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3.6.4
Homeostasis
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
39
Total marks
89
Marks / Q
2.3
Accessibility
65.5%
Mastery
32.2%
Student strength
45.9%
01
At a glance
3.6.4 · Homeostasis
8YRSYNTHESIS
3.6.4 (Homeostasis) appeared in 8 of the 8 years between 2017 and 2024, contributing 39 questions and 89 marks across Papers 1, 2 and 3. APPLICATION dominates the mark distribution at 55.1% of total marks. The accessibility–mastery gap sits at 33.2 percentage points (65.5% vs 32.2%) — 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 (21.7%) and highest in 2017 (46.3%). Calculation marks are a small share (9.0%) but typically sit at the lower end of the mastery distribution.
Access–mastery gap
+33 pp
Lowest mastery
2018 · 21.7%
Highest mastery
2017 · 46.3%
02
Question type split
By marks · compound to dominant
89MARKS
89
marks
Application55.1%49 marks
Knowledge36.0%32 marks
Calculation9.0%8 marks
(by marks; compound rows assigned to dominant type):
03
Credit terms — quick reference
Mark scheme tier-locked
29TERMS
Tier 1 · Always credit
insulin productionreceptorsosmosiscollecting duct
Tier 2 · Sometimes credit
blood glucoselong-term effectswater potentialstandard deviationssignificant differenceblood volumechannel proteinsglucosefacilitated diffusionactive transportcreatinine-detecting solutiontertiary structuremembraneobesitywater reabsorption
Reject · Never credit
diabetes is caused by diet/exercise (direction confusion)osmotic movement of fluid (not water)further production (doesn't imply more)re-stating diagram onlyinsulin opens existing channelsglycogen formationfat metabolismglucagon converts (acts as enzyme); glycogen (glucogenolysis not gluconeogenesis); produced in pancreasactive transport (glucose uptake is by facilitated diffusion); 'active site' for receptorglycolysis (wrong pathway); glucolysis/glucogenesis (not biological terms)
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 | 7 | 86.3% | 46.3% |
| 2018 | 7 | 13 | 56.7% | 21.7% |
| 2019 | 8 | 17 | 64.6% | 35.4% |
| 2020 | 3 | 8 | — COVID | — COVID |
| 2021 | 3 | 8 | — COVID | — COVID |
| 2022 | 7 | 16 | 60.0% | 33.6% |
| 2023 | 4 | 9 | 60.8% | 28.2% |
| 2024 | 4 | 11 | 81.2% | 35.5% |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
34TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| insulin production | 3 | 2017, 2022, 2024 | |
| receptors | 3 | 2017, 2019, 2022 | |
| osmosis | 3 | 2018, 2019, 2022 | |
| collecting duct | 3 | 2018, 2019, 2023 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| blood glucose | 2 | 2017, 2022 | |
| long-term effects | 2 | 2017, 2022 | |
| water potential | 2 | 2018 | |
| standard deviations | 2 | 2018, 2022 | |
| significant difference | 2 | 2018, 2022 | |
| blood volume | 2 | 2018, 2023 | |
| channel proteins | 2 | 2018, 2022 | |
| glucose | 2 | 2019 | |
| facilitated diffusion | 2 | 2019, 2023 | |
| active transport | 2 | 2019, 2023 | |
| creatinine-detecting solution | 2 | 2020 | |
| tertiary structure | 2 | 2021, 2023 | |
| membrane | 2 | 2021, 2022 | |
| obesity | 2 | 2022, 2024 | |
| water reabsorption | 2 | 2022, 2023 |
Commonly rejected language
| Term | Times rejected | Years | Why rejected |
|---|---|---|---|
| diabetes is caused by diet/exercise (direction confusion) | 1 | 2017 | |
| osmotic movement of fluid (not water) | 1 | 2018 | |
| further production (doesn't imply more) | 1 | 2018 | |
| re-stating diagram only | 1 | 2018 | |
| insulin opens existing channels | 1 | 2018 | |
| glycogen formation | 1 | 2018 | |
| fat metabolism | 1 | 2018 | |
| glucagon converts (acts as enzyme); glycogen (glucogenolysis not gluconeogenesis); produced in pancreas | 1 | 2019 | |
| active transport (glucose uptake is by facilitated diffusion); 'active site' for receptor | 1 | 2019 | |
| glycolysis (wrong pathway); glucolysis/glucogenesis (not biological terms) | 1 | 2019 | |
| 'increasing/higher blood pressure' (not same as high); blood proteins listed as filtrate | 1 | 2019 | |
| water reabsorbed INTO the loop (wrong direction); 'ions' without specifying sodium; 'long diffusion pathway' | 1 | 2019 | |
| calorimeter (for colorimeter) | 1 | 2020 | |
| none (ignore nephron) | 1 | 2020 | |
| change in tertiary structure of receptor; 'active site'; reference to enzyme or substrate | 1 | 2021 |
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
- Insulin described as opening existing channel proteins rather than causing insertion of new ones — in 2018, the specification content requires understanding that insulin regulates glucose uptake by controlling the inclusion of channel proteins in the cell-surface membrane; students who wrote "insulin opens channels" described a different mechanism (direct gating) and were not credited; only 19% scored both marks on this question (2018 P3 Q01.3)
- Gluconeogenesis defined as the conversion of glycogen to glucose rather than from non-carbohydrate precursors — in 2019, only 44% scored at least one mark for gluconeogenesis; the most common error was defining it as the same process as glycogenolysis (glycogen breakdown); gluconeogenesis is the synthesis of glucose from glycerol, amino acids, or lactate; conflating these two processes demonstrates a fundamental confusion between the two routes to blood glucose elevation (2019 P2 Q05.1)
