| Date | May 2013 | Marks available | 1 | Reference code | 13M.3.SL.TZ1.7 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | State | Question number | 7 | Adapted from | N/A |
Question
The nitrogen content of a leaf is mainly due to the proteins contained in the chloroplasts. These proteins are either in the thylakoids or in the stroma, where most enzymes are found. The quantity of nitrogen from the thylakoids is directly proportional to the amount of chlorophyll; a ratio of approximately 50 mmol nitrogen : 1 mmol chlorophyll would represent 100 % of the leaf nitrogen content. Scientists hypothesized that the higher leaf percentage nitrogen content resulting from a decrease in light intensity is due mainly to an increase in chlorophyll in many plant species, three of which are represented in the following graph.
State the difference in chlorophyll content for Phaseolus between high and low light intensity, giving the units.
State the percentage value of total leaf nitrogen in chlorophyll for Spinacia at low light intensity.
......................%
Suggest one advantage for plants to increase their leaf chlorophyll content per surface area when light intensity is lower.
Evaluate the hypothesis that lower light intensity increases thylakoid nitrogen.
Markscheme
0.08 mmol m–2 (Accept answers between 0.07 and 0.09 mmol m–2. Units required.)
22.5 (%) (Percentage symbol is not required. Accept answers between 22.0 and 23.0)
to absorb the same quantity of light (as high intensity)
a. (hypothesis is supported as) there is an increase in chlorophyll/nitrogen content for the three species at lower light intensity;
b. (hypothesis is not supported as) total leaf nitrogen decreases in low light for Alocasia and Phaseolus but not for Spinacia/remains the same for Spinacia;
c. greatest difference in Alocasia / smaller differences in the other two species;
d. chosen species may not be representative of all plants;
e. (hypothesis is not supported as) increase in nitrogen may be due to stroma protein;
Reject unqualified answers suggesting only that there are insufficient data.
Examiners report
For C1 (a) and (b) most candidates had these answers correct.
A relatively small number of candidates answered this option, but those who did generally achieved well.
For C1 (a) and (b) most candidates had these answers correct.
A relatively small number of candidates answered this option, but those who did generally achieved well. There were many good answers in 1c.
A relatively small number of candidates answered this option, but those who did generally achieved well.
Few candidates evaluated the hypothesis. To evaluate, students are required to have arguments for and against. Few did.
