Photorespiration: C2 Cycle

Photorespiration: C2 cycle

Photorespiration is a process which involves oxidation of organic compounds in plants by oxygen in the presence of light.  Like normal respiration, this process also releases carbon from organic compound in the form of carbon dioxide but does not produce ATP.  Thus, it seems to be a wasteful process.  Photorespiration occurs only in C3 plant during daytime usually when there is high concentration of oxygen.

Photorespiration was first demonstrated by Dicker and Tio (1959) in tobacco and the term, photorespiration, was given by Krotkov in the year 1963.  It occurs in temperate C3 plants such as rice, wheat, barley, bean etc.  The process of photorespiration takes place in chloroplast, peroxisome and mitochondria.

Photorespiration increases with increase in the intensity of light, high temperature, high oxygen, low CO2 concentration and age of the leaf.  On a warm, bright day when plant is undergoing rapid photosynthesis, upto 50% of the CO2 fixed in photosynthesis is lost in photorespiration without generating any substantial amount of energy.  Like normal respiration (i.e., dark respiration), photorespiration also takes in oxygen and gives out CO2.

Following are some of the important characteristics of photorespiration:

  1. It takes place in the photosynthetic cells in presence of light.
  2. It depends upon the photochemical activity of chloroplasts and  also the biochemical activity of peroxisomes and mitochondria.
  3. Although photorespiration utilizes ATP and NADPH2, but produces only a very small amount of ATP. Thus it is an energy consuming process whereas normal respiration is energy producing process.

Mechanism of photorespiration /C2 cycle:

The process of photorespiration at the initial stage occurs inside the chloroplast.  Its primary substrate is an early product of photosynthesis, the glycolate.  Since glycolate is a 2-carbon compound the process is also termed as C2 cycle.

In presence of light, with high concentration of oxygen and low concentration of carbon dioxide in the atmosphere the photosynthetic enzyme RuBP carboxylase develops a high affinity for oxygen than carbon dioxide and it functions as RuBP oxygenase.  It catalyzes RuBP into 3 phosphoglyceric acid and phosphoglycolate or phosphoglycolic acid in the presence of oxygen.  Glyoxylate then converts into amino acid, glycine by transamination reaction.

Glycine goes to mitochondria, where two molecules of glycine interact to form one molecule of amino acid, serine along with the release of ammonia and carbon dioxide.  Serine then returns back to peroxisomes and thus contribute to the formation of glyceric acid.  Glyceric acid then passes into chloroplasts where it is phophorylated to form 3PGA.  3PGA enters into the C3 cycle of photosynthesis.

Following are the steps involved in photorespiration in C3 plant :

  1.  When carbon dioxide concentration in the atmosphere becomes less and oxygen concentration inside the plant increases, ribulose 1-5 diphosphate combines with oxygen to form one molecule each of 3 phosphoglyceric acid and 2 phosphoglycolic acid (2 carbon compound) in the presence of enzyme RuBP oxygenase.

Ribulose 1-5 diphosphate+O2 → 3PGA (phosphoglyceric acid) +2 phosphoglycolic acid

  1. 2 phosphoglycolic acid loses its phosphate group in the presence of enzyme phosphatase and thus converts it into glycolic acid.

2 phosphoglycolic acid+H2O → Glycolic acid + H3PO4

  1. The glycolic acid synthesized in chloroplast is then transported to peroxisome, inside the peroxisome, it reacts with oxysome to form glyoxylic acid and H2O2 in the presence of enzyme, glycolic acid oxidase. H2O2 converts into water and oxygen in the presence of enzyme, catalase.

Glycolic acid+O2 → Glyoxylic acid + H2O2

2 H2O2 → 2 H2O+O2

  1. Glyoxylic acid then converts into an amino acid, glycine by transamination reaction with glutamic acid.

Glyoxylic acid+ Glutamic acid → Glycine

  1. Glycine enters into mitochondria where 2 molecules of glycine interact to form 1 molecule each of serine, carbon dioxide, and ammonia. NH3 is then transported to cytoplasm where it is synthesized into glutamic acid.

2 Glycine+ H2O+ NAD → Serine+ CO2 +NH3 +NADH2

  1. Serine returns to peroxisome where it is deaminated and reduced to hydroxy pyruvic acid and finally to glyceric acid.

Serine+ Glyoxylic acid→  hydroxy pyruvic acid

Hydroxy pyruvic acid + NADH2 → Glyceric acid+NAD

  1. Glyceric acid finally enters into chloroplast where it is phosphorylated to 3 phosphoglyceric acid, which enters into C3 cycle.

Glyceric acid+ATP → 3Phosphoglyceric acid+ADP

Photorespiration /C2 Pathway

Fig: Photorespiration /C2 Pathway

Significance of photorespiration:

Photorespiration does not produce energy or reducing power.  Rather it consumes energy. Further it undoes the work of photosynthesis.  Thus there is 25% loss of fixed CO2.  Therefore it is a highly wasteful process.  This happens only in case of C3 plants.

C4 plants have overcome the problem of photorespiration by performing light reaction in mesophyll cells and Rubisco mediated CO2 fixation by Calvin cycle in the interior of leaves ie., in the bundle sheath cells where both temperature and oxygen are lower.

Photorespiration protects the plants from photoxidative damage by dissipating excess of excitation energy.

Difference between Photorespiration and Normal (dark) respiration

Following are the differences between Photorespiration and Normal (dark) respiration

Photorespiration Normal (dark) respiration
1.  It only occurs inside photosynthetic cells 1.  It is found in all living cells.
2.  It takes place only in the presence of light. 2.  It takes place both in dark and in light.
3.  In Photorespiration  both uptake of oxygen and evolution of CO2 are light dependent. 3.  Exchange of gases is independent of light.
4.  It occurs in the chloroplasts and may also require the help of peroxisomes and mitochondria. 4.  It occurs in both cytoplasm and mitochondria.
5.  The substrate is RuBP. 5.  The substrate is commonly glucose though other food materials (like fat, protein, organic acids) can also be used.
6.  End products are CO2 and PGA. 6.  End products are CO2 and water.
7.  It is a wasteful method and thus does not produce energy. 7.  It produces energy
8.  It is prominent in C3plants and negligible in C4 plants. 8.  It is prominent in all organisms except the anaerobic ones.
9.  The substrate forms at the time of utilization. 9.  The substrate is already present in the cells.
10. It is not essential. 10.  It is essential for survival of organisms