<P> Photorespiration (also known as the oxidative photosynthetic carbon cycle, or C photosynthesis) refers to a process in plant metabolism where the enzyme RuBisCO oxygenates RuBP, causing some of the energy produced by photosynthesis to be wasted . The desired reaction is the addition of carbon dioxide to RuBP (carboxylation), a key step in the Calvin--Benson cycle, however approximately 25% of reactions by RuBisCO instead add oxygen to RuBP (oxygenation), creating a product that cannot be used within the Calvin--Benson cycle . This process reduces the efficiency of photosynthesis, potentially reducing photosynthetic output by 25% in C plants . Photorespiration involves a complex network of enzyme reactions that exchange metabolites between chloroplasts, leaf peroxisomes and mitochondria . </P> <P> The oxygenation reaction of RuBisCO is a wasteful process because 3 - phosphoglycerate is created at a reduced rate and higher metabolic cost compared with RuBP carboxylase activity . While photorespiratory carbon cycling results in the formation of G3P eventually, there is still a net loss of carbon (around 25% of carbon fixed by photosynthesis is re-released as CO) and nitrogen, as ammonia . Ammonia must be detoxified at a substantial cost to the cell . Photorespiration also incurs a direct cost of one ATP and one NAD (P) H . </P>

What happens to oxygen in the calvin cycle
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