Photorespiration was once thought to be a wasteful process, but it may serve useful functions in plants. While photorespiration requires energy and reducing equivalents, it rescues 3/4 of the carbon from phosphoglycolate that would otherwise be inaccessible. Photorespiration also removes toxic intermediates, protects from photoinhibition under stress, and supports plant defense reactions. Many photorespiratory intermediates are involved in other metabolic pathways as well, and photorespiration contributes to amino acid synthesis. Overall, photorespiration appears to provide benefits beyond just recovering carbon and is integrated with primary plant metabolism.

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Photorespiration :
Wasteful or Useful
Created By - AJAY PRAKASH UNIYAL
M. Sc Plant Science
Central University Of Punjab

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Introduction
 Photorespiration is the process of light-dependent uptake of
molecular oxygen (O2) concomitant with release of carbon dioxide
(CO2) from organic compounds.
 During oxygen fixation, one molecule of PGA and one molecule of
2-phosphoglycolate are formed. The latter is converted back to PGA
in the photorespiratory cycle. The pathway requires energy (ATP)
and reducing (NAD(P)H) equivalents.

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Taiz and Zeiger

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Possible alternative photorespiratory pathways in Arabidopsis kleibish et al, 2010

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Transgenic pathways for the reduction of photorespiratory losses in Arabidopsis. (Kleibeish
et al , 2010)

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THE LIGHT SIDE AND THE DARK SIDE OF PHOTORESPIRATION
Keibish et al, 2010
Costs for each carbon fixation reaction are 3 ATP and 2 NADPH, or 8 ATP in total.

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On the other hand, the pathway rescues ¾ of the carbon in phosphoglycolate
that would be otherwise inaccessible for further metabolism.
There are several additional arguments for a positive function of
photorespiration in plant metabolism:
1. Photorespiration Removes Toxic Metabolic Intermediates.
2. Photorespiration Protects from Photoinhibition (Kozaki et al, 1997 and
Murata et al., 2007).
photorespiration can act as an electron sink especially under stress conditions (Wingler et al.,2000)
by consuming reducing equivalents during the refixation of released ammonia and by
exporting reduced components from the chloroplast to the mitochondrion.

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3. Photorespiration Supports Plant Defense Reactions
H2O2 triggers the hypersensitive response and the attacked cell eventually
undergoes programmed cell death (Heath, 2000)
4. Photorespiration is Intimately Integrated into Primary Metabolism
Chaouch et al,
2010.
Most photorespiratory intermediates are also part of other metabolic pathways
and photorespiration significantly contributes to the synthesis of several amino
acids (Novitskaya et al., 2002).

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