11th Class Biology Photosynthesis in Higher Plants Photorespiration

Decker and Tio (1959) reported that light induces oxidation of photosynthetic intermediates with the help of oxygen in tobacco. It is called as photorespiration. The photorespiration is defined by Krotkov (1963) as an extra input of \[{{O}_{2}}\] and extra release of \[C{{O}_{2}}\] by green plants is light.

Photorespiration is the uptake of \[{{O}_{2}}\] and release of \[C{{O}_{2}}\] in light and results from the biosynthesis of glycolate in chloroplasts and subsequent metabolism of glycolate acid in the same leaf cell. Biochemical mechanism for photorespiration is also called glycolate metabolism.

Loss of energy occurs during this process. The process of photorespiration involves the involvement of chloroplasts, peroxisomes and mitochondria. RuBP carboxylase also catalyses another reaction which interferes with the successful functioning of Calvin cycle.

Biochemical mechanism

(1) Ribulose-1, 5-biphosphate \[\xrightarrow{{{O}_{2}}}\] 2 Phoshoglycolic acid + 3 Phoshoglyceric acid

(2) 2 Phosphoglycolic acid \[+{{H}_{2}}O\xrightarrow{Phosphatase}\] Glycolic acid + Phosphoric acid.

(3) Glycolic acid \[+{{O}_{2}}\underset{\text{Oxidase}}{\mathop{\xrightarrow{\text{Glycolic}\,\text{acid}}}}\,\] Glyoxylic acid\[+{{H}_{2}}{{O}_{2}}\] \[2{{H}_{2}}{{O}_{2}}\xrightarrow{\text{Catalase}}2{{H}_{2}}O+{{O}_{2}}\]

(4) Glyoxylic acid + Glutamic acid \[\underset{\text{transaminase}}{\mathop{\xrightarrow{\text{Glutamate}-\text{glyoxylate}}}}\,\]

Glycine \[+\,\,\alpha -\]keto glutaric acid

(5) 2 Glycine \[+{{H}_{2}}O+NA{{D}^{+}}\xrightarrow{{}}\]Serine\[+C{{O}_{2}}+N{{H}_{3}}+NADH\]

(6) Serine + Glyoxylic acid \[\xrightarrow{{}}\] Hydroxypyruvic acid + Glycine Hydroxypyruvic acid \[\xrightarrow{{}}\] Glyceric acid

(7) Glyceric acid + ATP ® 3 phosphoglyceric acid + ADP + phosphate

Importance of photorespiration : Photorespiration is quite different from respiration as no ATP or NADH are produced. Moreover, the process is harmful to plants because as much as half the photosynthetically fixed carbon dioxide (in the form of RuBP) may be lost into the atmosphere through this process.

Any increase in \[{{O}_{2}}\] concentration would favour the uptake of \[{{O}_{2}}\] rather than \[C{{O}_{2}}\] and thus, inhibit photosynthesis for this rubisco functions as RuBP oxygenase. Photorespiration is closely related to \[C{{O}_{2}}\] compensation point and occurs only in those plants which have high \[C{{O}_{2}}\] compensation point such as \[{{C}_{3}}\] plants.

Photorespiration generally occurs in temperate plants. Few photorespiring plants are : Rice, bean, wheat, barley etc. Inhibitors of glycolic acid oxidase such as hydroxy sulphonates inhibit the process of photorespiration. Unlike usual mitochondria respiration neither reduced coenzymes are generated in photorespiration nor the oxidation of glycolate is coupled with the formation of ATP molecules. Photorespiration (\[{{C}_{2}}\] cycle) is enhanced by bright light, high temperature, high oxygen and low \[C{{O}_{2}}\]concentration.