What is photorespiration and why can it be detrimental to C3 plants under high O2 and low CO2?

• A. Photorespiration is a beneficial process that increases carbon fixation efficiency.
• B. Photorespiration only occurs in C4 plants and has no effect on C3 plants.
• C. Photorespiration consumes energy and releases fixed CO2 when RuBP is oxygenated by Rubisco, reducing photosynthetic efficiency.
• D. Photorespiration uses light energy to directly synthesize ATP.

Answer: (C)

Photorespiration happens when Rubisco fixes oxygen instead of carbon dioxide. In C3 plants, when O2 is relatively high and CO2 is low, Rubisco is more likely to react with O2, producing phosphoglycolate. This tripped-off metabolite triggers a costly salvage pathway across chloroplasts, peroxisomes, and mitochondria that uses ATP and reducing power but releases CO2 and yields little net gain of fixed carbon. The result is wasted energy and carbon that reduce the overall efficiency of photosynthesis. This is especially a problem for C3 plants under high O2 and low CO2, whereas C4 plants minimize it by concentrating CO2 near Rubisco, lowering the chance of oxygenation. The other statements aren’t accurate because photorespiration isn’t beneficial, isn’t exclusive to C4 plants, and doesn’t directly produce ATP from light.