Q8 - Which Kinetic and Environmental Data Explain Rubisco's Carboxylase/Oxygenase Balance

Theoretical 2 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license

Calvin Cycle, Photorespiration & C4/CAM Biochemistry

Carbon assimilation in photosynthesis begins when Ribulose-bisphosphate carboxylase/oxygenase (Rubisco) binds one molecule of CO2 to Ribulose 1,5-bisphosphate (RuBP) to form two molecules of 3-phosphoglycerate. O2 can bind to the active site of Rubisco instead of CO2, in which case one molecule of 3-phosphoglycerate and one molecule of 3-phosphoglycolate are formed. Thus, CO2 and O2 function as antagonists. (a) Kinetic characteristics of Rubisco (substrate concentration at 50% of saturation at 25C): KM[CO2] = 9 uM, KM[O2] = 535 uM, KM[RuBP] = 28 uM. (b) Maximum activity: kcat[CO2] = 3.3/s, kcat[O2] = 2.4/s. (c) Concentration in water in equilibrium with air (0.035% CO2, 21% O2) at 25C: CO2 = 11 uM, O2 = 253 uM. RuBP concentration in chloroplast stroma is 4 to 10 mM. Which properties from (a) to (c) above are necessary to explain the following facts from A to D? Choose the most suitable set from: (1) (a)(b)(c), (2) (a)(b), (3) (a)(c), (4) (b)(c), (5) (a), (6) (b), (7) (c).

Using the information and data, determine which of the statements are true or which are false.

A. The carboxylase activity of Rubisco increases as the oxygen concentration in the air decreases.

B. In the current global environment, the carboxylase activity of Rubisco is higher than the oxygenase activity.

C. Plants must have large amounts of Rubisco to maintain the full capacity of photosynthesis.

D. Increasing the concentration of CO2 in the air increases the carboxylase activity of Rubisco.

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