Question

Assertion (A) Twelve molecules of water are used in the equation of photosynthesis.
Reason (R) The equation of photosynthesis is
$6C{O}_2+12H_{2}O\stackrel{\text{ Light }}{\to }{C}_6{H}_{12}{O}_6+6H_{2}O+6O_2$

• A. If both A and R are true and R is the correct explanation of A.
• B. If both A and R are true, but R is not the correct explanation of A.
• C. If A is true, but R is false.
• D. If A is false, but R is true

Answer 1

Answer: (B)

Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
The net reaction of photosynthesis is
$6C{O}_2+12H_{2}O\stackrel{\text{ Light }}{\to }{C}_6{H}_{12}{O}_6+6H_{2}O+6O_2$
From this reaction it is clear that to form one molecule of glucose, six molecules of $C{O}_2$ are required.
This is because the product 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 $1$ molecules of oxygen and the respective number of electrons and hydrogen ions are also formed. Now for producing $12$ molecules of $NADPH$, total $12$ molecules of water are required.