DP Biology (first assessment 2025)

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Question 22M.2.HL.TZ1.6c

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Date May 2022 Marks available [Maximum mark: 7] Reference code 22M.2.HL.TZ1.6c
Level HL Paper 2 Time zone TZ1
Command term Explain Question number c Adapted from N/A
c.
[Maximum mark: 7]
22M.2.HL.TZ1.6c
(c)

Explain how water balance is restored in mammals when they are dehydrated.

[7]

Markscheme
  1. thirst;
  2. more water drunk / more water reabsorbed from feces (in the colon/large intestine);
  3. osmoreceptors in the hypothalamus detect dehydration/high solute concentration in blood;
  4. ADH secreted;
  5. by the pituitary gland;
  6. ADH signals to collecting duct/DCT (cells) to increase permeability to water;
  7. more aquaporins (in plasma membranes of collecting duct/distal convoluted tubule cells);
  8. more water reabsorbed from filtrate (in collecting ducts/distal convoluted tubules);
  9. reabsorption by osmosis / reabsorption due to medulla being hypertonic;
  10. reabsorbed water passes into the blood/reduces the solute concentration of blood;
  11. smaller volume/more concentrated/hypertonic urine formed;
  12. less sweating;
Examiners report

This was poorly answered by many candidates. The discrimination index was very high but the mean mark was only 1.5 out of 7. Often candidates did not get beyond the basic ideas of thirst and drinking water to rehydrate. The best candidates gave concise but detailed accounts of the roles of the hypothalamus, pituitary gland, collecting duct and aquaporins.

Syllabus sections