What is Calvin Cycle?
The chloroplast of plant and algae contain specific enzymatic machinery to reduce atmoshpheric CO2 into sugar, which autotrophs essentially require to live.
It has been discovered in 1950 by Nobel Laureate Melvin Calvin and colleagues using radioisotope 14C with Chlorella pyrenoidosa and Scenedesmusobliques algae.
It is also known as C3 cycle, biosynthetic phase, dark reactions, photosynthetic carbon reduction cycle, or light-independent reactions of photosynthesis.
In plant leaves stomata is the route of entry of CO2 to reach chloroplast via diffusion. Stroma is the place where Calvin cycle takes place, here in the absence of light synthesis of sucrose and starch, from CO2 and H2O occurs by utilizing NADPH and ATP molecules produced in light reactions of photosynthesis.
Phases of Calvin Cycle
Glucose is not directly generated by Calvin cycle instead three-carbon sugar molecule glyceraldehydes 3-phosphate (G3P) is produced following a battery of reactions. Calvin’s cycle is completed in the following three phases to yield a final product.
1. Carboxylation (Fixation)
2. glycolytic reversal (Reduction)
3. regeneration of RuBP
Phase 1. Carboxylation (Carbon fixation)
To initiate the cycle two components, ribulose-1,5 bisphosphate carboxylase (Rubisco- a most abundant enzyme present on Earth) and five-carbon sugar ribulose bisphosphate (RuBP) are required in addition to CO2.
The first step of carbon fixation is the binding of CO2 to RuBP facilitated by Rubisco.
This reaction forms a highly unstable six- carbon intermediate compound which readily converts into two molecules of 3- phosphoglycerate (3-PGA). This transformation of inorganic CO2 into organic form is termed carbon fixation.
Phase 2. Glycolytic Reversal (Reduction)
In this stage, the end product of carbon fixation, 3-PGA receives electrons and converts into glucose by assimilating the energy from ATP and NADPH.
Six molecules of 3-PGA forms six molecules of glyceraldehydes 3 phosphate (G3P).
ATP transfers a phosphate group to each molecule of 3-PGA , and forms 1,3 bisphosphoglycerate which is subsequently reduced by electron donor NADPH and results in the formation of G3P.
In glycolysis splitting of glucose molecules also forms the same compound. Three molecule of CO2 forms six molecules of G3P.
One molecule of this three-carbon sugar (G3P) exits the cycle and form carbohydrate mostly glucose because the rest are required to regenerate three RuBP acceptor molecules.
At the end of the reaction, six molecules of both ATP and NADPH are used.
ATP releases energy while converting to ADP by loosing terminal phosphate, NADPH dispenses both posphate and electron and finally forms NADP+.
Both of these molecules enter again in light dependent reactions to be reused and recycled.
Phase 3: Regeneration of RuBP
This is the last step of Calvin cycle. As mentioned above one molecule of G3P involves in the synthesis of carbohydrates so the rest five molecules should be recycled to accept new carbon molecules to continue the Calvin cycle.
In this direction three molecules of RuBP regenerates by reshuffling five molecules of G3P, following the array of reactions, and investing three more ATP molecules.
Regulation of Calvin Cycle
Changes in gene expression, protein biosynthesis, and post-translational modification regulate an adequate amount of required intermediates during the cycle and turned off the cycle when it is not needed.
Short term regulation of Calvin cycle includes a transformation of covalent bonds such as reduction of disulphides, the modification of non covalent interactions for eg. Binding of enzymes to the thylakoid membranes increases the efficiency of Calvin cycle.
Key Points of Calvin Cycle
Calvin cycle refers to the process of producing carbohydrates from the energy stored by the light-dependent reactions of photosynthesis.
Carbon fixation is the first stage of the Calvin cycle.
In the second step of the cycle, 3-PGA converts to G3P after the reduction in the presence of ATP and NADPH which further changes into ADP and NADP+.
RuBP recycling is the last phase of cycle , RuBP to fix more carbon.
Calvin cycle utilizes one carbon at each turn in carbon fixation.
One molecule of glyceraldehyde-3 phosphate is synthesized after three turns of Calvin cycle by fixing 3CO2 .
The combining of two molecules of glyceraldehyde-3 phosphate takes place to form one molecule of glucose.
3 ATP and 2NADH were consumed at each turn of Calvin cycle to regenerate RuBP and to reduce 3-phosphoglyceric acid into glyceraldehyde-3 phosphate.
It can be concluded that a total of 18 ATP and 12 NADPH are utilized to produce a single molecule of glucose via the Calvin cycle.
Significance of the Calvin Cycle
Calvin cycle produces G3P which subsequently converts into glucose and forms cellulose and starch. Plants use these carbohydrate molecules for energy production and as structural building materials.
Calvin cycle play important role in regulating the level of atmospheric CO2 .
This makes the Calvin cycle vital for the existence of most ecosystem.
The carbon produced in Calvin cycle is the primary source of protein, lipid, nucleic acid synthesis in most plants and animal
Calvin Cycle Citations
- Variations in the Calvin-Benson cycle: selection pressures and optimization? J Exp Bot . 2019 Mar 27;70(6):1697-1701.
- The minimal model of Hahn for the Calvin cycle. Math Biosci Eng . 2019 Mar 15;16(4):2353-2370.
- The importance of the photosynthetic Gibbs effect in the elucidation of the Calvin-Benson-Bassham cycle. Biochem Soc Trans . 2018 Feb 19;46(1):131-140.
- Discovery of the canonical Calvin-Benson cycle. Photosynth Res . 2019 May;140(2):235-252.