Answer :
Final answer:
c. CO2 and RuBP join forming 3PG
The Calvin cycle begins with the carbon fixation of CO2 and RuBP to form 3PG, catalyzed by the enzyme RuBisCO. While the cycle is continuous, this is the recognized starting point. Other steps like the reduction of 3PG to G3P or the regeneration of RuBP are subsequent parts of the cycle.
Explanation:
Which of the following begins the Calvin cycle that results in the entire pathway being carried out under environmental conditions? The correct answer to this is c. CO2 and RuBP join forming 3PG. This step marks the commencement of the Calvin cycle.
During this stage, the enzyme RuBisCO catalyzes the incorporation of carbon dioxide into an organic molecule, 3-phosphoglycerate (3-PGA), which is then reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH.
As the Calvin cycle is indeed cyclic, the other statements reflect subsequent steps: a. 3PG is reduced to G3P using ATP and NADPH + H+ corresponds to the reduction phase, where 3-PGA is converted to G3P. d. G3P is converted into glucose and fructose describes the process by which carbohydrate molecules are produced from G3P. b. RuBP is regenerated refers to the final phase of the cycle where the initial molecule RuBP is reformed, allowing the cycle to continue.
Therefore, the Calvin cycle can logically proceed from any of these points, but the starting point for the pathway under environmental conditions is the carbon fixation of CO2 and RuBP.