| Date | May 2012 | Marks available | 1 | Reference code | 12M.3.SL.TZ2.7 |
| Level | Standard level | Paper | Paper 3 | Time zone | Time zone 2 |
| Command term | Determine | Question number | 7 | Adapted from | N/A |
Question
A key reaction in photosynthesis occurs when ribulose bisphosphate carboxylase (Rubisco) catalyses the fixation of carbon dioxide to ribulose bisphosphate (RuBP). To be effective, Rubisco must be activated by another enzyme called activase. The activities of Rubisco and activase (each isolated from tobacco leaves) were independently investigated in a laboratory, under conditions of increasing temperature.
State the relationship between Rubisco activity and temperature.
Calculate the percentage decrease of activase activity from the optimum temperature to 50°C.
......................%
Determine which enzyme shows overall greater activity from 25°C to 42°C.
Explain the change in activase activity at temperatures higher than 42 °C.
In a leaf, both enzymes are present together. Predict, with a reason, how the rate of photosynthesis would change from 35°C to 50°C.
Markscheme
as temperature rises, (Rubisco) activity rises / direct correlation/proportional
(accept answer in the range of 73 % to 79 %)
Rubisco
as an enzyme activase is a protein;
42°C is the optimal temperature (for activase);
activity is decreased because the protein denatures;
denaturation changes enzyme/active site shape reducing catalytic activity;
initial increase in photosynthesis rate up to 42°C;
decrease in photosynthesis rate after 42°C;
reduced activity of activase lowers the activation of Rubisco;
less Rubisco activity means less fixation of carbon (dioxide);
Examiners report
Most candidates had this answer correct.
In C1 (b) there were many good answers.
N/A
Most candidates explained the change in activase activity at temperatures higher than 42°C by the denaturation of the enzyme.
Most candidates failed to analyse the data, they simply described the information on the graph without making any prediction.
