Why is RuBisCO considered inefficient, and how do plants mitigate this?

Why is RuBisCO considered inefficient, and how do plants mitigate this?

• A. It has dual affinity for CO2 and O2, causing photorespiration; plants mitigate via C4/CAM pathways or by concentrating CO2 around RuBisCO.
• B. It is highly specific to CO2 and never reacts with O2.
• C. It is located in the mitochondria, not the chloroplast.
• D. It is unaffected by temperature changes.

Answer: (B)

RuBisCO isn’t a perfect carbon-fixer. It can catalyze two different reactions: fixing carbon dioxide to start the Calvin cycle, and reacting with oxygen in a competing oxygenase reaction. When oxygen binds, RuBisCO initiates photorespiration, a pathway that wastes energy and can release previously fixed carbon as CO2. This inefficiency stems from the enzyme’s modest specificity for CO2 versus O2 and its relatively slow turnover, so oxygenation happens with appreciable frequency in normal air. Plants deal with this by raising the local CO2 concentration around RuBisCO. In C4 plants, CO2 is first fixed in the mesophyll cells by a different enzyme (PEP carboxylase) into a four-carbon compound, which is transported to bundle-sheath cells and decarboxylated to release CO2 near RuBisCO. This high CO2 environment makes carboxylation more likely than oxygenation. In CAM plants, CO2 is fixed at night into organic acids and stored, then released as CO2 during the day when RuBisCO is active but stomata are closed, again boosting the CO2/O2 ratio around the enzyme. So the inefficiency comes from RuBisCO’s oxygenase activity and relatively slow pace, and the plant strategies—C4 and CAM—concentrate CO2 to minimize wasteful photorespiration. The idea that RuBisCO is highly specific to CO2 and never reacts with O2 isn’t accurate, and other options about its location or temperature insensitivity don’t describe the enzyme’s behavior.

RuBisCO isn’t a perfect carbon-fixer. It can catalyze two different reactions: fixing carbon dioxide to start the Calvin cycle, and reacting with oxygen in a competing oxygenase reaction. When oxygen binds, RuBisCO initiates photorespiration, a pathway that wastes energy and can release previously fixed carbon as CO2. This inefficiency stems from the enzyme’s modest specificity for CO2 versus O2 and its relatively slow turnover, so oxygenation happens with appreciable frequency in normal air.

Plants deal with this by raising the local CO2 concentration around RuBisCO. In C4 plants, CO2 is first fixed in the mesophyll cells by a different enzyme (PEP carboxylase) into a four-carbon compound, which is transported to bundle-sheath cells and decarboxylated to release CO2 near RuBisCO. This high CO2 environment makes carboxylation more likely than oxygenation. In CAM plants, CO2 is fixed at night into organic acids and stored, then released as CO2 during the day when RuBisCO is active but stomata are closed, again boosting the CO2/O2 ratio around the enzyme.

So the inefficiency comes from RuBisCO’s oxygenase activity and relatively slow pace, and the plant strategies—C4 and CAM—concentrate CO2 to minimize wasteful photorespiration. The idea that RuBisCO is highly specific to CO2 and never reacts with O2 isn’t accurate, and other options about its location or temperature insensitivity don’t describe the enzyme’s behavior.