DP Biology (first assessment 2025)

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Question 23M.2.HL.TZ2.2

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Date May 2023 Marks available [Maximum mark: 6] Reference code 23M.2.HL.TZ2.2
Level HL Paper 2 Time zone TZ2
Command term Explain, Identify, Outline, Predict Question number 2 Adapted from N/A
2.
[Maximum mark: 6]
23M.2.HL.TZ2.2

The reaction that produces urea in liver cells is shown.

(a)

Arginine and ornithine are in the same group of biochemicals. Identify this group.

[1]

Markscheme

amino acids;

(b)

This reaction forms part of a metabolic cycle. Outline one feature of a metabolic cycle that distinguishes it from a chain.

[1]

Markscheme
  1. initial molecule/substrate/intermediates are regenerated;
  2. products become substrates/reactants;

Accept clearly annotated diagrams.

(c)

Predict what effect arginase has on the activation energy of this reaction.

[1]

Markscheme

reduced/lowered (activation energy);

(d)

The concentration of urea in blood plasma is typically about 30 mg per 100 ml. In urine it can be as high as 1800 mg per 100 ml. Explain how this increase in concentration is achieved.

[3]

Markscheme
  1. urea is toxic/ a (excretory) waste product removed from the body/ blood (plasma) by the kidneys/in the urine (to be excreted in the urine);
  2. urea filtered out from blood in glomerulus/Bowman’s capsule;
  3. water reabsorbed from filtrate (by osmosis);
  4. in proximal convoluted tubule/descending loop of Henle/collecting duct;
  5. loop of Henle maintains hypertonic conditions in the medulla;
  6. little/no urea reabsorbed from filtrate;

 

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