About the calvin cycle

The Calvin Cycle

The Calvin cycle is a light-independent reaction that occurs during the photosynthesis process. It occurs as carbon dioxide (CO2) diffuses through the stroma, where the light-independent photosynthesis reaction occurs. The three stages concerned are referred to as the Calvin cycle. Carbon dioxide (CO2) fixation is the first step, followed by a reduction reaction stage, and eventually ribulose 1, 2 -bisphosphate regeneration (Gong & Li, 2016).

Carbon Fixation

Carbon fixation refers to the mechanism by which carbon dioxide is converted from an inorganic to an organic state. Carbon dioxide (CO2), an enzyme referred to as ribulose bisphosphate carboxylase (RuBisCO) and three ribulose bisphosphate molecules (RuBP) are present in the stroma to initiate the light-independent reaction. Bisphosphate molecules (RuBP) has five carbon atoms, flanked by other two phosphates. A reaction between bisphosphate molecules (RuBP) and Carbon dioxide (CO2) is catalyzed by ribulose bisphosphate carboxylase (RuBisCO). For every CO2 molecule which reacts with one RuBP, two 3-phosphoglyceric acids (3-PGA) molecules are formed. Every turn within the cycle, the same molecules combine to form the 3-phosphoglyceric acid (3-PGA). The number of carbon atoms, however, remain unchanged as they form new bonds (3 atoms from 3CO2 + 15 atoms from 3RuBP = 18 atoms in 3 atoms of 3-PGA), during the reactions.

Reduction Reaction

During reduction reaction, six molecules of 3-phosphoglyceric acid (3-PGA) are converted by six molecules of both ATP and NADPH to form six molecules of a chemical referred to as glyceraldehyde 3-phosphate (G3P). It is a reduction reaction due to the fact that 3-phosphoglyceric acid (3-PGA) gains electrons. Energy is released when ATP losses a terminal phosphate atom, which is converted to ADP. On the contrary, a hydrogen atom and energy are lost when NADPH is converted to NADP+. Both of the molecules are however taken back to the light-dependent reactions where they are reenergized ad used again.

Ribulose 1, 2 -Bisphosphate Regeneration

During this stage, glyceraldehyde 3-phosphate (G3P) is released from the Calvin cycle. Glyceraldehyde 3-phosphate (G3P) moves to the cytoplasm. It contributes to the formation of other important compounds crucial to the plant. Glyceraldehyde 3-phosphate (G3P) from the chloroplast has three carbon atoms as it is exported. It, therefore, takes three cycles of the Calvin process to fix enough carbon that would be able to export one molecule of glyceraldehyde 3-phosphate (G3P). Even so, each cycle only makes two glyceraldehyde 3-phosphate (G3P) s thus three turns make six. Only one, glyceraldehyde 3-phosphate (G3P) molecule is exported while the remaining five molecules are left behind in the cycle in order to be used in regenerating bisphosphate molecules (RuBP). This, in turn, helps prepare the system to receive more oxygen which will be fixed. Three more ATP molecules are used during the regeneration process.

The Overall Equation

The overall equation for the Calvin cycle chemical reaction is:

3 CO2 + 6 NADPH + 5 H2O + 9 ATP → glyceraldehyde-3-phosphate (G3P) + 2 H+ + 6 NADP+ + 9 ADP + 8 Pi (Pi = inorganic phosphate)

In order to produce one glucose molecules, six runs of the cycle are needed. Surplus glyceraldehyde 3-phosphate (G3P) is used to form a number of carbohydrates depending on what the club needs (Gong & Li, 2016).

Reference

Gong, F., & Li, Y. (2016). Fixing carbon, unnaturally. Science, 354(6314), 830-831.