What is Calvin Cycle?
“The cyclic series of reactions, catalyzed by respective enzymes by which carbon is fixed and reduced, resulting in the synthesis of sugar during the dark reactions of the photosynthesis is called Calvin cycle.”
The dark reaction takes place in the stroma of the chloroplast. These reactions do not require light energy directly. They can occur in the presence or absence of light. They need only assimilatory power (stored compounds) in the form of ATP and NADPH. These compounds are produced during light reactions. So energy is stored in these compounds.
This stored energy of these compounds is used for the synthesis of carbohydrates from CO2. These reactions can be summarized as follows:
The detail of the path of the carbon in these reactions was discovered by Melvin Calvin and his colleagues at the University of California. Calvin was awarded Nobel Prize in 1961.
The Calvin cycle can be divided into three phases: Carbon fixation, Reduction, and regeneration of CO2 acceptor.
Phase 1 in Calvin cycle: Carbon fixation
In this step, the initial incorporation of three molecules of CO2 takes place into the organic material. The molecule of CO2 reacts with a highly reactive phosphorylated five-carbon sugar called ribulose bisphosphate (RUBP). This reaction is catalyzed by an enzyme called ribulose bisphosphate carboxylase. This enzyme is commonly called Rubisco.
The rubisco is the most abundant protein in the chloroplast. It is perhaps the most abundant protein on the earth. The product of this reaction is a highly unstable six-carbon compound. This compound immediately breaks into two molecules of three-carbon compounds, called 3-phosphoglycerate (phosphoglyceric acid PGA).
The product of the initial carbon compound is a three-carbon compound. Therefore, The Calvin cycle is also known as the “C3 pathway”.
Phase 2: Reduction
- Each molecule of the PGA receives an additional phosphate molecule from the ATP (coming from light energy) and forms 1, 3 biphosphoglycerates.
- The NADPH (coming from the light reaction) donates its electrons to 1, 3 biphosphoglycerates. So it is reduced to form glyceraldehyde 3-phosphate (G3P). This G3P sugar is also formed by splitting the glucose molecule during glycolysis. In this way, fixed carbon is reduced to energy-rich G3P. The G3P (not glucose) is the carbohydrate produced during the Calvin cycle.
During the Calvin cycle, three molecules of CO2 and three molecules of RUBP (five-carbon compounds) are used. They produce six molecules of G3P (containing 18 carbon in all). Only one molecule of G3P out of six molecules leaves the cycle and is used for making glucose, sucrose, starch, or other carbohydrates.
The other five molecules are recycled to regenerate the three molecules of five carbons RuBP. The RuBP is a CO2 acceptor. So, only one molecule of G3P can be counted as the net gain of carbohydrates.
Phase 3: Regeneration of CO2 acceptor, RuBP
The carbon skeletons of the five remaining molecules of G3P (three-carbon compound) are rearranged through a complex series of reactions.
They form a five-carbon compound ribulose phosphate (RUP). The RUP is phosphorylated and becomes ribulose bisphosphate (RuBP). The RuBP is the five-carbon CO2 acceptor. It is the same compound with which the cycle was started. More molecules of ATP (coming from the light reaction) are used for the phosphorylation of three RuP molecules. These RuBP are now prepared to receive more CO2 again. In this way, this cycle continues.