Question

What happens during the Calvin-Benson cycle, or dark reaction?

Answer 1

In Calvin cycle or dark reaction, sugars are formed by using the chemical energy of `ATP` and `NADPH` formed during light reaction.

Explanation:

Equation for dark reaction:

`3CO_2` + `6NADPH` + `9ATP to (CH_2O)_3` + `6NADPH` +
`9ADP` +`9`Pi

Diagram of Calvin cycle:

https://www.khanacademy.org/science/biology/photosynthesis-in-plants/the-calvin-cycle-reactions/a/calvin-cycle

Steps in Calvin Cycle:

1st step: Carbon fixation:

It refers to the initial incorporation of `CO_2` into organic material. We will follow `3` molecules of `CO_2` through the reaction because we want sugars(carbohydrates) as the end product of cycle. And for this, at least 3 molecules of `CO_2` are necessary.
Because;
`3` molecules of `CO_2`→ contain `3` C→ so at least they will give `1` carbohydrate molecule( a triose).

• Calvin cycle begins by the reaction of CO2 with highly reactive phosphorylated five carbon-sugar named ribulose bisphosphate(RuBP).

• This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase also known as Rubisco .

• The product of this reaction is highly unstable six-carbon intermediate that immediately breaks into two molecules of three-carbon-compound named 3-phosphoglycerate(PGA).
• The carbon that was originally a part of `CO_2` molecule is now a part of an organic compound which means carbon has been fixed .

2nd step: Reduction:

In this step, fixed carbon is reduced to energy rich G3P with the energy and reducing power of ATP and NADPH respectively.

• Each molecule of PGA recieves an additional phosphate from ATP of light reaction and form 1,3-bisphosphoglycerate.

• It is reduced to glyceraldehyde 3-phosphate(G3P) by recieving a pair of electrons donated from NADPH of light reactions.
  Actually G3P is the carbohydrate that is produced directly from Calvin cycle.

• For every `3` molecules of `CO_2` entering the cycle and combining with `3` molecules of RuBp, `6` molecules of G3P are produced. But only `1` molecule of G3P can be counted as a net gain of carbohydrate.
  Because out of every `6` molecules of G3P formed, only one molecule leaves the cycle to be used by plants for making glucose and other carbohydrates; the other five molecules are recycled to regenerate the three molecules of five carbon RuBP, the `CO_2` acceptor.

3rd step: Regeneration of `CO_2` acceptor, RuBp:

• Through a complex series of reactions, the carbon skeletons of five molecules of three-carbon G3P are rearranged into three molecules of five-carbon ribulose phosphate(RuP).

• Each RuP is phosphorylated to RuBP. Again `3` molecules of ATP of light reactions are used for this phosphorylation of `3`
  RuP.

• These RuBP are now prepared to recieve `CO_2` again, and the cycle continues.