Date | November 2012 | Marks available | 1 | Reference code | 12N.3.SL.TZ0.7 |
Level | Standard level | Paper | Paper 3 | Time zone | TZ0 |
Command term | Identify | Question number | 7 | Adapted from | N/A |
Question
Sour orange trees (Citrus aurantium L.) were grown outdoors in Phoenix, Arizona (USA) in chambers with clear plastic sides and open tops. These chambers were continuously maintained at mean atmospheric CO2 concentrations of either 400 or 700 ppm (parts per million) for several years. Both the rate of CO2 absorption of sunlit leaves and the temperature of the leaves were measured on some of the hottest days.
Identify the relationship between temperature and CO2 absorption shown in both graphs.
The line on each graph indicates the mean net photosynthesis rate. Calculate the difference in net photosynthesis at 34°C between plants grown at 400 ppm and 700 ppm CO2.
Compare the data for the sour orange trees growing at 400 ppm with those growing at 700 ppm.
Identify, with a reason, whether CO2 concentration or temperature is the limiting factor on photosynthesis at a temperature of 34°C and 400 ppm CO2.
State two products that pass from the light-dependent to the light-independent stages of photosynthesis.
1. ..................................................................
2. .................................................................. .
Markscheme
negative correlation / as temperature increases the CO2 absorption decreases / inverse relationship
4.6(μ mol m−2 s−1)
Accept answers in the range of 4.3(μ mol m−2 s−1) to 4.9(μ mol m−2 s−1).
both show a decrease in CO2 absorption/net photosynthesis as temperature rises;
at each temperature the 700 ppm CO2 sample shows greater rate;
at higher temperatures the 700 ppm CO2 sample can continue to photosynthesis / at higher temperatures the 400 ppm CO2 sample loses CO2/goes to net respiration;
the 700 ppm CO2 trendline/net photosynthesis is (slightly) steeper;
at high temperatures/above 43°C CO2 absorption at 400 ppm CO2 goes below zero while it never goes below zero at 700 ppm CO2;
more variation/greater scatter of results at 700 ppm CO2; (accept converse)
carbon dioxide is a limiting factor at 400 ppm CO2 as rate increases at same temperature with increased CO2
ATP and NADPH2/NADPH+ H+ (both needed)
Examiners report
Most correctly stated that there was a negative correlation in (a).
Many candidates carried out the calculation in (b) correctly.
Often 2 marks were awarded for (c) but seldom 3. Candidates saw that both show a decrease in net photosynthesis as temperature rises and that at each temperature the 700 ppm CO2 sample shows a greater rate. Only stronger candidates were able to get a third mark.
Most candidates saw that CO2 was the limiting factor in (d) and why.
There were many wrong answers to part (e) which was a recall question from the syllabus.