Relate nephron structure to urine formation
Connect filtration surfaces and proximal-tubule adaptations with their functions.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not a practical skills certification.
Before you begin
- A control system detects a change, communicates information and changes an effector response.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which proximal-tubule adaptation increases surface area?
What you will learn
- Relate Bowman’s capsule and glomerular barrier to ultrafiltration
- Relate proximal-tubule surface area and mitochondria to reabsorption
Build the explanation
The glomerulus has many capillaries and a high pressure relative to Bowman’s capsule. The filtration barrier has fenestrated capillary endothelium, a basement membrane and filtration slits between podocyte processes; these permit water and small solutes while retaining cells and most large proteins. The proximal convoluted tubule has a long folded surface with microvilli, many mitochondria and epithelial transport proteins, supporting selective reabsorption.
Apply the limits
Structure and function should be linked: filtration barrier and pressure for ultrafiltration; microvilli increase surface area and ATP supports active transport in proximal cells. Do not imply that every solute crosses without selectivity.
Pause and explain
An inhibitor reduces ATP production in proximal-tubule cells. Predict one effect on selective reabsorption and explain.
Worked example
How do microvilli help the proximal convoluted tubule?
Show the worked solution
- They increase the epithelial surface area.
- More membrane area supports transport of useful solutes from filtrate.
Answer Microvilli increase surface area for selective reabsorption.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Link two proximal-tubule adaptations to reabsorption.
Give me a hint
Name the relevant structure or signal, then state its effect.
Compare my reasoning
- Microvilli increase surface area.
- Many mitochondria supply ATP for active transport.
Microvilli increase the area available for transport, while mitochondria supply ATP for active transport of useful substances from filtrate into blood.
Look for these in your work
- Correct mechanism
- Direction of response explained
Why do red blood cells normally remain in glomerular capillaries?
Give me a hint
Separate the observed change from the body's response.
Compare my reasoning
- The filtration barrier is selective.
- Cells are too large to pass through the barrier into Bowman’s capsule.
Red blood cells are retained because they are too large to pass the glomerular filtration barrier.
Look for these in your work
- Evidence interpreted
- Control step identified
An inhibitor reduces ATP production in proximal-tubule cells. Predict one effect on selective reabsorption and explain.
Give me a hint
Apply the mechanism to the new context without claiming more than the evidence shows.
Compare my reasoning
- Active transport may be reduced.
- Less glucose or ion transport can reduce their reabsorption from filtrate.
Reduced ATP can impair active transport in proximal-tubule cells, lowering reabsorption of substances such as glucose or ions and leaving more of them in the tubule fluid.
Look for these in your work
- New context applied
- Limits preserved
Common mistakes
- Saying microvilli create filtration pressure
- Confusing the filtration barrier with proximal-tubule transporters
What can you explain now?
Which proximal-tubule adaptation increases surface area?
Compare with the explanation
Microvilli
They increase membrane area for reabsorption.
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Make it stick
Which structures support ultrafiltration, and which support selective reabsorption?
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