Introduction

When many green plants are exposed to light, they exhibit specific respiration called photorespiration. Adequate dark respiration (i.e., normal mitochondrial respiration) is usually light-independent, with the same rate in light and dark.

Like normal mitochondrial respiration, photorespiration is an oxidative process in which glycolate is oxidised, and then CO₂ is released (post illumination burst of CO₂).

Table of Contents

• Definition of Photorespiration
• Function and Significance of Photorespiration
• Photorespiration and Glycolate Metabolism
• Frequently Asked Questions – FAQs

Definition of Photorespiration

In oxygen-producing photosynthetic organisms, photorespiration is one of the key carbon metabolism pathways. When ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) utilises oxygen instead of carbon dioxide, it recycles 2-phosphoglycolate (2-PG), a hazardous metabolite, to 3-phosphoglycerate.

The C₂ cycle of photosynthesis, also known as photorespiration or glycolate-glyoxylate metabolism, is another name for the glycolate pathway. It lowers the effectiveness of C₃ plants’ photosynthesis. Unicellular green algae have a glycolate metabolism as well.

This cycle aids in the removal of 2-phosphoglycolate, a harmful byproduct of RuBisCO’s oxygenation reaction. It finally creates PGA, although 25 percent of the carbon is lost as CO₂, and ATP is also used.

Function and Significance of Photorespiration

In 1963, Krotkov et al created the terminology photorespiration to distinguish between these seemingly distinct modes of CO₂ evolution. Both forms of respiration have varying sensitivity to O₂, temperature, and metabolic inhibitors, as well as different respiratory substrate-specific activity after photosynthesis in ¹⁴CO₂.

Photorespiration is strongly connected to CO₂ adaptation point and is found only in plants with a high CO₂ compensation point, such as wheat, tomato, oats, and the green alga Chlorella (C₃-plants). It is negligible or non-existent in plants with a low CO₂ compensation point, such as maize and sugarcane (C₄-plants).

(i) Photorespiration takes place only in chlorophyllous tissues of the plant.

(ii) Photorespiration is carried out by three types of cell organelles: chloroplasts, peroxisomes, and mitochondria.

(iii) Glycolate (glycolic acid) is the primary metabolite and substrate of photorespiration. Both amino acids glycine and serine are also significant metabolites.

(iv) Photorespiration, like normal mitochondrial respiration, is an oxidative process in which glycolate is oxidised and CO₂ is released.

Although the favourable effects of photorespiration and glycolate metabolism have yet to be discovered, it is known that glycolate metabolism plays a protective role. By oxygenation, two molecules of phosphoglycolate with 2 + 2 carbon atoms are produced after two turns of this cycle.

Read Also:

Photorespiration in C3 and C4 plants

Photorespiration and Glycolate Metabolism

The following are the several processes of glycolate metabolism, namely glycolate production and oxidation with subsequent CO₂ release (photorespiration):

(i) Glycolate is produced as a byproduct of some photosynthetic intermediates in chloroplasts. It is likely deduced from C₁ and C₂ of a ketose sugar phosphates of the carbon-reducing cycle (i.e., Calvin cycle).

Phosphoglyceric acid enters the Calvin cycle again, while phosphoglycolic acid is dephosphorylated to produce glycolate in the presence of the enzyme Phosphatase.

(ii) The glycolate migrates from chloroplasts to peroxisomes, where it is oxidised (photo-respired) to glyoxylate by the enzyme glycolic acid oxidase.

The enzyme catalase removes the hydrogen peroxide produced.

H₂O₂ → H₂O + ½O₂

(iii) Glyoxylate is now transformed to glycine, an amino acid. This is a transamination reaction that takes place in the presence of the enzymes L-Glutamate glyoxylate transaminase and at the expense of L-Glutamate.

(iv) The glycine generated in peroxisomes migrates to the mitochondrion, where two molecules of glycine interact to form one molecule of amino acid serine, releasing CO₂ and NH₃. This enzyme serine hydroxymethyl transferase catalyses this process.

(v) In the presence of Serine: Glyoxylate aminotransferase, the serine is transaminated to hydroxypyruvate in the peroxisome.

(vi) NAD⁺ now reduces hydroxypyruvate in the peroxisome, enabling hydroxypyruvate reductase to generate glyceric acid.

(vii) In the presence of the enzyme glycerate kinase, the glyceric acid (glycerate) diffuses into the chloroplast, where it would be phosphorylated to 3-phosphoglyceric acid (PGA). PGA is a well-known Calvin cycle intermediate.

As a result of the production of glycolate from Calvin cycle intermediates, serine is produced, which is then transformed back into Calvin cycle intermediates, completing the glyco­late cycle.

The glycolate cycle or glycolate metabolism is sometimes known as the C₂—cycle because glycolate and several other metabolites of this cycle, such as glyoxylate and glycine, are 2-C molecules. Photosynthetic carbon oxidation cycle is another name for it (PCO-cycle).

Read Also:

• Short Notes on Glycolate Pathway
• Short Notes on CAM Plants