 | 2017, 2023 | |
| temperature | 2 | 2017, 2020 | |
| channel protein | 2 | 2017, 2024 | |
| aquaporin | 2 | 2017 | |
| water moves in | 2 | 2018, 2024 | |
| phospholipid bilayer | 2 | 2018, 2020 | |
| channel proteins | 2 | 2020, 2023 | |
| water potential gradient | 2 | 2021, 2024 | |
| carrier protein | 2 | 2021, 2024 | |
| fatty acids | 2 | 2022, 2025 | |
| bilayer | 2 | 2022, 2025 | |
| H+ into vacuole | 2 | 2025 |
Commonly rejected language
| Term | Times rejected | Years | Why rejected |
|---|---|---|---|
| villi | 2 | 2017, 2020 | |
| thin membranes | 1 | 2017 | |
| incorrect chemical symbols for Na⁺ and H⁺ | 1 | 2017 | |
| results being significant/due to chance (reject once only) | 1 | 2017 | |
| probability is 0.05% or 5 | 1 | 2017 | |
| amount (for mineral ions | 1 | 2017 | |
| light | 1 | 2017 | |
| water | 1 | 2017 | |
| CO₂) | 1 | 2017 | |
| nutrients (not acceptable for mineral ions) | 1 | 2017 | |
| pH alone | 1 | 2017 | |
| enzyme-catalysed reactions (for temperature effect) | 1 | 2017 | |
| polar | 1 | 2017 | |
| non-polar | 1 | 2017 | |
| polarity | 1 | 2017 |
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
- "Thin membranes" described as a membrane adaptation — this conflates the phospholipid bilayer with the epithelial layer it forms; the bilayer is nanometres thin by nature and is not adapted by thinning; referencing villi rather than microvilli generated the same confusion and blocked mark point 1 in 2017 (2017 P1 Q02.3)
- Facilitated diffusion described as an active process requiring ATP — a significant minority of students wrote this explicitly; ATP is exclusive to active transport; the error compounds when channel proteins are then named for active transport, which uses only carriers (2024 P1 Q02.1)
- "Turgidity" applied to animal cells — animal cells have no cell wall and cannot become turgid; students reaching for this GCSE schema in osmosis contexts lost marks regardless of other accuracy (2018 P1 Q01.3)
- Water "concentration" used throughout instead of water potential — AQA requires water potential terminology at A-level; writing about relative water concentrations rather than water potential gradients was penalised in multiple years including 2024 (2024 P1 Q04.4)
- Fructose co-transport described as simple active transport or facilitated diffusion — co-transport uses the proton gradient established by active H⁺ pumping; describing fructose as entering by its own active transport or passive facilitated diffusion ignores the coupling mechanism that defines co-transport (2025 P1 Q05.1)
Vocabulary errors
- "Amount of light" written instead of "light intensity"; "nutrients" instead of "mineral ion concentration" — AQA explicitly rejects imprecise quantitative language at A-level; these are not equivalent phrasings, and both were penalised examples in 2017 (2017 P1 Q06.1)
- "Results are significant" rather than "the difference between the means is significant" — the distinction is enforced every year; students who reached the correct statistical conclusion but used "results" rather than "difference" lost the mark; this was penalised in the 2017 P-value interpretation question (2017 P1 Q03.2)
- Detergent described as "breaking down" or "hydrolysing" the membrane — the mark scheme required "dissolves"; detergent dissolves lipids in water; "breaks down" implies a chemical reaction that is not happening, and students who repeated this language from the question stem without the key verb scored one mark not two (2018 P3 Q05.1)
- Phosphate groups called hydrophobic; fatty acid tails called hydrophilic — inverted recall of membrane properties; students writing this were penalised in 2025 even when the bilayer structure itself was correctly described elsewhere in the answer (2025 P1 Q01.2)
Application errors
- Whole-cell adaptations described for a question that asked specifically about cell-surface membranes — "one cell thick", "many mitochondria", "a good blood supply", and "villi" are epithelial or organ-level adaptations, not membrane adaptations; the examiner noted this in 2017 as the most common reason for losing marks on an otherwise manageable question (2017 P1 Q02.3)
