Final answer:
The Calvin cycle begins with the 5-carbon molecule ribulose bisphosphate (RuBP), which is fixed with carbon dioxide by the enzyme RuBisCO to produce 3-PGA. The cycle then generates G3P, which is used to create glucose and regenerate RuBP with the aid of ATP and NADPH. The answer is A) RuBP.
Step-by-step explanation:
The molecule required to start the Calvin cycle is ribulose bisphosphate (RuBP), which is a 5-carbon molecule. During the Calvin cycle, carbon dioxide is fixed to RuBP by the enzyme ribulose bisphosphate carboxylase-oxygenase, commonly known as RuBisCO. This process yields two molecules of 3-phosphoglycerate (3-PGA), which are then reduced to glyceraldehyde-3-phosphate (G3P). Some of the G3P molecules go on to form carbohydrates like glucose, but the majority are used to regenerate RuBP, thus perpetuating the cycle. The regeneration of RuBP requires energy which comes from ATP and NADPH produced by the light-dependent reactions of photosynthesis. Therefore, the correct answer to the question is A) RuBP.