Which two pathways are cited as plants’ strategies to mitigate RuBisCO inefficiency?

Which two pathways are cited as plants’ strategies to mitigate RuBisCO inefficiency?

• A. C4 and CAM pathways
• B. C3 and CAM pathways
• C. C4 and C3 pathways
• D. CAM pathways only

Answer: (A)

The main idea is how plants reduce RuBisCO’s tendency to fix oxygen instead of carbon dioxide, which leads to photorespiration. To avoid this inefficiency, some plants use carbon-concentrating strategies that raise the CO2 concentration around RuBisCO. One strategy is the C4 pathway. Here CO2 is first fixed in the mesophyll by an enzyme that strongly prefers CO2, forming a four-carbon compound. This compound is then shuttled to specialized bundle-sheath cells where CO2 is released for fixation by RuBisCO in the Calvin cycle. By concentrating CO2 in the cells where RuBisCO operates, the chance of RuBisCO fixing O2 instead of CO2 drops, reducing photorespiration and boosting photosynthetic efficiency in hot, sunny environments. The other strategy is CAM, which temporally separates carbon fixation from the Calvin cycle. At night, stomata open to collect CO2 and fix it into a storage compound (malate). During the day, when stomata are closed to conserve water, CO2 is released from the stored malate inside the leaf, providing a high local CO2 concentration for RuBisCO. This is especially advantageous in arid conditions where water loss would be too costly if stomata stayed open during the day. Together, these two pathways are the plant strategies cited to mitigate RuBisCO inefficiency, whereas the standard C3 pathway lacks this CO2 concentration mechanism.

The main idea is how plants reduce RuBisCO’s tendency to fix oxygen instead of carbon dioxide, which leads to photorespiration. To avoid this inefficiency, some plants use carbon-concentrating strategies that raise the CO2 concentration around RuBisCO.

One strategy is the C4 pathway. Here CO2 is first fixed in the mesophyll by an enzyme that strongly prefers CO2, forming a four-carbon compound. This compound is then shuttled to specialized bundle-sheath cells where CO2 is released for fixation by RuBisCO in the Calvin cycle. By concentrating CO2 in the cells where RuBisCO operates, the chance of RuBisCO fixing O2 instead of CO2 drops, reducing photorespiration and boosting photosynthetic efficiency in hot, sunny environments.

The other strategy is CAM, which temporally separates carbon fixation from the Calvin cycle. At night, stomata open to collect CO2 and fix it into a storage compound (malate). During the day, when stomata are closed to conserve water, CO2 is released from the stored malate inside the leaf, providing a high local CO2 concentration for RuBisCO. This is especially advantageous in arid conditions where water loss would be too costly if stomata stayed open during the day.

Together, these two pathways are the plant strategies cited to mitigate RuBisCO inefficiency, whereas the standard C3 pathway lacks this CO2 concentration mechanism.