Photosynthesis involves the transfer and harvesting of energy from sunlight and the fixation of carbon dioxide into carbohydrates. This process occurs in higher plants, including Arabidopsis thaliana. Oxygenic photosynthesis requires water, which acts as an electron donor molecule. The reactions which involve the trapping of sunlight are known as "light reactions", and result in the production of NADPH, adenosine triphosphate, and molecular oxygen. The "dark reactions" are known as the Calvin cycle, and involve the use of the products of the light reactions to fix carbon dioxide and produce carbohydrates. The light-independent Calvin-Benson cycle consist of nine reactions that take place in the chloroplast stroma. Beginning with the enzyme RuBisCO, D-ribulose-1,5-bisphosphate is converted into 3-phosphoglyceric acid. It requires magnesium ion as a cofactor. Next, chloroplastic glyceraldehyde 3-phosphate dehydrogenase catalyzes the conversion of glyceric acid 1,3-biphosphate into D-glyceraldehyde 3-phosphate. Then triose-phosphate isomerase catalyzes the conversion of D-glyceraldehyde 3-phosphate into dihydroxyacetone phosphate. Next, the enzyme fructose-bisphosphate aldolase catalyzes the conversion of dihydroxyacetone phosphate into fructose 1,6-bisphosphate. Then fructose-1,6-bisphosphatase catalyzes the conversion of fructose 1,6-bisphosphate into fructose-6-phosphate. It requires magnesium ion as a cofactor. Next, transketolase catalyzes the conversion of fructose-6-phosphate into xylulose 5-phosphate. It requires a divalent metal cation and thiamine diphosphate as cofactors. Then the enzyme ribulose-phosphate 3-epimerase is catalyzes the interconverson of xylulose 5-phosphate and D-ribulose 5-phosphate. Lastly, phosphoribulokinase catalyzes the conversion of D-ribulose 5-phosphate to regenerate D-ribulose-1,5-bisphosphate. An alternative pathway intersects the Calvin-Benson cycle providing another route to synthesize D-ribulose 5-phosphate and D-xylulose 5-phosphate, which both feed back into the main cycle, from dihydroxyacetone phosphate. This subpathway begins with the predicted enzyme sedoheptulose-1,7-bisphosphate aldolase theorized to catalyze the converson of glycerone phosphate and D-erythrose 4-phosphate into sedoheptulose-1,7-bisphosphate. Next, sedoheptulose-1,7-bisphosphatase catalyzes the conversion of sedoheptulose-1,7-bisphosphate into D-sedoheptulose 7-phosphate. Next, transketolase catalyzes the converson of D-sedoheptulose 7-phosphate into D-ribose 5-phosphate and D-xylulose 5-phosphate (which feeds back into the main cycle). Lastly, ribose-5-phosphate isomerase is the probable enzyme that catalyzes the interconverson of D-ribose 5-phosphate and D-ribulose 5-phosphate. D-ribulose 5-phosphate feeds back into the main cycle.

Calvin-Benson Cycle References