Photorespiration: Definition, Process, Significance

Photorespiration: The oxygenase activity of ribulose-1,5-bisphosphate-carboxylase/oxygenase (RUBISCO), the same enzyme responsible for CO2 fixation in practically all photosynthetic organisms, initiates photorespiration. In the photorespiratory pathway, phosphoglycolate produced by oxygen fixation is recycled to the Calvin cycle intermediate phosphoglycerate. 

This chemical cycle uses up energy and reduces equivalents, and some of the previously fixed carbon is liberated as CO2. As a result, photorespiration was frequently regarded as a wasteful procedure. Photorespiration is a light-dependent cyclic respiration mechanism that results in CO2 removal in photosynthetic cells. Dicker and Tio discovered photorespiration in tobacco plants in 1959 and coined the term. Read this article to learn more about photorespiration.

Photorespiration Definition

Photorespiration is a light-dependent cyclic respiration process that occurs in photosynthetic cells resulting in a loss of CO₂. The term photorespiration was discovered by Dicker and Tio in the year 1959 in tobacco plants. The term photorespiration is the combination of two words ‘Photo’ means ‘Light’ and ‘Respiration’ means ‘Take in O₂ and give out CO₂’. Photorespiration is also called the C₂ cycle or oxidative photosynthesis cycle, or glycolate pathway because the first stable product is phosphoglycolate which contains two carbon compounds.

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Photorespiration Sites

Photorespiration takes place in three cell organelles:

A. Chloroplast
B. Peroxisome
C. Mitochondria

Photorespiration Occurrence

Photorespiration generally takes place in C₃ plants like beet, bean, cotton, rice, wheat, barley, etc. C₄ plants like maize, sugarcane, Sorghum, etc., lack this process.

Photorespiration Diagram

Fig: Photorespiration

Photorespiration Process

Ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) acts as an oxygenase enzyme when the concentration of CO₂ is low, the temperature is high, and the concentration of O₂ is high. The RuBisCO acts as carboxylase when the concentration of CO₂ is high, the temperature is low, and the concentration of O₂ is low. Different steps of photorespiration occur in different cell organelles like chloroplast, peroxisome, and mitochondria in the cycling of different organic substances. These are described below:

A. In Chloroplast

1. Photorespiration begins in the chloroplast by forming a bond between RuBP and O₂ molecules with the help of the RuBisCO enzyme. This RuBP on attaching with O_2, splits up into phosphoglyceric acid (PGA) (3C) and phosphoglycolate (2C). PGA is used in the Calvin cycle.
2. Then 2-phosphoglycerate loses its phosphate group in the presence of enzyme phosphatase and is converted into glycolate. Glycolate is released from the chloroplast and enters the peroxisome by diffusion.

B. In Peroxisome

3. Glycolate is oxidised by glycolate oxidase enzymes to form glyoxylate and H₂O₂.
4. Glyoxylate is then converted to an amino acid glycine by a transamination reaction catalysed by the enzyme glutamate-glyoxylate transaminase. Now this glycine, by diffusion, will enter into the mitochondria.

C. In Mitochondria

5. Two molecules of glycine combined to form one molecule of serine (3C) and one molecule of CO₂. This molecule of CO₂ is released as a result of loss of 25% of CO₂ in mitochondria

D. In Peroxisome

6. Serine is transported back to peroxisome by diffusion where it is converted into hydroxypyruvate with the help of a transamination reaction (release of NH₃).
7. Hydroxypyruvate is reduced to glycerate with the help of NADH.

E. In Cytosol

8. Glycerate in the cytosol is phosphorylated with ATP into 3PGA which then diffuses into the chloroplast and enters the Calvin cycle.

Photorespiration Significance

The process of photorespiration is found significant when the concept of the evolution of plants is considered. Photorespiration is most common in C₃ plants or dicot plants in which the loss of CO₂ reduces the photosynthetic deficiency and as a result, it leads to the evolution of the C₄ plants.

Photorespiration Demerits

Some of the demerits of photorespiration are given below:

1. Photorespiration is regarded as a wasteful process in crop plants which ultimately leads to a reduction in the final yield of the crop.
2. Photorespiration reduces photosynthetic efficiency because it leads to the loss of fixed CO₂.

Summary

Photorespiration is a light-dependent cyclic respiration process that occurs in photosynthetic cells resulting in loss of CO₂. The term photorespiration was discovered by Dicker and Tio in the year 1959 in tobacco plants. Different steps of photorespiration occur in different cell organelles like chloroplast, peroxisome, and mitochondria in the cycling of different organic substances. Photorespiration generally takes place in C₃ plants like beet, bean, cotton, rice, wheat, barley, etc. The process of photorespiration interferes with the successful functioning of the Calvin cycle. The presence of photorespiration is considered a wasteful and energy-consuming process in crop plants which ultimately leads to a reduction in the final yield of crops.

Frequently Asked Questions (FAQs) on Photorespiration

Q.1. What is Photorespiration?
Ans: Photorespiration is a light-dependent cyclic respiration process that occurs in photosynthetic cells resulting in loss of CO₂.

Q.2. What are the sites of photorespiration?
Ans: The sites of photorespiration are chloroplast, peroxisome, and mitochondria.

Q.3. Why is photorespiration considered a wasteful process?
Ans: Photorespiration is considered a wasteful process for two reasons:
a) There is no synthesis of sugar and ATP or NADPH.
b) The CO₂ is released with the utilisation of ATP.

Q.4. How many oxygen molecules are used in the process of photorespiration?
Ans: Two molecules of oxygen are used. One molecule is used by RuBP and one molecule is used by glyoxylate.

Q.5. What is the significance of photorespiration?
Ans: The process of photorespiration is found significant when the concept of the evolution of plants is considered. Photorespiration is most common in C₃ plants or dicot plants in which the loss of CO₂ reduces the photosynthetic deficiency and as a result, it leads to the evolution of the C₄ plants.

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