Leaving Certificate Photosynthesis Practice Test

Photorespiration happens when RuBisCO binds oxygen to RuBP instead of carbon dioxide. In this situation, RuBP is oxygenated and splits into products that later lead to the release of CO2 and require energy to process, producing two-carbon by-products along the way. The crucial idea is that O2 competes with CO2 for RuBisCO’s active site, so photorespiration rises when CO2 is scarce and O2 is relatively abundant. This situation often occurs at higher temperatures, especially when stomata close to reduce water loss, which lowers CO2 intake and shifts the internal gas balance toward O2. That combination—O2 competing with CO2 and the resulting CO2 release with two-carbon by-products—describes photorespiration accurately. The other statements describe different processes (efficient CO2 fixation under high CO2, the light-dependent production of ATP, or dark glycolysis), which don’t capture this oxygenation pathway.