What is photorespiration? In Biology

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The inhibition of photosynthesis by high concentration of atmospheric oxygen is called Warburg’s effect. This inhibition is found in most of C3 plants. Different concentration of oxygen have no specific effect on C4 plants. Extensive studies were made to explain Warburg’s effect and these studies led to the discovery of photorespiration. Generally, plants respire both in light and dark but in some plants such as tobacco, pea, rice the rate of respiration is much more in light than in dark. It is estimated that in green leaves and algae, the rate of respiration in light is 3-4 times more than in dark.

Mechanism of photorespiration:-

Glycolic acid the intermediatory product of photosynthesis in C3 plants serves as the primary substrate for photorespiration. Since the glycolic acid is a 2C-compound, Photorespiration cycle is also called C2 Cycle. Three organelles are associated with this cycle. They are chloroplasts peroxisomes and mitochondria. The process of photorespiration is possible due to the dual nature of enzyme RuDP or RuBP carboxylase or RUBISCO. When the concentration of CO2 is more in the atmosphere this enzyme acts as carboxylase i.e., acts as acceptor of co2. But when the concentration of O2 is more this enzyme acts as oxygenase i.e., acts as acceptor of O2 . under normal condition this enzyme catalyses the process of CO2 fixation.

In C3 plants following steps are involved in photorespiration

  1. When there is high concentration of oxygen (O2) or low concentration of carbon dioxide (CO2) in atmosphere, ribulose 1,5-diphosphate (RuDP) combines with O2 to form one molecule of 3-phosphoglyceric acid and one molecule of 2-phosphoglycolic acid. This reaction is catalysed by enzyme RuDP oxygenase.

Ribulose 1,5-diphosphate + O2 → 3-Phosphoglyceric acid

                                          (3C- Compound) +2-Phosphoglycolic acid


2. The 2-Phosphoglycolic acid loses one molecule of phosphate group in presence of enzyme phosphotase and gets converted into glycolic acid. The 3-Phosphoglyceric acid formed in the above reaction enters the Calvin cycle.

Glycolic acid or Glycolate is the main metabolic substance (Substrate) for photorespiration which is a 2C-Compound. Both the above two reaction occur in chloroplasts.

2-Phosphoglycolic acid+ H2O → Glycolic acid + Phosphoric acid

3. Glycolic acid synthesized in chloroplast as an early product of photosynthesis is then transferred to peroxisomes. oxidase enzyme is present in peroxisomes. Therefore the glycolic acid reacts with oxygen to glyoxylic acid and hydrogen peroxide (H2O2). This reaction is catalysed by enzyme glycolate oxidase.

Glycolic acid + O2 → Glyoxylic acid +H2O2

4. Glyoxylic acid is then converted into an amino acid- Glycine by transamination reaction. This reaction is catalysed by the enzyme glutamate glyoxylate transaminase.

Glyoxylic acid + Glutamic acid → Glycine + α-Ketoglutaric acid

5. From peroxisome glycine is transferred into mitochondria. Two molecules of glycine interact to form one molecule each of serine (amino acid) CO2 and ammonia (NH3).

2 Glycine + H2O +NAD† → Serine + CO2 + NH3 + NADH.

Significance of photorespiration:-

  1. This process works as amino acid synthesis cycle and also exhibits that how amino acids are synthesized from photosynthetic product.
  2. From energy production point of view this process is not very important as energy (ATP) is not produced in it.