AQA A-Level Biology Paper 3, June 2025: Question 2

9 marks · Medium difficulty · Extended Answer

Identify nephron features, explain why proteins are not in urine, and evaluate a claim relating high-protein diet to kidney damage using rat study data.

Practise this question

Question

Question 02. 02.1 features Table 1 with four nephron parts (Glomerulus, Proximal convoluted tubule, Distal convoluted tubule, Collecting duct) and four statements to tick: ADH receptors are present, Water can be absorbed from the filtrate, Blood is under high pressure, Glucose is absorbed from the filtrate. 02.2 asks to explain why protein is not normally present in the urine of a person (2 marks). 02.3 describes an experiment where 5-day-old underweight rats were fed either a high-protein or normal-protein milk diet, followed by solid food until 160 days old, finding significantly more glomerular sclerosis (GS) in the high-protein group (P < 0.01). Students are asked to evaluate a journalist's statement linking high-protein milk in underweight human babies to adult kidney damage (5 marks).
Question text

02.1 Table 1 shows four statements and the names of four parts of a nephron.

Put a tick ( ) in each box where the statement is true for that part of the nephron.

[2 marks]

Table 1

Part of the nephron

Statement Proximal Distal

Collecting

Glomerulus convoluted convoluted

duct

tubule tubule

ADH receptors are

present

Water can be

absorbed from the

filtrate

Blood is under high

pressure

Glucose is

absorbed from the

filtrate

02.2 Protein is not normally present in the urine of a person.

Explain why.

[2 marks]

02.3 Glomerular sclerosis (GS) is a type of kidney damage that can cause protein to be

present in the urine.

High-protein diets are linked to GS in adults.

Scientists investigated the development of GS in underweight, 5-day-old rats.

They fed:

• 14 rats a high-protein milk diet

*06•*14 rats a normal-protein milk diet.

From day 21, they fed both groups of rats the same diet of solid food. At 160 days

old, they examined nephrons in the rats’ kidneys for GS.

They identified the number of nephrons with GS for each group of rats.

Rats fed a high-protein diet had a larger number of nephrons with GS than rats fed a

normal-protein diet. The scientists performed a statistical test on these data and they

calculated P < 0.01

A journalist reported on this investigation and stated ‘feeding high-protein milk to

underweight human babies could cause kidney damage in adults’.

Use all of the information provided to evaluate this statement.

[5 marks]

Mark scheme

Show the mark scheme Mark scheme for Question 02. 02.1 table ticks: ADH receptors (Distal convoluted tubule, Collecting duct); Water absorbed (Proximal convoluted tubule, Distal convoluted tubule, Collecting duct); Blood under high pressure (Glomerulus); Glucose absorbed (Proximal convoluted tubule). 02.2: Plasma proteins too large to pass through basement membrane (2 marks). 02.3: Up to 5 marks for evaluating the statement with supporting points (P < 0.01 means significant / probability < 1% due to chance; both are mammals) and limitations (done on rats not humans; not known if 160-day rats are adults; small sample size; only two protein amounts; other factors causing damage).

Question Marking Guidance Mark Comments

All four rows correct = 2 marks;;

3 or 2 rows correct = 1 mark;

Part of the nephron

Statement

Proximal Distal

Collecting

Glomerulus convoluted convoluted

duct 2

tubule tubule

02.1 ADH receptors (2 x

are present AO1)

Water can be

absorbed from

the filtrate

Blood is under

high pressure

Glucose is

absorbed from

the filtrate

1. (Plasma) proteins are (too) large;

02.2 (2 x

2. To filter/pass through basement membrane;

AO1) 2. Ignore any other

structures

Max 4 for MP3-7

1. Accept probability

(Yes – no mark) of less than

1%/0.05/5% that

1. Probability of less than 0.01 that the increase the increase is due

(in GS) is due to chance to chance

1. Accept probability

OR of more than

99%/0.99/95%/0.95

Increase (in GS) is significant; that the increase is

not due to chance

2. Rats and humans are both mammals, so 1. Accept P for

similar; probability

1. Accept difference

(No – no mark)

for increase

1. Ignore correlation

3. (Investigation only) done on rats

is significant

1. Ignore results are

OR

significant

(Investigation) not done on humans; unqualified

5 max

4. Do not know if (160-day-old) rats are adults; 4. Ignore long terms

8 02.3 (5 x

AO3) effects unknown

5. Small sample size, so not representative;

5. Accept only 14/28

6. Only two (different) quantities of protein (used) (rats), so not

representative

OR

6. Ignore answers

Only high and normal protein (milk/s) relating to protein

content of solid

OR food diet

Protein content (of milk/s) not known

OR

Types of protein (in the milk/s) not known;

7. Only GS studied 7. Accept other

named factors (can

OR cause kidney

damage) e.g.

