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

1 Answer
Nov 7, 2017

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:

enter image source here
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.