What is “photorespiration” and why it is needed by plants?
Photorespiration occurs when carbon dioxide levels within the leaf tissue dwindle to about 50 parts per million, usually on a hot, dry day where a plant has closed its stomates to prevent water loss. At this concentration of CO2, oxygen reacts with ribulose bisphosphate (RuBP) in the presence of RubisCO, an enzyme used in the Calvin cycle of photosynthesis, and creates 2 molecules of 3-phosphoglycerate and 1 molecule of 2-phosphoglycolate. Under normal oxygen and carbon dioxide concentrations, the reaction is between CO2, RuBP, and RubisCO to produce 12 molecules of 3-phosphoglycerate. To top it all off, 2-phosphoglycolate is toxic, so the plant has to convert it into a non-detrimental compound through a number of steps, each of which costs energy and CO2. Photorespiration is wasteful, as well, in that it does not produce any 3-phosphoglyceraldehyde (PGAL), which is used by the plant for a large number of functions. If you take a look at the Calvin cycle (
