Question 23M.2.HL.TZ2.2
| 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 |
The reaction that produces urea in liver cells is shown.
Arginine and ornithine are in the same group of biochemicals. Identify this group.
[1]
amino acids;
This question was answered well by many candidates, with correct use of terminology indicating "amino acids". However, some answers mentioned "proteins". Several candidates seemed to have confused what the term "group of biochemicals" represented, and gave responses related to carboxyl or amino groups.
This reaction forms part of a metabolic cycle. Outline one feature of a metabolic cycle that distinguishes it from a chain.
[1]
- initial molecule/substrate/intermediates are regenerated;
- products become substrates/reactants;
Accept clearly annotated diagrams.
In general, vague answers were given regarding cycles being continuous. There was a low proportion of candidates being specific with terms such as substrates, intermediates and products.
Predict what effect arginase has on the activation energy of this reaction.
[1]
reduced/lowered (activation energy);
While a great number of students correctly answered about the reduced activation energy, it was not uncommon to find scripts stating that the enzyme "increased" the activation energy. Some other answers were vague and just referred to the fact that enzymes speed up chemical reactions, which did not answer the question that specifically asked the effect of arginase on the activation energy of the reaction.
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]
- 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);
- urea filtered out from blood in glomerulus/Bowman’s capsule;
- water reabsorbed from filtrate (by osmosis);
- in proximal convoluted tubule/descending loop of Henle/collecting duct;
- loop of Henle maintains hypertonic conditions in the medulla;
- little/no urea reabsorbed from filtrate;
Many candidates failed to explain the process of why urea concentration is higher in the urine compared to the blood plasma. The idea of water reabsorption was absent in many responses. Some candidates referred to ADH, though the question focused on the process that normally occurs in the nephron. Few candidates were able to indicate that little or no urea is to be reabsorbed from the filtrate as it is to be excreted.
