What is Photorespiration
Photosynthetic Carbon Oxidation or PCO Cycle or C_2 cycle:
It was discovered by Dicker and Tio (1959) in Tobacco leaves. Subsequently, photorespiration was found to be universal in C_3 plants. It involves three cell organelles – chloroplasts, peroxisomes and mitochondria. The various evidences in support of occurrence of photorespiration are
(i) Decrease in the rate of photosynthesis with the increase in light intensity.
(ii) Decrease in the rate of photosynthesis when O_2 concentration is raised from 2.5% to 21%.
Mechanism
RUBP carboxylase functions as RUBP oxygenase with the decrease in CO_2:O_2 ratio. In photorespiration two molecules of phosphoglycolate formed by oxygenation of RUBP is changed to one molecule of RUBP is changed to one molecule of phosphoglycerate (PGA) and one molecule of CO_2. Thus, 75% of carbon lost in oxygenation of RUBP is recovered by photorespiratory carbon oxygenation or the loss is only 25%.
Mechanism of photorespiration
Photorespiration does not give any energy or reducing power. It consumes energy. So it is a wasteful process, especially in C3 plants but negligible or absent in C4 plants. Photorespiration also increases with temperature and age of the leaf. Glycine and serine are two amino acids formed in photorespiration or C2 cycle.
Differences between Photorespiration and Normal (Dark) Respiration
1. It occurs in green photosynthetic cells only It occurs in all living cells (vital process)
2. It is light dependent Light independent
3. It increases with increase in O_2 conc.
(2–21%) Saturated at low O_2 concentration
4. 3 organelles-chloroplast, peroxisomes, mitochondria are involved Only mitochondria is involved
5. Respiratory substrate is glycolic acid (2C) Respiratory substrate is generally hexose sugar or glucose. Sometimes proteins and lipids also
6. No ATP or NADH_2 produced ATP or NADH_2 produced
7. It is a wasteful phenomenon Useful phenomenon
8. It occurs in only C_3 plants It occurs both in C_3 and C_4 plants
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