Photosynthesis and transpiration occur in leaves. Explain how temperature affects these processes.

photosynthesis rate increases as temperature rises (up to an optimum temperature);
(due to) increase in the rate of enzyme catalysed reactions/light independent reactions/the Calvin cycle;
(steep) drop in rate of photosynthesis above the optimum;
at high temperatures enzymes/Rubisco/RuBP carboxylase denature(s);
graph with correctly labelled axes showing relationship between temperature and rate of photosynthesis;
transpiration rate increases as temperature rises;
(energy/heat leads to more) to more evaporation of water (in the leaf);
faster diffusion of water vapour at higher temperatures;
relative humidity falls as temperature rises / warmer air can hold more water vapour;
stomata may close at very high temperatures reducing the transpiration rate;
some plants open their stomata at very high temperatures to cool by transpiration;

Here candidates were able to outline the effects of temperature on photosynthesis and transpiration, but explanations failed to adequately address mechanisms such as the role of enzymes in photosynthesis and the role of evaporation in transpiration.

A very high number of candidates stated that higher temperatures meant more light and it was this that increased the rate of photosynthesis.  Others went off into the idea that transpiration was a type of sweating and was used normally by plants to keep them cool.  Few linked increased rate to enzyme activity and a further increase beyond optimum to a decrease in rate because of denaturation.