Suggest one method other than measuring CO₂ uptake by which the rate of photosynthesis could have been measured in these experiments.

Suggest why heat treatment may reduce photosynthesis rates.

Outline the effect of drought and of re-watering on the rate of photosynthesis.

Describe the isoprene emissions during the drought and recovery periods at 25°C.

Compare the effect of the two temperatures on the emission of isoprene.

State the effect of heat treatment on the rate of photosynthesis.

Using the results in the graph, deduce the effect of the presence of fosmidomycin on the rate of photosynthesis in the leaves.

Suggest possible conclusions for this experiment.

State the difference in percentage recovery of photosynthesis 1 hour after heat treatment between the 22 μL dm^–3 isoprene treatment and the 0 μL dm^–3 isoprene treatment.

Explain the evidence provided by the data in the bar chart for the hypothesis that isoprene improves plants’ tolerance to high temperatures.

Suggest two reasons for some plant species synthesizing and emitting isoprene, but not other plant species such as common beans.

oxygen production/release; (not count bubbles)
production/increase/change/ measurement of biomass;

high/higher than optimum temperatures denature enzymes (of Calvin cycle);
ribulose bisphosphate carboxylase/rubisco stops working/does not bind substrate;
wilting / withering / loss of water / decrease in turgor / increased transpiration;
closure/reduced aperture of stomata;
lower CO₂ level inside leaf / reduced CO₂ diffusion/uptake into leaf;

rate decreases/drops (to zero) with drought and increases when re-watered/recovering

slight decrease/constant initially then falls / falls increasingly rapidly / decreases exponentially (in drought/up to Day 35);
increases almost to original level/ but doesn’t reach original level / rapidly at first then less rapidly / increases then reaches plateau (during recovery/after Day 35);

higher/greater (emission) at 35°C than 25°C during both drought and recovery;
both at (approximately) same level at end of drought period/at 35 days;
both increase during recovery but not to original level;
less/little difference in emission between temperatures during recovery/after watering / converse;

decreases (rate of photosynthesis);

no effect before (the first) heat treatment;
lower rate/greater reduction in rate during heat treatments with fosmidomycin;
lower photosynthesis/fosmidomycin reduces recovery after heat treatments;
Ignore statements that fosmidomycin reduces the rate of photosynthesis if this is not related to heat treatments.

high temperature/heat stress/treatment reduces rate of photosynthesis;
repeated heat treatments cause greater reduction in photosynthesis;
isoprene causes less change/less reduction in photosynthesis due to heat/46°C /higher rate of photosynthesis during heat treatment with isoprene (than without);
isoprene helps photosynthesis to rise again after heat (treatments);

26 (%) (Allow a range of 25 % to 27 %)

faster recovery with isoprene than without/than with water treatment;
recovery faster/better/improved with higher isoprene concentration (than lower);
after both time periods / after 24 hours and 1 hour;

different plants live in/evolved in/are adapted to different temperature regimes;
(selective) advantage for plants that produce isoprene in high temperature regions;
isoprene synthesis uses energy/materials/only beneficial at high temperatures;
some plants do not have the enzymes/genes for making isoprene;

This question had parts that ranged from easy to very challenging and there was a wide range of scores on it. 

In (a) candidates were tested on their knowledge of methods of measuring the rate of photosynthesis. Most answered it correctly. A few candidates made vague statements about growth or suggested that use of water could be a measure of photosynthesis. Neither of these answers was accepted.

This question had parts that ranged from easy to very challenging and there was a wide range of scores on it.  

Part (b) was also testing knowledge rather than data analysis skills. There were some good explanations of why heat may reduce photosynthesis rates, including the idea that stomatal closure would reduce carbon dioxide uptake. Enzyme denaturation was accepted although photosynthesis rates drop at much lower temperatures in most plants than could be due to denaturation. Candidates were not expected to know about photorespiration and the reactions that are catalysed by rubisco at high temperatures.  

This question had parts that ranged from easy to very challenging and there was a wide range of scores on it.  

Part (c) was intended to be an easy question and almost all candidates answered it correctly. 

Question 1: This question had parts that ranged from easy to very challenging and there was a wide range of scores on it.

Answers to (d) were more mixed. The command term "describe" requires a detailed account so the marks were not awarded simply for stating that there was a fall in isoprene emissions during drought and a rise during recovery. There had to be some qualification, such as the changes in the rate of rise or fall, or an indication of whether recovery was complete. Some candidates stated that the emissions "spiked upwards" during recovery. This was allowed but strictly speaking a spike is a sharp rise and fall, not just a rise.

Question 1: This question had parts that ranged from easy to very challenging and there was a wide range of scores on it.

Part (e) was quite well answered but relatively few candidates scored both marks. Fewer candidates than in the past simply described the results for 25°C and then for 35°C, without proper comparison, but there were some simple comparisons on the mark scheme that most candidates missed. The best approach was to think about the difference between the results at each time during drought and recovery, not to overcomplicate things by trying to compare rates of change. 

Question 1: This question had parts that ranged from easy to very challenging and there was a wide range of scores on it.

Part (f) was intended to be an easy lead in to the third graph, but a higher than expected proportion of candidates stated that heat treatment increased the rate of photosynthesis rather than decreased it. The arrows on the graph show when the heat treatments were administered and at these times there is clearly a decrease but some candidates thought that the rise following the times indicated with an arrow showed that the heat treatment had positive effects on photosynthesis.  

Question 1: This question had parts that ranged from easy to very challenging and there was a wide range of scores on it.

By part (g) of the question some candidates were starting to struggle. There were two independent variables in this experiment; temperature and presence or absence of fosmidomycin. Marks were only awarded if the effects of fosmidomycin were related to the heat treatments.

Question 1: This question had parts that ranged from easy to very challenging and there was a wide range of scores on it.

Part (h) of the question was also found difficult by some candidates. Marks were awarded for conclusions about the effect of heat on photosynthesis, but not for conclusions about fosmidomycin. This chemical was used in the experiment as a means of investigating the effects of isoprene so the expected conclusions were about the protective effect of isoprene during heat treatments. There were some excellent answers from the stronger candidates who understood the experiment and were able to analyse its results effectively.  

Question 1: This question had parts that ranged from easy to very challenging and there was a wide range of scores on it.

Few candidates had any problem with part (i) and calculated the difference in percentage recovery successfully.

Question 1: This question had parts that ranged from easy to very challenging and there was a wide range of scores on it.

Answers to part (j) were very varied, with fewer candidates scoring both marks than expected. There were separate marks for stating that recovery was faster with isoprene than without (an all or nothing effect) and for stating that the higher the isoprene concentration the faster the recovery. There was also a mark for stating that these trends were evident both after one hour and 24 hours.

Question 1: This question had parts that ranged from easy to very challenging and there was a wide range of scores on it.

Part (k) was another two mark question where most candidates scored either one or no marks. Two reasons were required, with four interrelated reasons on the mark scheme. Few candidates suggested that some plants might lack the genes for isoprene synthesis and almost none that there is a cost to synthesis in terms of energy or resources so there will be selection against it in areas where hot conditions are never experienced.