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3.1.3
Lipids
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
9
Total marks
22
Marks / Q
2.4
Accessibility
76.2%
Mastery
52.5%
Student strength
70.0%
01
At a glance
3.1.3 · Lipids
8YRSYNTHESIS
3.1.3 (Lipids) appeared in 4 of the 8 years between 2017 and 2024, contributing 9 questions and 22 marks across Papers 1, 2 and 3. KNOWLEDGE dominates the mark distribution at 54.5% of total marks. The accessibility–mastery gap sits at 23.8 percentage points (76.2% vs 52.5%) — 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 2018 (15.0%) and highest in 2024 (65.0%).
Access–mastery gap
+24 pp
Lowest mastery
2018 · 15.0%
Highest mastery
2024 · 65.0%
02
Question type split
By marks · compound to dominant
22MARKS
22
marks
Knowledge54.5%12 marks
Application45.5%10 marks
Calculation0.0%0 marks
(by marks; compound rows assigned to dominant type):
03
Credit terms — quick reference
Mark scheme tier-locked
16TERMS
Tier 1 · Always credit
glycerol
Tier 2 · Sometimes credit
ester bondsfatty acidscondensation reactionwater removedester bond
Reject · Never credit
separate descriptions without explicit comparisonenergetic valuesapplicationsprecisionglycosidicpeptidehydrogen bondH from C-H circledoleiclinoleic
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 | 0 | 0 | — COVID | — COVID |
| 2018 | 1 | 5 | 65.0% | 15.0% |
| 2019 | 0 | 0 | — COVID | — COVID |
| 2020 | 1 | 3 | — COVID | — COVID |
| 2021 | 4 | 9 | — COVID | — COVID |
| 2022 | 0 | 0 | — COVID | — COVID |
| 2023 | 0 | 0 | — COVID | — COVID |
| 2024 | 3 | 5 | 80.0% | 65.0% |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
18TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| glycerol | 4 | 2018, 2020, 2021, 2024 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| ester bonds | 2 | 2018, 2020 | |
| fatty acids | 2 | 2018, 2021 | |
| condensation reaction | 2 | 2021, 2024 | |
| water removed | 2 | 2021, 2024 | |
| ester bond | 2 | 2021, 2024 |
Commonly rejected language
| Term | Times rejected | Years | Why rejected |
|---|---|---|---|
| separate descriptions without explicit comparison | 1 | 2018 | |
| energetic values | 1 | 2018 | |
| applications | 1 | 2018 | |
| precision | 1 | 2021 | |
| glycosidic | 1 | 2024 | |
| peptide | 1 | 2024 | |
| hydrogen bond | 1 | 2024 | |
| H from C-H circled | 1 | 2024 | |
| oleic | 1 | 2024 | |
| linoleic | 1 | 2024 | |
| hydrogen molecules | 1 | 2024 | |
| boiling point | 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
2CASE STUDIES
Conceptual errors
- Glycerol omitted from the structure of phospholipids — many students described triglycerides correctly (glycerol + 3 fatty acids) but failed to include glycerol when describing the phospholipid; the phospholipid structure is glycerol + 2 fatty acids + phosphate group, and omitting glycerol loses a critical structural mark (2018 P1 Q10.2)
- Direction of the melting point–unsaturation relationship reversed — some students stated that more double bonds raise the melting point rather than lower it; increasing unsaturation decreases the melting point because double bonds prevent close molecular packing (2024 P1 Q01.3)
- "Glycosidic" named as the bond in a triglyceride — the correct term is "ester bond"; glycosidic bonds link monosaccharides in polysaccharides and do not apply to lipid chemistry (2024 P1 Q01.1)
Vocabulary errors
- "Hydrogen molecules" written instead of hydrogen atoms — students describing the relationship between hydrogen content and melting point used "hydrogen molecules" (H₂), which is incorrect; the relevant unit is hydrogen atoms within C–H and O–H bonds in the fatty acid chain (2024 P1 Q01.3)
- "Boiling point" written when the question asked about melting point — a reading error flagged by the examiner; the question explicitly asked about the temperature at which solid becomes liquid (2024 P1 Q01.3)
- Limited examiner data available for this category beyond the above.
Application errors
- Separate descriptions written instead of comparative statements for compare/contrast questions — in 2018 Q10.2, every mark point required explicit pairing of triglyceride and phospholipid; examiners noted they would not infer links between separate statements about each molecule (2018 P1 Q10.2)
- Energetic values and functional applications included instead of structural comparison — discussions of why lipids store energy or how they function in membranes did not earn marks in 2018 Q10.2, which asked only for structural and property comparisons (2018 P1 Q10.2)
- Both OH groups circled in the ester bond diagram rather than H from one molecule and OH from the other — this fails to show that water is removed in the condensation reaction; the diagram must identify the atoms that leave as water, not circle identical groups on both molecules (2024 P1 Q01.1)
High-impact failures · examiner narrative
2018 P1 Q10.25 marks15%full marks
Tested structural and property comparison of triglycerides and phospholipids. Only 15% scored full marks. The examiner identified two systemic failures: despite knowing each molecule individually, students could not produce comparative statements under the "compare and contrast" command — examiners explicitly noted they would not infer links between separate descriptions; and glycerol was omitted from the phospholipid structure by "many students" despite being a direct structural component. Functional and energetic content further diluted answers. This is the joint-lowest mastery question in 3.1.3, and its failure mode — correct but non-comparative content — recurs across the subtopic wherever comparison questions appear.
2024 P1 Q01.13 marks
Tested diagram-based identification of the atoms removed in ester bond formation, and naming the bond and reaction type. Most students correctly named the ester bond and condensation reaction. The diagram element was the discriminating part: many circled the H from a C–H group of glycerol rather than the H from the O–H group, and others circled OH from both molecules, failing to show that water is the removed product. The examiner flagged "glycosidic" as a common bond-name error. This question shows that procedural knowledge (condensation removes water) does not guarantee structural knowledge (which specific atoms constitute that water in a given molecule).
08
Difficulty signals
Performance metric synthesis
24PP GAP
Mean accessibility
76.2%
Mean mastery
52.5%
Mean student strength
70.0%
The accessibility–mastery gap of 23.8 percentage points characterises this sub-section's difficulty profile. Most students reach partial credit; full marks remain harder to achieve. Within 3.1 (Biological molecules), 3.1.3 ranks 7 of 7 sub-sections by mean mastery (1 = hardest). Mean mastery was lowest in 2018 (15.0%) and highest in 2024 (65.0%).