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  Photorespiration

Photorespiration or C2 cycle:

In bacteria and animals, just one type of respiration termed as dark respiration takes place. This is not influenced by the existence or nonexistence of light. Although in specific green plants, there are two dissimilar kinds of respiration– photorespiration and dark respiration. Respiration which takes place in photosynthetic tissues in the existence of light and results in increased rate of carbondioxide evolution is termed as photorespiration / light respiration.
Photorespiration includes three organelles. They are:

1. Chloroplasts,
2. Peroxisomes and
3. Mitochondria.

Oxidation of RuBP in the existence of high oxygen is the 1st reaction of photorespiration. This reaction is catalysed through the Rubisco* enzyme termed as carboxylase. It leads to the creation of 2C compound they are–phosphoglycolic acid and 3C compound PGA. While PGA is finished in the Calvin cycle, the phosphoglycolic acid is dephosphorylated to create glycolic acid within the chloroplasts.

The glycolic acid diffuses into the peroxisome in which it is oxidised to glyoxalic acid and hydrogen peroxide from the chloroplast. In peroxisome from glyoxalic acid, glycine is created.

Note: * Rubisco = Ribulose bisphosphate carboxylase

864_photorespiratory pathway.jpg

 

Glycine molecules come into mitochondria where two molecules of glycine merge to provide a molecule of serine, NH3 and CO2.  Throughout this process, NAD+ is reduced to NADH2.

The aminoacid serine is occupied to peroxisome in which, it is converted into hydroxy pyruvic acid. Hydroxy pyruvic acid is reduced through NADH2 to create glyceric acid.

The glyceric acid left out peroxisome and come into chloroplast, in which it is phosphorylated to PGA that enters into Calvin cycle. Throughout the photorespiratory pathway, one CO2 molecule released in mitochondria is to be re-fixed.

Photorespiration is as well termed as photosynthetic carbon oxidation cycle or C2 cycle. Within the circumstances of high light and limited CO2 supply, photorespiration guards the plants from photooxidative damage. The meaning of this is that, if sufficient CO2 is not obtainable to utilize light energy, excess energy causes damage to plant. Though, photorespiration uses part of the light energy and saves the plant from photooxidative damage. Raised O2 level increases photorespiration while increased CO2 level decreases photorespiration and raises photosynthesis.

 

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