Question

What is the first step of the Calvin-Benson cycle?

Answer 1

`Carbon fixation"`

Explanation:

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` `to` contain `3` `C to` so at least they will give `1` carbohydrate molecule( a triose).

• Calvin cycle begins by the reaction of `CO_2` 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.
  

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

`Note:`
Rubisco is the most abundant protein in the chloroplast and probably the most abundant protein on Earth.