A patient is diagnosed with a condition in which their nephrons have been damaged, meaning that glucose and amino acids are lost in their urine. Explain why glucose and amino acids are normally absent from urine, describing the region of the nephron involved and the process that prevents their loss.

WJEC A-Level Biology (Wales) — 4.5 Homeostasis and the kidney · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

In a healthy kidney, the filtrate produced in the capillary knot contains small molecules including glucose, amino acids, urea, water and mineral ions. In a healthy person, glucose and amino acids do not appear in the final urine.

Model answer (5 marks)

Glucose and amino acids are present in the filtrate after filtration at the capillary knot (glomerulus). They are then removed from the filtrate by selective reabsorption in the kidney tubules, mainly the proximal convoluted tubule. In a healthy kidney all glucose and amino acids are reabsorbed, so none remain in the urine. In the damaged kidney selective reabsorption cannot occur properly, so glucose and amino acids remain in the urine.

Examiner tips

  • Mention the glomerulus and proximal convoluted tubule explicitly
  • Explain that reabsorption is selective and complete in health
  • Show the link between damage and loss of reabsorption

Common mistakes

  • Saying the collecting duct is where reabsorption occurs
  • Claiming glucose is filtered but not reabsorbed in a healthy kidney
  • Confusing filtration with secretion

Mark scheme (5 marks)

  1. Glucose and amino acids are present in the filtrate after filtration at the capillary knot / glomerulus
  2. Glucose and amino acids are removed from the filtrate by selective reabsorption
  3. Selective reabsorption occurs in the kidney tubules / collecting duct
  4. All glucose and amino acids are reabsorbed so none remain in the urine
  5. In the damaged kidney, selective reabsorption cannot occur properly, so glucose and amino acids remain in the urine / are lost in urine

Key terms in this question

nephron

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