Answer :
Final answer:
The first step of the Calvin cycle is the combination of carbon dioxide with RuBP to produce PGA. This step is catalyzed by the enzyme RuBisCO and is the initial act of carbon fixation in the photosynthetic process of converting atmospheric carbon dioxide into glucose.
Explanation:
The first step of the Calvin-Benson cycle, also known simply as the Calvin cycle, involves the enzyme RuBisCO fixing carbon dioxide to a five-carbon molecule called ribulose biphosphate (RuBP). This reaction produces two molecules of 3-phosphoglycerate (PGA). Therefore, the correct first step of the Calvin-Benson cycle is: Carbon dioxide and RuBP combine to make PGA.
The Calvin cycle is a process of photosynthesis where ATP and NADPH, produced in the light-dependent reactions, provide the energy to convert carbon dioxide from the atmosphere into organic compounds like glucose. Throughout the cycle, the energy is used to regenerate RuBP, allowing the cycle to prepare for more carbon fixation and continue the process of photosynthesis.