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Photosynthesis in Higher Plants

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CBSE Gujarat Board Haryana Board

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Class 10 Class 12
What led to the evolution of C4 plants or C4 pathway of photosynthesis ? Describe in detail.

C4 pathway is an adaptation to tolerate strong light, high temperature ; high quantity of oxygen etc. Evolution of C4 plants from C3 plants can be described as follows :

1. C3 plants undergo photorespiration in strong light or at high temperature which is a wasteful process in which no energy is produced.

2. In C3 plants RUBP carboxylase get denatured in strong light, but in C4 plants it does not.

3. These C4 plants have Kranz anatomy. In this case mesophyll cells have grana thylakoid and bundle sheath cells have agranal thylakoids present in stroma of chloroplast.

4. In C4 plant, light reaction occurs in mesophyll cells and RuBisCo is found in bundle sheath cells in which CO2 fixation occurs. Thus, in C4 plants RuBisCo remains protected from sunlight and is also protected from oxygenation, because in bundle sheeth cells only dark reaction occurs.

Thus, to avoid photorespiration, oxygenation of RuBisCo etc. C4 plants were evolved from C3plants. The C4 plants are maize, sugar cane, grasses etc.

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Name the first stable compound formed in C3 cycle.

3-phosphoglyceric acid (PGA)
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Where CO2 assimilation occurs in CPlants ?

In bundle sheath cells.
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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.
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What is stroma ?

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