No, photorespiration does not occur at night.
Photorespiration, also known as the C2 cycle or oxidative photosynthetic carbon cycle, is a metabolic pathway that occurs in plants when the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) oxygenates RuBP (ribulose-1,5-bisphosphate) instead of carboxylating it. This process is highly dependent on light, as it is intricately linked to photosynthesis and its associated processes.
Why Photorespiration Doesn't Occur at Night:
- Light Dependency: Photorespiration is directly linked to photosynthetic activity. The oxygenation of RuBP by RuBisCO is favored under conditions of high oxygen concentration and low carbon dioxide concentration within the chloroplast. These conditions are more likely to occur during the day when photosynthesis is actively producing oxygen. During the day, the Calvin cycle consumes carbon dioxide. If carbon dioxide becomes limiting (e.g., due to stomatal closure during hot, dry conditions), oxygenation becomes more prevalent.
- RuBisCO Activity: RuBisCO's oxygenase activity is enhanced by high oxygen levels and low carbon dioxide levels, typically conditions that arise under intense light and heat, conditions present only during the daytime.
- Photosynthetic Products: The glycolate pathway, which is a central part of photorespiration, relies on products and processes generated during photosynthesis. Without the light-dependent reactions of photosynthesis, the necessary substrates and energy required for the glycolate pathway are not available.
Photorespiration in Detail
Photorespiration involves a series of reactions occurring in the chloroplast, peroxisome, and mitochondria. The process begins when RuBisCO, instead of fixing carbon dioxide, fixes oxygen to RuBP. This leads to the formation of one molecule of 3-phosphoglycerate (which can enter the Calvin cycle) and one molecule of 2-phosphoglycolate. The 2-phosphoglycolate is then converted to glycolate, which is transported to the peroxisome, where it is converted to glyoxylate and then to glycine. Glycine is then transported to the mitochondria, where two molecules of glycine are converted to one molecule of serine, releasing carbon dioxide and ammonia. The serine is then returned to the peroxisome and eventually converted back to glycerate, which is transported back to the chloroplast and phosphorylated to regenerate RuBP.
Because this process consumes ATP and NADPH without producing any sugar, and also releases carbon dioxide, it is considered wasteful compared to normal carbon fixation.
