Question

Equation of photosynthesis :

$6CO_2\: + \:12 H_2O \rightarrow C_6H_{12}O_6 \:+\: 6H_2O \: + \: 6O_2$

Explain why 12 molecules of water as substrate are used in the equation given above .

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Answer 1

Answer : The reaction of photosynthesis is :6CO2 + 12H2O (light) --> C6H12O6 + 6H2O + 6O2

From this reaction, it is clear that to form one molecule of glucose, six molecules of carbon dioxide are required.

Products of light reaction required in dark reactions to form one molecule of glucose are 12 molecules of NADPH and 18 molecules of ATP.

For forming 2 molecules of NADPH from NADP+,2 molecules of water are required so that demand of 4 electrons and protons can be satisfied.

Two molecules of water split to form a molecule of oxygen and the respective number of electrons and hydrogen ions are also formed.

Now for producing 12 molecules of NADPH, a total of 12 molecules of water are required.