- LSD (least significant difference) misidentified as a form of standard deviation — in 2017 P1 Q06.2 the LSD threshold was explained in the question stem, but many students treated it as a measure of spread rather than a significance threshold, producing incorrect conclusions about whether herbicide effects were distinguishable (2017 P1 Q06.2)
- Potometer described for an osmosis investigation — the potometer measures transpiration rate, not water potential differences between cells and solutions; any method relying on a potometer scored zero regardless of surrounding accuracy (2022 P1 Q06.3)
- Water movement direction stated unclearly or not stated — "water moves in the ileum" and "water moves out" were both cited as imprecise in 2023; AQA expects explicit directionality (into or out of the named compartment, with reference to water potential) for the movement mark (2023 P1 Q04.1)
High-impact failures · examiner narrative
2017 P1 Q06.24 marks
Tested evaluation of herbicide effectiveness using ion release data alongside a novel least-significant-difference threshold. Very few students achieved full marks. The first loss: the LSD was misread as standard deviation, leading to incorrect conclusions about whether group differences were meaningful. The second: the causal chain — ion release → membrane disruption → cell death — was not completed; students identified that 2,4-D increased ion release from wild oats but stopped before explaining what that ion loss meant for plant survival. Some compounded the error by confusing ions released from leaf discs with soil ions, suggesting that higher external ion concentration would promote plant growth.
2018 P1 Q02.23 marks
Tested rate of osmosis per unit surface area, requiring surface area calculation for one large cube and eight small cubes, a rate reading from a graph, and combination into a rate per unit area. 56.4% failed to score any mark. The most common failure was calculating the surface area for one small cube rather than all eight, or reading the rate from the wrong portion of the graph. Critically, many students showed only a final numerical answer without intermediate steps — meaning any arithmetic error produced zero from what may have been a correct method. A student who showed working at each step could earn a mark despite numerical error; showing only the final wrong answer earned nothing.
2024 P1 Q02.13 marks
Tested definitions of facilitated diffusion and active transport. Only about one in five students achieved all three marks. Failures concentrated on three points: carrier proteins in active transport were omitted or replaced with channel proteins (only carriers operate in active transport); facilitated diffusion was described as active or ATP-dependent by a significant minority; and movement of charged and polar molecules was not mentioned despite being a mark point. The examiner noted these weaknesses reflected imprecise recall of definitions rather than lack of understanding — the same students who wrote that facilitated diffusion requires ATP typically also correctly stated that it moves molecules down a concentration gradient.
2024 P1 Q04.43 marks12%full marks
Tested osmosis in a historical animal-bladder experiment with molasses. Only 12% scored full marks. Two principal losses: students wrote "water concentration" rather than water potential throughout, losing the terminology mark; and the mark distinguishing whether air or solution volume changed was missed by students who correctly described water movement but did not identify the consequence in the apparatus. The "partially permeable membrane" mark required naming the animal bladder as that membrane rather than describing a membrane generically — a specificity demand consistent with the application question pattern throughout this sub-section.
08
Difficulty signals
Performance metric synthesis
39PP GAP
Mean accessibility
66.0%
Mean mastery
26.9%
Mean student strength
43.8%
The accessibility–mastery gap of 39.1 percentage points characterises this sub-section's difficulty profile. Most students reach partial credit; full marks remain harder to achieve. Within 3.2 (Cells), 3.2.3 ranks 1 of 4 sub-sections by mean mastery (1 = hardest). Mastery trajectory is falling across the cohort window: 30.4% in 2017 → 18.8% in 2025 (-11.6 percentage points). Mean mastery was lowest in 2024 (18.0%) and highest in 2019 (40.0%).