| Date | May 2011 | Marks available | 2 | Reference code | 11M.3.SL.TZ1.1 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Compare | Question number | 1 | Adapted from | N/A |
Question
Vitamin C is an important component of a healthy diet. Because it is water-soluble it cannot be effectively stored and excess vitamin C is released in the urine. In order to investigate the ability of the body to retain different chemical forms of vitamin C, 17 healthy, female university students, between the ages of 18 and 22, were placed on a low vitamin C diet (< 5 mg per day) for three days. The test subjects were divided into two groups and were given an oral vitamin C loading of either ascorbic acid (176 mg) or dehydroascorbic acid (174 mg). Levels of vitamin C in the urine were measured six hours before and at regular intervals over a 24 hour period following vitamin C loading. The negative value indicates time before vitamin C loading.
State the urinary vitamin C content for each of the two study groups six hours before vitamin C loading.
Ascorbic acid: ..........................................................................................
Dehyrdoascorbic acid: .............................................................................
Calculate the percentage increase in urine levels of vitamin C for the ascorbic acid study group during the first three hours after vitamin C loading.
Compare the trends in vitamin C release for the two test groups during the first twelve hours after vitamin C loading.
Large individual differences in the urine level of ascorbic acid and dehydroascorbic acid were recorded between test subjects after 24 hours. Suggest one possible reason for these large individual differences.
Scurvy is a disease that is due to vitamin C deficiency. Evaluate the importance of this investigation for finding ways to combat vitamin C deficiency.
Markscheme
Ascorbic acid: 2 mg (units are needed)
Dehydroascorbic acid: 2 mg (units are needed)
Both required for the mark.
400 (%) (working not required)
more ascorbic acid (40 mg) than dehydroascorbic acid (33 mg) is released/excreted;
more dehydroascorbic acid (16 mg) than ascorbic acid (10 mg) is released/excreted in the first three hours / dehydroascorbic acid peaks before ascorbic acid;
from six hours onwards more ascorbic acid is released than dehydroascorbic acid;
maximum release of dehydroascorbic acid at three hours whereas maximum release of ascorbic acid at six hours;
ascorbic acid release rises then falls whereas dehydroascorbic release falls (until 12 hours after loading);
genetic variability / differences in vitamin C metabolism / differences in vitamin C requirements / differences in levels of uptake into blood / size/weight of individuals
both chemical forms are released/excreted so supplements need to be given regularly;
using dehydroascorbic acid instead of ascorbic acid does not improve vitamin C retention / dehydroascorbic acid could be marginally better as less is excreted;
study only looks at young/healthy/female subjects;
period of vitamin C deficiency is too short / need a longer period to allow scurvy to develop;
Examiners report
Most candidates gave the correct answer, but many omitted the units, and a few simply gave the title of the horizontal axis.
There were a lot of incorrect answers of 500%. As mentioned earlier, calculation of percentage change continues to present difficulties for many.
The majority of students managed to gain 2 marks, but again, weaker responses did not give comparative statements about the two vitamin C supplements.
Few candidates could offer a sensible reason such as size/weight of individuals, genetic variability or differences in vitamin C requirements/metabolism.
It seems that many students did not understand the question, as many were writing about the importance of vitamin C in the fight against scurvy, with no reference to the data provided.