- Water reabsorption in the loop of Henle described as occurring in the wrong direction — in 2019, students stated that water moved into the loop rather than out of it; water leaves the descending limb of the loop of Henle by osmosis into the medullary interstitium; students who wrote it moves inward reversed the osmotic gradient created by the high sodium ion concentration in the medulla (2019 P3 Q01.4)
- Hypothalamus named as the site of baroreceptors for blood pressure detection — in 2023, students who identified the hypothalamus as the location where blood volume changes are detected were penalised; baroreceptors are in the walls of the aorta and carotid arteries; the hypothalamus contains osmoreceptors that detect blood water potential; these are distinct receptors in distinct locations (2023 P2 Q06.4)
Vocabulary errors
- "Fluid" used instead of "water" in osmosis descriptions — in 2018, "osmotic movement of fluid" was rejected; osmosis is the movement of water specifically, not fluid in general; this distinction was enforced in the nephron context where water reabsorption from the collecting duct is the mark requirement (2018 P2 Q05.3)
- "Glucagon converts" used to describe gluconeogenesis, implying glucagon acts as an enzyme — in 2019, stating "glucagon converts amino acids to glucose" was rejected; glucagon is a hormone that activates enzymes; it does not itself perform the conversion; the phrasing implies an enzymatic role for a signalling molecule (2019 P2 Q05.1)
- "Active site" used instead of "receptor" when describing insulin binding — in 2019, "insulin binding to active site" was rejected; insulin binds to specific receptor proteins on target cell membranes; the active site is a feature of an enzyme's catalytic function; using active site in a receptor-binding context conflates two different types of protein–molecule interaction (2019 P2 Q05.3)
Application errors
- Plasma flow rate calculation missing the 60% conversion factor — in 2018, only 20% scored the kidney filtration calculation mark; the question provided mean blood flow rate into the kidney and stated that plasma constitutes 60% of blood volume; students who did not convert from total blood flow to plasma flow obtained 20,400 cm³ rather than the correct value; the 60% factor was stated in the question but not applied (2018 P2 Q05.5)
- Group A and Group B misidentified on a graph by not reading the key — in 2017, a significant minority of students confused the two treatment groups by misreading the key, restricting themselves to a maximum of two marks on a four-mark question; accuracy of data extraction before answering was the limiting factor, not conceptual understanding (2017 P2 Q06.3)
- ADH mechanism described without naming aquaporins — in 2022, a quarter of students scored full marks on the ADH collecting duct question; many correctly stated that ADH increases water reabsorption but did not explain the aquaporin mechanism; the mark required that vesicles containing aquaporins are inserted into the cell-surface membrane, increasing its permeability to water; stating only that "permeability increases" without the aquaporin mechanism earned one but not both marks (2022 P2 Q09.3)
High-impact failures · examiner narrative
2019 P3 Q01.43 marks
Role of loop of Henle length in medullary water potential. Only 2% scored all three marks. The question required: longer loops extend further into the medulla (mp1); the medulla has a lower water potential due to high sodium ion concentration (mp2, with "sodium ions" specifically required — not just "ions"); water therefore moves out of the loop into the medulla by osmosis (mp3). Students who described a long diffusion pathway (triggered by seeing "thickness" on the graph) failed mp1. Students who wrote "ions" without specifying sodium failed mp2. Students who described water moving into the loop rather than out of it failed mp3. All three marks required precision at different levels.
2022 P2 Q08.55 marks
Evaluation of rat model validity for type 2 diabetes research. Very few students achieved full marks. The question required evaluating both supporting evidence (high-fat diet rats showed significantly higher blood glucose than controls; the difference exceeded the standard deviations) and limitations (rats are not humans; type 2 diabetes mechanisms may differ; the rat model uses beta-cell destruction which models type 1, not type 2). Students who described the data without evaluating its validity, or who failed to use standard deviation data in their evaluation, scored partial credit only. The type 1 versus type 2 distinction — beta cells destroyed in the model but present in type 2 — was identified by only a fifth of students.
08
Difficulty signals
Performance metric synthesis
33PP GAP
Mean accessibility
65.5%
Mean mastery
32.2%
Mean student strength
45.9%
The accessibility–mastery gap of 33.2 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), 3.6.4 ranks 4 of 4 sub-sections by mean mastery (1 = hardest). Mastery trajectory is broadly flat across the cohort window: 46.3% in 2017 → 35.5% in 2024 (-10.8 percentage points). Mean mastery was lowest in 2018 (21.7%) and highest in 2017 (46.3%).