Which statement about CO2 fixation in the Calvin cycle is correct?

Which statement about CO2 fixation in the Calvin cycle is correct?

• A. CO2 is fixed by Rubisco to RuBP to form 3-PGA
• B. CO2 is reduced to methane
• C. CO2 is fixed by RuBP to form glucose
• D. CO2 is released as a waste product

Answer: (A)

In the Calvin cycle, carbon dioxide is captured in the stroma by the enzyme Rubisco, which adds CO2 to RuBP, a five-carbon sugar. This creates a six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate. This carboxylation step is the essential start of incorporating inorganic carbon into organic molecules. From there, 3-PGA is phosphorylated and reduced to glyceraldehyde-3-phosphate, and later these triose phosphates are used to make glucose after several more steps. This explains why the fixation step is described this way. CO2 is not reduced to methane in plants, glucose isn’t formed directly in a single fixation event, and CO2 isn’t released as a waste product during this fixation step (though CO2 can be released in other processes like photorespiration under certain conditions).

In the Calvin cycle, carbon dioxide is captured in the stroma by the enzyme Rubisco, which adds CO2 to RuBP, a five-carbon sugar. This creates a six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate. This carboxylation step is the essential start of incorporating inorganic carbon into organic molecules. From there, 3-PGA is phosphorylated and reduced to glyceraldehyde-3-phosphate, and later these triose phosphates are used to make glucose after several more steps. This explains why the fixation step is described this way. CO2 is not reduced to methane in plants, glucose isn’t formed directly in a single fixation event, and CO2 isn’t released as a waste product during this fixation step (though CO2 can be released in other processes like photorespiration under certain conditions).