3 Phases of the Carbon Cycle
3 Phases of Carbon Cycle
Phase 1: Carbon fixation
The Calvin cycle incorporates each CO2 molecule by attaching it to a five-carbon sugar named ribulose biophosphate (abbreviated RuBP).
The enzyme that catalyzes this step is RuBP carboxylase, or rubisco.
The product of the reaction is a six carbon intermediate so unstable that it immediately splits in half to form two molecules of 3-phoshpyglycerate (for each CO2)
Phase 2: Reduction
Each molecule of 3-phsophoglycerate receives an additional phosphate group from ATP, becoming 1,3-biophosphoglcerate.
Next, a pair of electrons donated from NADPH reduced 1,3 biophosphoglycerate to the carbonyl group of G3P, which stores more potential energy.
G3P is a sugar-the same three-carbon sugar formed in glycolysis by the splitting of glucose.
Remember! For every three molecule of CO2 there are six molecules of G3P. But only one molecule of this three-carbon sugar can be counted as a net gain of carbohydrate
The cycle began with 15 carbons’ worth of carbohydrate in the form of three molecule of the five-carbon sugar RuBP.
Now there is 18 carbons’ worth of carbohydrate in the form of six molecules of G3P.
One molecule exits the cycle to be used by the plant cell, but the other five molecules must be recycled to regenerate the three molecules of RuBP.
Phase 3: Regeneration of CO2 acceptor (RuBP)
In a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged by the by the last step of the Calvin Cycle into three molecule of RuBP.
To accomplish this, the cycle spends three more molecules of ATP.
The RuBP is now prepared to receive CO2 again, and the cycle continues!
Phase 1: Carbon fixation
The Calvin cycle incorporates each CO2 molecule by attaching it to a five-carbon sugar named ribulose biophosphate (abbreviated RuBP).
The enzyme that catalyzes this step is RuBP carboxylase, or rubisco.
The product of the reaction is a six carbon intermediate so unstable that it immediately splits in half to form two molecules of 3-phoshpyglycerate (for each CO2)
Phase 2: Reduction
Each molecule of 3-phsophoglycerate receives an additional phosphate group from ATP, becoming 1,3-biophosphoglcerate.
Next, a pair of electrons donated from NADPH reduced 1,3 biophosphoglycerate to the carbonyl group of G3P, which stores more potential energy.
G3P is a sugar-the same three-carbon sugar formed in glycolysis by the splitting of glucose.
Remember! For every three molecule of CO2 there are six molecules of G3P. But only one molecule of this three-carbon sugar can be counted as a net gain of carbohydrate
The cycle began with 15 carbons’ worth of carbohydrate in the form of three molecule of the five-carbon sugar RuBP.
Now there is 18 carbons’ worth of carbohydrate in the form of six molecules of G3P.
One molecule exits the cycle to be used by the plant cell, but the other five molecules must be recycled to regenerate the three molecules of RuBP.
Phase 3: Regeneration of CO2 acceptor (RuBP)
In a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged by the by the last step of the Calvin Cycle into three molecule of RuBP.
To accomplish this, the cycle spends three more molecules of ATP.
The RuBP is now prepared to receive CO2 again, and the cycle continues!