Calvin Cycle
Now that we have stored light from the sun in ATP and NADPH, we can now use them to make sugars using the Calvin Cycle.
Phase 1: Carbon fixation: Rubisco catalyzes the reaction of 3 molecules of carbon dioxide joining to 3 molecules of RuBP. This forms 3 short-lived intermediates. They quickly decompose into 6 molecules of 3-PGA. For simplicity's sake, the diagrams below divide all quantities by a factor of 3.
Phase 1: Carbon fixation: Rubisco catalyzes the reaction of 3 molecules of carbon dioxide joining to 3 molecules of RuBP. This forms 3 short-lived intermediates. They quickly decompose into 6 molecules of 3-PGA. For simplicity's sake, the diagrams below divide all quantities by a factor of 3.
Phase 2: Reduction: 6 molecules of ATP and 6 molecules of NADPH are added (in that order) to change the 6 molecules of 3-PGA to 6 molecules of G3P.
Phase 3: Regeneration of RuBP: 5 of the 6 G3P molecules are converted to 3 molecules of RuBP through the addition of 3 ATP molecules. This makes sense because there are 3 carbon atoms in each G3P, so 5 of them would have 15 carbons. On the other hand, each RuBP has 5 carbons, and since 3 of them are made, the total number of carbon atoms at the end would still be 15. The other G3P molecule is left over as the net gain. The 3 extra carbons from the carbon dioxide which were put into the reaction are accounted for by the net gain of one G3P, which will be used to form glucose later.
Overall Result
What is the overall result of the Calvin Cycle?
The price: 3 molecules of carbon dioxide, 9 molecules of ATP, and 6 molecules of NADPH
The gain: 1 molecule of G3P
The byproducts: 9 molecules of ADP, 8 molecules of inorganic phosphate, and 6 molecules of NADP+
Since ATP and NADPH are consumed in a 3:2 ratio, now you see why we need cyclic electron flow to produce extra ATP in the light reactions. Additionally, the G3P molecule that we gain will be converted to glucose or other carbohydrates, which are the starting materials for cellular respiration.
The price: 3 molecules of carbon dioxide, 9 molecules of ATP, and 6 molecules of NADPH
The gain: 1 molecule of G3P
The byproducts: 9 molecules of ADP, 8 molecules of inorganic phosphate, and 6 molecules of NADP+
Since ATP and NADPH are consumed in a 3:2 ratio, now you see why we need cyclic electron flow to produce extra ATP in the light reactions. Additionally, the G3P molecule that we gain will be converted to glucose or other carbohydrates, which are the starting materials for cellular respiration.