Other kidney damage (can) occur/s; genetic factors

How to answer it

Nephron Anatomy, Ultrafiltration & Study Evaluation

WHAT THIS QUESTION TESTS

Core Synoptic Assessment: Homeostasis, Osmoregulation & Scientific Data Evaluation

  • Structural Specialisation of the Nephron: Identifying specific sites for ultrafiltration, selective reabsorption of glucose, variable water permeability, and ADH action.
  • Mechanism of Ultrafiltration: Explaining molecular size exclusion with precise anatomical terminology (the role of the basement membrane).
  • Critical Scientific Evaluation (AO3): Assessing a newspaper claim by critically balancing statistical significance (P < 0.01) against experimental limitations (animal model validity, sample size, confounding variables).
PART 02.1 • 2 MARKS

Nephron Regions and Physiological Processes

Matching physiological statements to the specific functional zones of a nephron

💡 Completed Reference Table

Statement Part of the nephron
Glomerulus Proximal convoluted tubule Distal convoluted tubule Collecting duct
ADH receptors are present ✓ ✓
Water can be absorbed from the filtrate ✓ ✓ ✓
Blood is under high pressure ✓
Glucose is absorbed from the filtrate ✓

✅ Mark Scheme Logic

  • All 4 rows completely correct: 2 marks.
  • 2 or 3 rows correct: 1 mark.
  • 0 or 1 row correct: 0 marks.

❌ Common Errors to Avoid

  • Forgetting the DCT has ADH receptors: ADH binds to receptors on both the distal convoluted tubule and the collecting duct to insert aquaporins.
  • Missing water reabsorption in the PCT: ~85% of filtrate water is reabsorbed in the PCT by osmosis following solute uptake.
  • Ticking Loop of Henle: Not an option here, but don't confuse water movement along the PCT with the hairpin countercurrent multiplier.
PART 02.2 • 2 MARKS

Absence of Protein in Healthy Urine

Explaining ultrafiltration and molecular sieving at the Bowman's capsule

✅ Correct Answer

  1. (Plasma / blood) proteins are too large; [1 mark]
  2. To pass / filter through the basement membrane; [1 mark]

💡 Key Knowledge: The Ultrafiltration Barrier

The filtration barrier consists of three layers:

  1. Fenestrated endothelium of glomerular capillaries (prevents RBCs escaping).
  2. Basement membrane: A mesh of negatively charged glycoproteins acting as the effective molecular sieve (blocks proteins > 69 kDa).
  3. Podocyte filtration slits: Epithelial cells of Bowman's capsule with interlocking pedicels.

🧠 Exam Technique: Be Exact

The mark scheme includes a strict note: "Ignore any other structures". If you write only "capillary wall", "podocytes", or "Bowman's capsule", you will not gain the second mark. The mark scheme demands explicit mention of the basement membrane.

❌ Common Student Misconceptions

  • Writing that protein is reabsorbed: Proteins are not selectively reabsorbed in the PCT; they never enter the filtrate in the first place.
  • Vague sizing: Writing "molecules are big" without specifying proteins are too large.
PART 02.3 • 5 MARKS

Evaluating the Journalist's Headline

Statement: "Feeding high-protein milk to underweight human babies could cause kidney damage in adults"

🧠 How to Structure AQA "Evaluate" Questions

A maximum of 4 marks can come from points against the statement (limitations). You must provide a balanced argument with points that support the claim and points that refute / limit the claim.

✅ Points Supporting the Journalist (Max 2)

  • Statistical significance: The probability of the increase in glomerular sclerosis (GS) being due to chance is less than 0.01 (P < 0.01) / the difference is statistically significant; [1 mark]
  • Mammalian physiology: Rats and humans are both mammals, so their kidneys/organ systems have similar physiology; [1 mark]

❌ Points Challenging the Claim (Max 4 from this group)

  • Species difference: Investigation was only done on rats, not on humans; [1 mark]
  • Age uncertainty: We do not know if 160-day-old rats represent adults; [1 mark]
  • Small sample size: Only 14 rats in each group (28 total), so the sample is not representative; [1 mark]
  • Dietary variables not controlled: Only two protein levels tested / exact protein content unknown / types of protein in milk unknown; [1 mark]
  • Specific pathology: Only glomerular sclerosis (GS) was studied, but other types of kidney damage exist / other factors could cause damage (e.g. genetics); [1 mark]

📐 Statistical Analysis Breakdown: What does P < 0.01 mean?

Whenever an exam question states P < 0.01 :

  • There is less than a 1% probability (p < 0.01) that the difference between the experimental and control groups is due to chance.
  • The difference is statistically significant.
  • Trap: Never say "the results are significant" or "correlation is significant" without naming the specific difference or increase in GS!
Examiner Insight: High-scoring candidates clearly paired limitation statements with consequences—e.g., writing "small sample size of 14 rats, so it is not representative" rather than just saying "small sample".

Topics

Biology · Practical skills · 3.6 Organisms respond to changes in their internal and external environments (A-level only) · Data analysis · Uncertainty and evaluation

Question and mark scheme from the AQA A-Level Biology examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.