What is Photorespiration?
The process of photorespiration was first observed by Warburg (1920), so is also called as Warburg’s effect. During the process, the rate of photosynthesis decreases at higher O2 concentration (than CO2) in C3 plants. The process of photorespiration takes place in chloroplasts, peroxisomes and mitochondria.
When the O2 concentration is more than CO2 inside the chloroplast stroma, then the RUBISCO enzyme of the Calvin cycle act as Ribulose diphosphate oxygenase enzyme. This enzyme splits a molecule of Ribulose 1, 5-biphosphate into one molecule of each of 3-phosphoglyceric acid and 2-phosphoglycolic acid.
The 2-phosphoglycolic acid loses its phosphate group in presence of enzyme phosphatase and converted into glycolic acid. The glycolic acid produced in the chloroplast stroma is then shifted into peroxisomes where it reacts with O2 and oxidised glyoxylic acid and H2O2 with the help of an enzyme glycolic acid oxidase. The H2O2, so, produced in the peroxisomes is destroyed by the enzyme catalase into water and O2. The glyoxylic acid is then converted into an amino acid- glycine by transamination reaction catalysed by an enzyme glutamate-glyoxylate transaminase.
The glycine is shifted out of peroxisomes into mitochondria, where two molecules of glycine interact to form one molecule each of serine, CO2 and NH3. The amino acid serine returns to peroxisomes where it is deaminated and reduced to hydroxy pyruvic acid and finally to glyceric acid.
The glyceric acid enters into chloroplasts, where it is phosphorylated to 3-phosphoglyceric acid which enters into C3 cycle.
Factors Promoting Photorespiration:
- The major factor that promotes photorespiration is lowering of concentration ratio between CO2 and O2. C4 plants do not show detectable photorespiration because they have a mechanism to concentrate CO2 while oxygen concentration is comparatively lower in the area of bundle sheath cells.
- A rise in temperature increases photorespiration while a drop in the same reduces photorespiration.
- It begins when light intensity is high, optimum or more than optimum.
- It is minimum in young leaves. The rate increases with the age of leaves.
Importance of Photorespiration:
The process of photorespiration interferes with the successful working of the Calvin cycle (dark reaction) in the C3 plants (and not in C4 plants). Due to this process, about 50% of photosynthetically fixed CO2 in the form of RUBP is lost into the atmosphere.