Photosynthesis in Higher Plants

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DÅRK RËÅÇTÏØÑ (BÏØSÝÑTHËTÏÇ PHÅSË)/ÜSË ØF ÅTP & ÑÅDPH

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Products of light reaction are ATP, NADPH and O2.

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In dark reaction ATP and NADPH is used to drive the processes for the

synthesis of food (sugars).

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It is the biosynthetic phase of photosynthesis.

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This phase does not directly depend on light but is dependent on the

products of light reaction. (i.e. ATP and NADPH besides CO2 and H2O)

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It can be verified as follows: Immediately after light becomes unavailable,

the biosynthetic process continues for some time, and then stops. If light

is available, the synthesis starts again.

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CO2 combines with H2O to form (CH2O)n or sugars.

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CO2 assimilation during photosynthesis is of 2 types :

1.

C3 pathway: In this, first stable product of CO2 fixation is a C3 acid

(PGA). Melvin Calvin studied algal photosynthesis using ¹⁴C. He

discovered that the first CO2 fixation product was 3-phosphoglyceric

acid (PGA), a 3-carbon organic acid.

2.

C4 pathway: In this, first stable product is oxaloacetic acid (OAA), a

4-carbon (C4) organic acid.

1. THË ÇÅLVÏÑ ÇÝÇLË (Ç3 PÅTHWÅÝ)

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It occurs in all photosynthetic plants.

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It has 3 stages: carboxylation, reduction and regeneration.

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RuBP (ribulose

bisphosphate - a

5-carbon ketose sugar) is

the primary CO2 acceptor.

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It is the most crucial step.

CO2 is fixed by RuBP to two

3-PGA in presence of the

enzyme RuBP carboxylase.

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Since this enzyme also has

an oxygenation activity it is

called RuBP carboxylase-

oxygenase (RuBisCO).

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ÇÅRBØXÝLÅTÏØÑ ØF RÜBP