What is the first step of the Calvin Cycle?

What is the first step of the Calvin Cycle?

• A. Reduction of 3-PG to G3P
• B. Carbon fixation CO2 + RUBP -> 2 3PG
• C. Regeneration of RUBP
• D. Phosphorylation of RuBP

Answer: (B)

The first step in the Calvin Cycle is carbon fixation: carbon dioxide is fixed onto the five-carbon sugar RuBP by the enzyme Rubisco, forming an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate. This step is the entry point of inorganic carbon into the cycle and sets up the subsequent reactions that use ATP and NADPH to convert 3-PGA into glyceraldehyde-3-phosphate and eventually regenerate RuBP. The other steps—the reduction of 3-PGA to G3P, the regeneration of RuBP, and the phosphorylation steps involved in rebuilding RuBP—occur after carbon fixation as the cycle proceeds.

The first step in the Calvin Cycle is carbon fixation: carbon dioxide is fixed onto the five-carbon sugar RuBP by the enzyme Rubisco, forming an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate. This step is the entry point of inorganic carbon into the cycle and sets up the subsequent reactions that use ATP and NADPH to convert 3-PGA into glyceraldehyde-3-phosphate and eventually regenerate RuBP. The other steps—the reduction of 3-PGA to G3P, the regeneration of RuBP, and the phosphorylation steps involved in rebuilding RuBP—occur after carbon fixation as the cycle proceeds.