Photorespiration : An Overview and its Significance MCQ

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Introduction to Photorespiration, Process of Photorespiration is considered one of the most asked concept.
19 Questions around this concept.

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Introduction to Photorespiration

Photorespiration was first observed by Otto Warburg in 1929 and demonstrated by Decker and Tijo in 1959.
The process of photorespiration creates a big difference between the C3 and C4 plants.
In order to understand the photorespiration, let us revise the first step in the C3 cycle, which is when RuBP combines with CO2 to form 2 molecules of 3PGA, catalyzed by RuBisCO.
RuBP is characterized by the fact that its active site can bind to both CO2 and O2.
RuBisCO has a much greater affinity for CO2 when the CO2:O2 is nearly equal.
It is the relative concentration of O2 and CO2 that determines which of the two will bind to the enzyme.
In C3 plants some O2 does bind to RuBisCO, and hence CO2 fixation is decreased.
Here the RuBP instead of being converted to 2 molecules of PGA binds with O2 to form one molecule of 3 carbon phosphoglycerate (PGA) and one molecule of 2 carbon phosphoglycolate in a pathway called photorespiration.
In the photorespiratory pathway, there is neither synthesis of sugars, nor of ATP.
Rather it results in the release of CO2 with the utilization of ATP.
In the photorespiratory pathway, there is no synthesis of ATP or NADPH.
The biological function of photorespiration is not known yet.
In C4 plants photorespiration does not occur. This is because they have a mechanism that increases the concentration of CO2 at the enzyme site.
This takes place when the C4 acid from the mesophyll is broken down in the bundle sheath cells to release CO2 – this results in increasing the intracellular concentration of CO2
In turn, this ensures that the RuBisCO functions as a carboxylase minimizing the oxygenase activity.

Process of Photorespiration

Phosphoglycolate is the substrate for the photorespiration process.
The process requires three cell organelles: chloroplasts, peroxisomes, and mitochondria.
It occurs in the following steps:
- Dephosphorylation of 2-Phosphoglycolate in chloroplast
- Glycolate metabolism in the peroxisome - Glycolate oxidase catalyzes the oxidation of glycolate to glyoxylate. Molecular oxygen is used as a co-substrate and, thus, H2O2 is produced by the enzyme.
- H2O2 produced by the glycolate oxidase reaction is disproportionated to H2O and O2 by catalase
- Transamination of glyoxylate to glycine by glutamate: glyoxylate aminotransferase in the peroxisome.
- Mitochondrial conversion of glycine to serine - Glycine is decarboxylated and deaminated by the multienzyme complex glycine decarboxylase yielding CO2, NH3, NADH.
- Formation of glycerate in the peroxisome - Back in the peroxisome, serine is deaminated to hydroxypyruvate which is reduced to glycerate.
- Regeneration of phosphoglycerate in the chloroplast.