Answer: It has low catalytic rate (kcat ~3/sec) and reacts with O 2 as well as CO 2 (oxygenase activity), causing photorespiration.
- A It is sometimes thought to function mainly at unusually high temperatures well above the normal physiological leaf range found in nature
- B It has low catalytic rate (kcat ~3/sec) and reacts with O<sub>2</sub> as well as CO<sub>2</sub> (oxygenase activity), causing photorespiration
- C It is sometimes thought to require rare transition metal cofactors that are absent from typical photosynthetic leaf tissue altogether
- D It is sometimes thought to become catalytically active mainly when CO<sub>2</sub> concentration far exceeds normal atmospheric levels found outdoors
Correct answer: B. It has low catalytic rate (kcat ~3/sec) and reacts with O<sub>2</sub> as well as CO<sub>2</sub> (oxygenase activity), causing photorespiration
Explanation: Rubisco: very slow turnover (~3 CO<sub>2</sub>/sec vs ~1000 for typical enzymes). Cannot fully discriminate between CO<sub>2</sub> and O<sub>2</sub>. At high O<sub>2</sub>/low CO<sub>2</sub>, adds O<sub>2</sub> (photorespiration), wasting energy.
Simplified photosynthesis diagram: light energy, CO2, and H2O enter the leaf and are converted in the chloroplast into glucose and O2.
Concept context
Light reactions, the Calvin cycle, C 3 /C 4 /CAM pathways, photorespiration, and factors affecting the rate of photosynthesis. One of the highest-yield NEET chapters.