In photorespiration, which enzyme binds to oxygen, leading to energy waste?

In photorespiration, which enzyme binds to oxygen, leading to energy waste?

• A. Lipase.
• B. ATP synthase.
• C. Rubisco.
• D. NADPH oxidase.

Answer: (C)

Rubisco is the enzyme that binds oxygen as well as carbon dioxide in the chloroplast. It can act as a carboxylase, fixing CO2 into organic matter, or an oxygenase, adding O2 to the same substrate. When O2 binds, the reaction produces a form that must be recycled through photorespiration, a costly process that uses ATP and NADPH and ultimately releases CO2. This wasteful pathway lowers the efficiency of photosynthesis, especially when CO2 is scarce or O2 is abundant. The other enzymes listed serve different roles—lipase digests fats, ATP synthase makes ATP in the chloroplast, and NADPH oxidase generates reactive oxygen species—so they don’t cause the oxygenation reaction that leads to energy loss.

Rubisco is the enzyme that binds oxygen as well as carbon dioxide in the chloroplast. It can act as a carboxylase, fixing CO2 into organic matter, or an oxygenase, adding O2 to the same substrate. When O2 binds, the reaction produces a form that must be recycled through photorespiration, a costly process that uses ATP and NADPH and ultimately releases CO2. This wasteful pathway lowers the efficiency of photosynthesis, especially when CO2 is scarce or O2 is abundant. The other enzymes listed serve different roles—lipase digests fats, ATP synthase makes ATP in the chloroplast, and NADPH oxidase generates reactive oxygen species—so they don’t cause the oxygenation reaction that leads to energy loss.