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3.3.3
Digestion and Absorption
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
5
Total marks
13
Marks / Q
2.6
Accessibility
58.3%
Mastery
16.7%
Student strength
41.7%
01
At a glance
3.3.3 · Digestion and Absorption
8YRSYNTHESIS
3.3.3 (Digestion and Absorption) appeared in 4 of the 8 years between 2017 and 2024, contributing 5 questions and 13 marks across Papers 1, 2 and 3. KNOWLEDGE dominates the mark distribution at 76.9% of total marks. The accessibility–mastery gap sits at 41.7 percentage points (58.3% vs 16.7%) — most students reach partial credit, but full marks remain harder to secure. Mastery varied year-to-year, lowest in 2025 (15.0%) and highest in 2024 (17.5%).
Access–mastery gap
+42 pp
Lowest mastery
2024 · 17.5%
Highest mastery
2024 · 17.5%
02
Question type split
By marks · compound to dominant
13MARKS
13
marks
Knowledge76.9%10 marks
Application23.1%3 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
Tier 2 · Sometimes credit
bile saltsfatty acidsdiffusionsurface areamicellescell-surface membrane
Reject · Never credit
facilitated diffusion (for fat absorption)hairsvillimembrane structures inside cellsincreased surface area (alone)ciliaemulsification produces micellesmicelles diffuse across membraneco-transport with sodiumchylomicrons
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 | 0 | 0 | — COVID | — COVID |
| 2019 | 0 | 0 | — COVID | — COVID |
| 2020 | 1 | 3 | — COVID | — COVID |
| 2021 | 1 | 2 | — COVID | — COVID |
| 2022 | 0 | 0 | — COVID | — COVID |
| 2023 | 0 | 0 | — COVID | — COVID |
| 2024 | 2 | 4 | 47.5% | 17.5% |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
19TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| bile salts | 2 | 2020, 2024 | |
| fatty acids | 2 | 2020, 2025 | |
| diffusion | 2 | 2020, 2025 | |
| surface area | 2 | 2021, 2025 | |
| micelles | 2 | 2024, 2025 | |
| cell-surface membrane | 2 | 2024, 2025 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| facilitated diffusion (for fat absorption) | 1 | 2020 | |
| hairs | 1 | 2024 | |
| villi | 1 | 2024 | |
| membrane structures inside cells | 1 | 2024 | |
| increased surface area (alone) | 1 | 2024 | |
| cilia | 1 | 2024 | |
| emulsification produces micelles | 1 | 2024 | |
| micelles diffuse across membrane | 1 | 2024 | |
| co-transport with sodium | 1 | 2024 | |
| chylomicrons | 1 | 2024 | |
| emulsification causes hydrolysis | 1 | 2025 | |
| triglyceride formation | 1 | 2025 | |
| facilitated diffusion of micelles | 1 | 2025 |
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
- Emulsification described as causing lipid hydrolysis directly — emulsification breaks fat droplets into smaller droplets to increase surface area for lipase; the hydrolysis is carried out by lipase, not by bile salts; stating "emulsification breaks down lipids" omits lipase entirely and was penalised in 2025 (2025 P1 Q01.1)
- Micelles described as diffusing across cell membranes intact — micelles are transport structures that release fatty acids and monoglycerides at the membrane surface; the individual molecules then diffuse through the phospholipid bilayer; the micelle unit does not cross the membrane (2024 P1 Q02.3, 2025 P1 Q01.1)
- Microvilli confused with villi — microvilli are single-cell folds of the cell-surface membrane; villi are multicellular finger-like projections of the gut lining involving many cells; most students described multicellular structures when the question asked about microvilli (2024 P1 Q02.2)
Vocabulary errors
- "Increased surface area" given as the structural description of microvilli — increased surface area is the function that results from microvilli; the structure is folds of the cell-surface membrane; the two are not interchangeable (2024 P1 Q02.2)
- Fatty acids and monoglycerides not named as the molecules that diffuse — "digested lipids" is insufficiently specific; the mark scheme requires fatty acids and/or monoglycerides to be named as the molecules released from micelles and absorbed across the membrane (2025 P1 Q01.1)
Application errors
- Sodium co-transport invoked for vitamin A absorption — vitamin A is a lipid-soluble molecule that diffuses through the phospholipid bilayer; sodium ion co-transport is the mechanism for glucose and amino acid absorption and is not relevant to lipid-soluble vitamin uptake (2024 P1 Q02.3)
- Golgi processing and chylomicron production described after absorption — the question in 2024 asked how vitamin A is absorbed into ileum cells; many students continued into intracellular processing, Golgi modification, and chylomicron release, none of which answered what was asked; the absorption step is where marks sat (2024 P1 Q02.3)
High-impact failures · examiner narrative
2024 P1 Q02.33 marks
Tested absorption of vitamin A (a lipid-soluble molecule) via micelles across the ileum cell-surface membrane. Multiple misconceptions stacked within single answers: emulsification was described as producing micelles (emulsification increases surface area for lipase action; micelles are formed separately by bile salt association); micelles were described as crossing membranes intact; and sodium co-transport was invoked for a lipid-soluble molecule. The examiners credited up to two marks even where students described lipid digestion generally rather than vitamin A specifically, but the combination of errors prevented many from reaching the third mark. The question required describing the intact vitamin A molecule entering via micelle → release → diffusion, and most answers either short-circuited or reversed this chain.
2025 P1 Q01.14 marks
Tested emulsification and micelle roles in lipid digestion and absorption. Fewer than 15% achieved all four marks. The most frequent loss at the emulsification stage was omitting lipase entirely — many stated that bile salts emulsify lipids and the lipids are then absorbed, skipping hydrolysis. At the micelle stage, "micelles diffuse to ileum cells" was common; the mark requires recognising that it is the fatty acids and monoglycerides released from micelles that diffuse, not the micelle itself. The final mark — specifying that fatty acids/monoglycerides diffuse across the membrane — was also frequently missed when students wrote only that "lipids are absorbed" without naming the molecules.
08
Difficulty signals
Performance metric synthesis
42PP GAP
Mean accessibility
58.3%
Mean mastery
16.7%
Mean student strength
41.7%
The accessibility–mastery gap of 41.7 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), 3.3.3 ranks 1 of 4 sub-sections by mean mastery (1 = hardest). Mean mastery was lowest in 2025 (15.0%) and highest in 2024 (17.5%).