Photorespiration Hypothesis

Abstract

Photorespiration, a process that results from the build up of oxygen concentration in the leaf inner space due to the closing of the stomata, involves enzyme RUBISCO. RUBISCO is essential for photosynthesis. It can catalyse the oxygenation and carboxylation of Ribulose-1, 5-biphoshate. When the concentration of oxygen is higher than that of carbon dioxide in the leaf inner space, the oxygenase activity of RUBISCO is enhanced. This oxygenase activity of RUBISCO leads to the formation of a C2 molecule, 2-phosphoglycolate (PG). The PG molecule, being an important inhibitor of the Calvin cycle, has to be converted to a useable molecule 3-phosphoglycolate (PGA) and this is done via the photorespiratory pathway /C2 pathway. This pathway is thought to be

The extra energy input created by the photorespiratory pathway

The oxygenase activity of RUBISCO does not result in the gain of carbon for the plant nor does it produce energy. Ribulose-1,5-bisphosphate used in photorespiration can be considered as wasted because it does not enter the normal cycle that assists in its regeneration (Taiz, L & Zeiger, E., 2010;Eckardt, N.A., 2005). The calculation of the energy lost is based on the assumption that ribulose-1,5-bisphosphate must be completely regenerated to reconstitute the metabolic status of the Calvin cycle before oxygenation occurs (Christoph P. et al.,2010).

It is as assumed that there is no energy needed from the production of phosphoglycolate to the glycine reaction. The energy cost starts with the production of CO2 and NH3 .For each molecule of phosphoglycolate 0.5 molecule of CO2 and NH3 are produced. One molecule of CO2 in the calvin cycle requires 3 ATP and 2 NADPH. The 0.5 NH3 produced need to be re-fixed in the Glutamine synthase/Fd-GOGAT cycle and requires one Fd reducing power with 0.5 ATP. One Fd reducing power is assumed to be equal to 0.5 NADPH , at this level an energy input of 2 ATP and 1.5 NADPH is needed (Christoph P. et al.,