Question

For synthesis of a molecule of glucose, the requirement of ATP and NADPH is respectively
(A) 15 and 10
(B) 33 and 22
(C) 12 and 8
(D) 18 and 12

Answer 1

Hint: The product of light reaction ATP and NADPH are used in the next phase called the biosynthetic phase (Though ${O_2}$ is also formed but it escaped into the environment). In this phase glucose (food) is formed through the Calvin cycle.

Complete step by step answer:
Glucose is formed from $C{O_2}$ through the Calvin cycle in which products of the light reaction are used. Calvin cycle is also called ${C_3}$ cycle because its first product is 3 carbon acid called 3-phosphogylceric acid or PGA.
Calvin cycle occurs in three stages:
1) Carboxylation: it is the first step of the Calvin cycle and it is the most crucial step. In this step carboxylation of RuBP by $C{O_2}$ occurs and this reaction is catalyzed by RuBP carboxylase. This results in the formation of two molecules of 3-PGA.
2) Reduction: This step leads to the formation of one molecule of glucose and this step consumes 2 molecules of ATP and 2 molecules of NADPH. So, for the formation of one molecule of glucose which consists of 6 carbon this cycle should run for 6 times as in each step one $C{O_2}$molecule is involved.
3) Regeneration: For this cycle to run uninterrupted, regeneration of $C{O_2}$acceptor (RuBP) is a must and this requires one ATP.
Calculation of ATP and NADPH:
For one cycle,
For 6 cycle (for one molecule of glucose)
3 ATP
$3 \times 6$$ = 18$ATP
2 NADPH
$2 \times 6 = 12$NADPH
So, the correct answer is option (D).

Additional information: This biosynthesis phase is also called dark reaction as it does not depend on light directly but it depends upon the product of the light reaction.

Note: Enzyme involved in the first step of the Calvin cycle is RuBP carboxylase. But it’s better to call it RuBP carboxylase Oxygenase (Ribulose bisphosphate carboxylase oxygenase, RuBisCO, it is one of the most abundant enzymes in the world ) as it is involved in the oxidation of RuBP to 3-PGA.