Photorespiration
From Free net encyclopedia
Photorespiration is an alternate pathway for Rubisco, the main enzyme of photosynthesis (specifically, the Calvin cycle). Although Rubisco favors carbon dioxide, it can also use oxygen, producing a glycolate and a glycerate. This usually occurs when oxygen levels are high; for example, when the stomata (tiny pores on the underside of the leaf) are closed to prevent water loss on dry days. It involves chloroplast peroxisomes and mitochondria.(Ashwani)
The reaction is catalyzed by RuBP oxygenase activity:
- RuBP + O2 → Phosphoglycolate + 3-Phosphoglycerate
The phosphoglycolate is salvaged by a series of reactions in the peroxisome and mitochondria whereby it is converted into serine and later glycerate and enters the Calvin cycle, at a cost of 1 ATP, by conversion to 3-phosphoglycerate within the chloroplast.
Photorespiration is a wasteful process because PGAL is created at a reduced rate and higher metabolic cost compared to RuBP carboxylase activity. PGAL, the end-product of the Calvin cycle, is used to create nearly all of the food and structures in the plant. Plants reduce photorespiration via CAM and C4 metabolism.
Photorespiration is said to be an evolutionary relic. The early atmosphere in which primitive plants originated contained very little oxygen, so it is hypothesized that the early evolution of rubisco was not influenced by its O2 fixing handicap.