# What is the first part of the Calvin cycle?

Nov 5, 2017

$C a r b o n$ fixation

#### Explanation:

It refers to the initial incorporation of $C {O}_{2}$ into organic material. We will follow $3$ molecules of $C {O}_{2}$ through the reaction because we want sugars(carbohydrates) as the end product of cycle. And for this, at least $3$ molecules of $C {O}_{2}$ are necessary.

Because;
$3$ molecules of $C {O}_{2}$$\to$ contain $3$ $C \to$ so at least they will give $1$ carbohydrate molecule( a triose).

• Calvin cycle begins by the reaction of $C {O}_{2}$ 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 $C {O}_{2}$ molecule is now a part of an organic compound which means carbon has been fixed.

$N o t e :$