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Photosynthesis in Higher Plants

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Class 10 Class 12
What is photorespiration ? Describe the process in detail and link it with the Calvin Cycle.

Photorespiration is the process where oxygen is consumed, food is oxidized and carbondioxide is liberated, but no energy is produced. It is a highly wasteful process which occurs in the presence of light. 

Mechanism of photorespiration:

It takes oplace in chloroplasts and completes with the assistance of peroxysomes and mitochondria.

1. At high temperature RUBP caThe enzyme has great affinity for oxygen than for carbon dioxide. It combines oxygen and RUBP to form PGA and phosphoglycolate.

2. Phosphoglycolate forms phosphoric acid and glycolate in the presence of hydrolase. It passes into peroxysome.

3. Glycolate molecule combines with oxygen to form glyoxylate and H2O2 which is very toxic. It occurs in peroxysome.

4. Glycoxylate undergoes amination to form glycine. The glycine molecules undergo condensation, decarboxylation, deamination to form serine.

5. Serine undergoes deammination to form PGA which enters into chloroplast and takes part in C3 cycle. Thus, it is linked with C3 cycle. Moreover RUBP carboxylase changes its behaviour at high tampereture and acts RUBPoxygenase and thus it delinks from C3 cycle.


Name the first stable compound formed in C3 cycle.

3-phosphoglyceric acid (PGA)

Where CO2 assimilation occurs in CPlants ?

In bundle sheath cells.

What are grana ?

Grana are the stacks of thylakoids embedded in the stroma of a chloroplast. They are the sites of light reaction of photosynthesis.

 What is stroma ?

Stroma is matrix or the cytoplasm of chloroplast where dark reaction of photosynthesis takes place.

Why Rubisco is not affected by O2 in C4 plants ?

O2 is formed in mesophyll cells while Rubisco is present in bundle sheath cells. Therefore Rubisco does not come in contact with O2 and is thus not affected by it.