Question

In \[{C_3}\] cycle for the fixation of every \[C{O_2}\] molecule, the reduction and regeneration steps require
A. \[3ATP\] and \[\;2NADP{H_2}\]
B. \[2ATP\] and \[\;2NADP{H_2}\]
C. \[2ATP\] and \[\;3NADP{H_2}\]
D. \[3ATP\] and \[\;3NADP{H_2}\]

Answer 1

Hint: The Calvin cycle or \[{C_3}\] cycle, is the most extensive \[C{O_2}\] fixation pathway among autotrophs. In \[{C_3}\] cycle, for the fixation of each carbon dioxide molecule, the reduction and regeneration stages need about \[3ATP\] and \[\;2NADP{H_2}\] molecules.

Complete answer:
The Calvin cycle is also known as Photosynthetic Carbon Reduction (PCR) cycle. The Calvin cycle is the cycle of chemical reactions performed by plants to fix carbon from \[C{O_2}\] into three-carbon sugars. It is divided into three major steps that are as follows:
Carboxylation (Carbon fixation): zero $ATP$ and zero $NADP{H_2}$
Reduction (Glycolytic reversal): two $ATP$ and two $NADP{H_2}$
Regeneration (Regeneration of RUBP): one $ATP$ and zero $NADP{H_2}$

Thus, each turn of the \[{C_3}\] cycle uses up \[3ATP\] and \[\;2NADP{H_2}\] in the processes of reducing (adding electrons to) 3-phosphoglyceric acid to produce glyceraldehyde-3 phosphate, and regenerating $RuBP$ so that they can accept a new atom of carbon from \[C{O_2}\] from the air.

So, the correct answer is '\[3ATP\] and \[\;2NADP{H_2}\]​'.

Additional Information:
By using the energy carriers created in the initial stage of photosynthesis, the Calvin cycle reactions fix \[C{O_2}\] from the air to build carbohydrate molecules. An enzyme, $RuBisCO$, catalyzes the fixation reaction, by combining \[C{O_2}\] with $RUBP$. The resultant six-carbon compound is broken down into two three-carbon compounds and the energy in $ATP$ and $NADPH$ is used to convert these molecules into$G3P$. One of the three-carbon molecules of $G3P$ leaves the cycle and becomes a part of a carbohydrate molecule. The remaining $G3P$ molecules keep on in the cycle to be formed back into $RUBP$, which is ready to react with more \[C{O_2}\].

Note: Calvin cycle reactions are also considered as the light-independent reactions of photosynthesis as they are not directly powered by light. Instead, the Calvin cycle is powered by $ATP$ and $NADPH$, which are created by harnessing the energy from photons in the light-dependent reactions.