9.6:
The Calvin Cycle
In autotrophic plants, the Calvin Cycle starts when atmospheric carbon dioxide eventually diffuses into the stroma of the chloroplast.
Here, one carbon atom from the carbon dioxide is added or fixed to a five-carbon acceptor sugar molecule, ribulose bisphosphate, or RuBP, in a reaction catalyzed by the enzyme Ribulose 1,5-bisphosphate-carboxylase-oxygenase, or RuBisCo for short. The resulting six-carbon molecule is highly unstable and splits into two three-carbon molecules of 3-phosphoglyceric acid, 3-PGA.
With ATP providing the energy, and NADPH affixing one hydrogen to each, the three PGA chains are converted into another three-carbon intermediate called glyceraldehyde-3-phosphate. One G3P then exits the cycle and waits for another one to build glucose with six carbon atoms.
Meanwhile, the remaining G3P must wait for four more cycles as carbons accumulate and ATP provides more energy to regenerate the RuBP acceptors. Overall, six turns of the Calvin Cycle fix six carbon dioxides from the atmosphere using the energy and reducing power of 18 ATPs and 12 NADPHs, respectively, to generate one molecule of glucose and rebuild RuBP to continue the loop.
9.6:
The Calvin Cycle
Overview
Oxygenic photosynthesis converts approximately 200 billion tons of carbon dioxide (CO2) annually to organic compounds and produces approximately 140 billion tons of atmospheric oxygen (O2). Photosynthesis is the basis of all human food and oxygen needs.
The photosynthetic process can be divided into two sets of reactions that take place in different regions of plant chloroplasts: the light-dependent reaction and the light-independent or “dark” reactions. The light-dependent reaction takes place in the thylakoid membrane of the chloroplast. It converts light energy to chemical energy, stored as ATP and NADPH. This energy is then utilized in the stroma region of the chloroplast, to reduce atmospheric carbon dioxide into complex carbohydrates through the light-independent reactions of the Calvin-Benson cycle.
The Calvin-Benson Cycle
The Calvin-Benson cycle represents the light-independent set of photosynthetic reactions. It uses the adenosine triphosphate (ATP) and nicotinamide-adenine dinucleotide phosphate (NADPH) generated during the light-dependent reactions to convert atmospheric CO2 into complex carbohydrates. The Calvin-Benson cycle also regenerates adenosine diphosphate (ADP) and NADP+ for the light-dependent reaction.
At the start of the Calvin-Benson cycle, atmospheric CO2 enters the leaf through openings called stomata. In the stroma region of the chloroplast, the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) adds one carbon atom from CO2 to a 5-carbon (5C) acceptor sugar molecule, ribulose-1,5- bisphosphate (RuBP). The resulting 6C molecule is highly unstable and splits into two molecules of 3-phosphoglyceric acid (3-PGA). The enzyme 3-phosphoglycerate kinase uses ATP to phosphorylate these 3-PGA molecules to form 1,3-bisphosphoglycerate. Glyceraldehyde 3-phosphate dehydrogenase uses NADPH to reduce these molecules to form glyceraldehyde 3-phosphate (G3P), a 3C sugar. This final product gives rise to the name C3 carbon fixation—an alias for the Calvin-Benson cycle.
To fix six CO2 molecules, the Calvin-Benson cycle reduces 12 NADPH and 18 ATP molecules. These energy sources are replenished by the light-dependent reactions of photosynthesis. The six CO2 are attached to six 5C molecules (RuBP) that break into 12 3C molecules (G3P). Ten of these G3P molecules regenerate six molecules of the RuBP acceptor, to continue the cycle. Two molecules of G3P are converted into one glucose. G3P may also be used to synthesize other carbohydrates, amino acids, and lipids.
Suggested Reading
Michelet, Laure, Mirko Zaffagnini, Samuel Morisse, Francesca Sparla, María Esther Pérez-Pérez, Francesco Francia, Antoine Danon, et al. “Redox Regulation of the Calvin–Benson Cycle: Something Old, Something New.” Frontiers in Plant Science 4 (2013). [Source]
Sharkey, Thomas D., and Sean E. Weise. “The Glucose 6-Phosphate Shunt around the Calvin–Benson Cycle.” Journal of Experimental Botany 67, no. 14 (July 1, 2016): 4067–77. [Source]