Which molecule must be regenerated to continue CO2 fixation in the Calvin Cycle?

Which molecule must be regenerated to continue CO2 fixation in the Calvin Cycle?

• A. G3P
• B. ATP
• C. NADPH
• D. RUBP (Ribulose-1,5-bisphosphate)

Answer: (D)

In the Calvin cycle, carbon fixation relies on a five-carbon sugar called RuBP. The enzyme Rubisco fixes CO2 by attaching it to RuBP, forming a six-carbon intermediate that splits into two molecules of 3-PGA. To keep fixing CO2 in subsequent rounds, RuBP must be regenerated from the products of the cycle. The energy from ATP and the reducing power of NADPH drive the conversion of 3-PGA into glyceraldehyde-3-phosphate, and most of that G3P is then used to rebuild RuBP so a new CO2 fixation step can occur. If RuBP isn’t regenerated, there’s no CO2 acceptor for Rubisco and CO2 fixation stops, even though the other molecules are being energized and reduced. So the molecule that must be regenerated to continue CO2 fixation is RuBP (Ribulose-1,5-bisphosphate).

In the Calvin cycle, carbon fixation relies on a five-carbon sugar called RuBP. The enzyme Rubisco fixes CO2 by attaching it to RuBP, forming a six-carbon intermediate that splits into two molecules of 3-PGA. To keep fixing CO2 in subsequent rounds, RuBP must be regenerated from the products of the cycle. The energy from ATP and the reducing power of NADPH drive the conversion of 3-PGA into glyceraldehyde-3-phosphate, and most of that G3P is then used to rebuild RuBP so a new CO2 fixation step can occur. If RuBP isn’t regenerated, there’s no CO2 acceptor for Rubisco and CO2 fixation stops, even though the other molecules are being energized and reduced. So the molecule that must be regenerated to continue CO2 fixation is RuBP (Ribulose-1,5-bisphosphate).