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.

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;

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.