| Date | May 2010 | Marks available | 2 | Reference code | 10M.3.SL.TZ1.9 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 1 |
| Command term | Explain | Question number | 9 | Adapted from | N/A |
Question
Global atmospheric carbon dioxide levels are rising. A study was carried out to test if the temperature dependence of photosynthesis was altered by elevated atmospheric CO2. Maize plants were grown in natural sunlight in controlled environmental chambers at different temperatures using current atmospheric and doubled CO2 levels.
State the time at which carbon dioxide exchange rate is maximal.
Explain the reasons for maximum carbon dioxide exchange rate at this time.
State the temperature that resulted in the highest rate of photosynthesis under current atmospheric conditions.
Using the data in the table, discuss whether rising carbon dioxide levels in the atmosphere will increase growth rates in maize.
Markscheme
12:00 (Accept 12:00 to 13:00)
photosynthesis in maize requires uptake of CO2 and light as energy source;
sunlight intensity strongest at midday therefore rate of photosynthesis is highest at this time;
light is limiting factor for photosynthesis so increased intensity increases photosynthesis;
high rate of photosynthesis means high exchange rate of CO2;
31ºC
data does not support idea that rising CO2 levels will increase growth rates in maize;
(at all temperatures) there appears to be no difference between exchange rate at current or elevated CO2 levels;
temperature has larger effect on growth of maize;
so if rising CO2 levels causes more of a greenhouse effect/larger temperature increase, this will affect growth of maize;
Examiners report
A range of answers were given, usually within the range acceptable in the mark scheme.
Many gained both marks for this question, but weaker candidates gave incomplete statements, mainly failing to link light intensity and CO2 exchange to photosynthesis.
This was answered correctly by most candidates.
Some candidates were able to interpret the data correctly, but there were few references to temperature and its effect on the growth of maize.
