Question

Explain the process of photorespiration.

Answer 1

PROCESS OF PHOTORESPIRATION

The process of photorespiration is a process that takes place in the presence of light.

The rate of respiration takes place in presence of light is three to five times higher than the rate of respiration takes place in dark.

In 1963 Krotkov introduced photorespiration term by the explaining “the release of carbon dioxide in respiration in the presence of light”.

The process of photorespiration depends on the two basic effects:

1. TEMPERATURE: plays an important role in photorespiration. The rate of the process increases in the range of 25 to 30°C.
2. CONCENTRATION OF OXYGEN: higher the concentration, higher the rate of photorespiration.

SITE OF PHOTORESPIRATION:

1. Initially it was discovered that the process of photorespiration takes place in only chloroplast, but after the discovery of peroxisomes it was suggested the link between photorespiration and peroxisomes.
2. Peroxisomes contain glycolate oxidase, catalase and glutamate-glyoxylate aminotransferase enzymes.
3. As the time passes the new discovery revealed that even mitochondria are the part of photorespiration process as it can evolve carbon dioxide.
4. So, the process of photorespiration takes place in three organelles: chloroplasts, peroxisomes and mitochondria.

PROCESS OF PHOTORESPIRATION:

1. In this process Ribulose bi phosphate (RuBP) is oxygenated in the presence of atmospheric oxygen and forms 2 carbon molecule containing phosphoglycolate and 3 carbon molecule containing phosphoglyceric acid (PGA). This reaction takes place in the presence of the enzyme carboxylase and oxygenase.
2. This above process takes place in chloroplasts, where phosphoglycolate is further converted into glycolate by the process of dephosphorylation in the presence of the enzyme glycolate oxidase.
3. In this reaction hydrogen peroxide (H₂O₂) is also produced.
4. Now the produced glycolate is then diffuses out from the chloroplast and enter to the peroxisome.
5. In peroxisome, glycolate is converted into glycine by the enzyme glutamate-glyoxylate aminotransferase that also produces α-ketoglutarate as a bi-product.
6. Now glycine diffuses from peroxisome to mitochondrion, where it combines with NAD+ and produces serine by the consequent release of NADH, carbon dioxide (CO₂) and ammonia (NH₃).
7. The produced serine is diffused to peroxisome, where it is converted to hydropyruvate and glycerate.
8. Now glycerate is transported to chloroplast where it is phosphorylated and produces ADP from ATP and RuBP.
9. During this reaction PGA is also produced that enters Calvin cycle.
10. These all reactions are seemed as cyclic process. So, it is also known as photorespiratory carbon oxidation cycle.
11. In this cyclic process, 75% of lost carbon is recovered by oxygenation of RuBP and 25% is wasted as CO₂.
12. The whole process depends on the balance of O₂ and CO_2, excess O₂ induces photorespiration whereas excess CO₂ inhibits it.