Explain how the ultrastructure of a proximal convoluted tubule (PCT) cell in the kidney is adapted for the reabsorption of solutes from the glomerular filtrate.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Microvilli on the apical surface form a brush border, which increases the surface area for absorption of solutes from the filtrate.
Abundant mitochondria supply ATP for the active transport of solutes such as glucose, amino acids and Na⁺ against concentration gradients.
Basolateral infoldings of the plasma membrane increase surface area on the basolateral side, allowing rapid transfer of reabsorbed solutes into the peritubular capillaries.
Tight junctions between adjacent PCT cells prevent leakage of reabsorbed solutes back into the filtrate, ensuring directional (transcellular) transport.
Abundant mitochondria supply ATP for the active transport of solutes such as glucose, amino acids and Na⁺ against concentration gradients.
Basolateral infoldings of the plasma membrane increase surface area on the basolateral side, allowing rapid transfer of reabsorbed solutes into the peritubular capillaries.
Tight junctions between adjacent PCT cells prevent leakage of reabsorbed solutes back into the filtrate, ensuring directional (transcellular) transport.
Examiner tips
- Use the word ‘increase’ for surface area changes. Mention ATP‑dependent transport for Na⁺, glucose, amino acids. Show the direction of transport: apical → basolateral. Include tight junctions to emphasise directionality.
Common mistakes
- Omitting the role of mitochondria. Confusing basolateral infoldings with apical microvilli. Forgetting tight junctions or describing them as ‘barriers’ only.
Mark scheme (4 marks)
- Microvilli / brush border on the apical surface greatly increase the surface area for absorption of solutes from the filtrate
- Abundant mitochondria provide ATP for active transport of solutes (e.g. glucose, amino acids, Na⁺) against concentration gradients
- Basal infoldings of the plasma membrane increase surface area on the basolateral side, allowing rapid transfer of reabsorbed solutes into the peritubular capillaries
- Tight junctions between adjacent PCT cells prevent leakage of reabsorbed solutes back into the filtrate / lumen, ensuring directional (transcellular) transport
Key terms in this question
proximal convoluted tubule
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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