By the end of this lesson
- Explain why glucose enters filtrate.
- Explain glucose reabsorption in the proximal tubule.
- Relate tubule-cell adaptations to active transport.
BIG QUESTION
Why is glucose filtered out of blood and then normally recovered almost completely?
Ultrafiltration is size based; selective transport in the proximal tubule prevents loss of a valuable respiratory substrate.
Filtration does not distinguish usefulness
Glucose is a small dissolved molecule, so it enters Bowman’s capsule during ultrafiltration. Losing it in urine would waste chemical energy.
The kidney therefore recovers it downstream.
The proximal tubule selectively transports glucose
In the proximal convoluted tubuleThe first coiled part of a nephron, where all the glucose and much of the water and salts are reabsorbed into the blood (selective reabsorption)., sodium–glucose co-transport moves glucose from filtrate into epithelial cells, using a sodium gradient maintained by active transport. Glucose then moves into tissue fluid and capillary blood.
At International GCSE level, this can be described as selective reabsorption involving active transport.
Cell structure supports high transport rate
Microvilli increase membrane area, mitochondria supply ATP and a thin epithelium shortens the route to nearby capillaries.
Under normal conditions essentially all filtered glucose is recovered. If filtered load exceeds transport capacity, glucose can remain in urine.
Filtration and glucose recovery
The two stages use different selectivity.
| Stage | Mechanism | Glucose outcome |
|---|---|---|
| Glomerulus | Pressure/size-based filtration. | Glucose enters filtrate. |
| Proximal tubule membrane | Carrier-mediated selective transport. | Glucose enters epithelial cells. |
| Surrounding capillary | Transfer to tissue fluid/blood. | Glucose returns to circulation. |
| Transport maximum exceeded | Carriers saturated. | Some glucose may remain in urine. |
Language that earns precision
- proximal convoluted tubule
- First coiled tubule where most selective reabsorption occurs.
- co-transport
- Linked movement of two substances through one membrane protein.
- transport maximum
- Highest rate at which available carriers can move a substance.
Common misconceptions
Claim: Glucose should never be filtered because it is useful.
Correction: It is filtered by size, then recovered selectively.
Claim: Glucose is reabsorbed in the bladder.
Correction: It is recovered mainly in the proximal tubule.
Claim: Glucose in urine always proves one specific disease.
Correction: It can have several causes and requires clinical assessment.
Match the answer to the command word
Filtered because small; reabsorbed because useful, using active transport/carriers.
Microvilli increase area and mitochondria supply ATP.
Recall, apply, then justify
Explain why and where glucose is selectively reabsorbed.
Hint
Value, location and mechanism.
Suggested marking points
- Glucose is useful for respiration.
- It is reabsorbed in the proximal convoluted tubule.
- Carrier proteins/co-transport move it into tubule cells.
- Active transport maintains gradients and glucose returns to blood.
Explain two adaptations of proximal-tubule cells.
Hint
Area and energy.
Suggested marking points
- Microvilli provide a large surface area.
- This allows more transport proteins/rapid reabsorption.
- Many mitochondria supply ATP.
- ATP supports active transport that drives selective reabsorption.
Explain how blood glucose above the renal transport maximum can produce glucose in urine.
Planning hint
Filter load versus carrier capacity.
What a strong design includes
- Higher blood glucose increases the amount filtered.
- Proximal-tubule carriers have a finite transport rate.
- At the transport maximum carriers become saturated.
- Not all filtered glucose is reabsorbed.
- Remaining glucose continues into urine.
- Urinary glucose alone does not establish the underlying cause.