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A-Level Biology October/November 2024 Q6(b): The glomerulus and Bowman's capsule of the nephron are important in the formation of ur…
A-Level Biology · Paper 9700/41 · October/November 2024 · Question 6(b) · [3 marks]
The glomerulus and Bowman's capsule of the nephron are important in the formation of urine. Outline the role of the glomerulus and Bowman's capsule in the formation of urine.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The main role of the glomerulus and Bowman's capsule is ultrafiltration, which forms the glomerular filtrate.
Due to the afferent arteriole being wider than the efferent arteriole, a high hydrostatic pressure builds up in the capillaries of the glomerulus.
This high pressure forces water and small solutes (e.g., glucose, amino acids, urea, mineral ions) out of the glomerular capillaries and into the Bowman's capsule. The fluid must pass through the fenestrations in the capillary wall, the basement membrane, and filtration slits between podocytes, which act as a filtration barrier.
How the marks are awarded
- B1 — Awarded for identifying the process as 'ultrafiltration' or stating that it 'forms glomerular filtrate'.
- B1 — Awarded for explaining the driving force is a 'high hydrostatic pressure' within the glomerulus.
- B1 — Awarded for stating that 'water and small solutes' are forced from the glomerulus into the Bowman's capsule. An alternative mark could be awarded for correctly naming two of the three filtration layers: fenestrations, basement membrane, or podocytes/filtration slits.
Common mistakes
- Confusing ultrafiltration with selective reabsorption, which occurs later in the nephron (in the proximal convoluted tubule).
- Incorrectly stating that large molecules like proteins or red blood cells are filtered into the Bowman's capsule; they are retained in the blood.
- Using vague terms like 'high pressure' without specifying it is 'hydrostatic' or 'blood' pressure.
- Failing to mention the structural reason for the high pressure (the afferent arteriole being wider than the efferent arteriole), which is a common AVP mark.
Examiner tip: Always link physiological processes to the specific structures involved and use precise scientific terms like 'hydrostatic pressure' and 'ultrafiltration' instead of vague descriptions.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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