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A Handbook of Biology

3. PHØTØRËSPÏRÅTÏØÑ

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Photorespiration creates an important difference between C3 and C4

plants.

Both O2 and CO2

show competitive

binding for the

active site of

RuBisCO

Most abundant

enzyme in the

world

Has a affinity for

both oxygen and

carbon dioxide

Has a greater

affinity for CO2

The relative

concentration of

O2 and CO2 decides

which one will bind to

(O2/CO2) enzyme

RuBisCO

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In C3 plants, some O2 binds to RuBisCO. Hence CO2 fixation is decreased.

Here RuBP binds with O2 to form one molecule of phosphoglycerate

and phosphoglycolate. This pathway is called photorespiration.

(NEET 2020)

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In this pathway, there is neither synthesis of sugars, nor of ATP and

NADPH. Hence photorespiration is a wasteful process. Rather it causes

the release of CO2 by using ATP.

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In C4 plants, photorespiration does not occur because they can

increase CO2 concentration at the enzyme site. (NEET 2016) This takes

place when C4 acid from the mesophyll is broken down in the bundle

cells to release CO2. This minimises the oxygenase activity of RuBisCO.

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Due to the lack of photorespiration, productivity and yields are

better in C4 plants. In addition, these plants show tolerance to higher

temperatures