In the Calvin cycle, which molecule is regenerated to allow continued CO2 fixation?

In the Calvin cycle, which molecule is regenerated to allow continued CO2 fixation?

• A. RuBP
• B. NADP+
• C. Glucose
• D. ADP

Answer: (A)

The main idea is that the cycle must continually rebuild the five‑carbon acceptor RuBP so Rubisco can fix more CO2. CO2 is added to RuBP to form a six‑carbon compound that splits into two molecules of 3‑PGA. Using ATP and NADPH, most of these 3‑PGA molecules are converted back into RuBP, while a portion goes on to form sugars. This regeneration of RuBP is essential because without it there would be no substrate for CO2 fixation and the cycle would stop. NADP+ and ADP are produced when energy carriers are used, and glucose is produced later from the carbon that leaves the cycle, but neither of those is the molecule that is regenerated to keep CO2 fixation going.

The main idea is that the cycle must continually rebuild the five‑carbon acceptor RuBP so Rubisco can fix more CO2. CO2 is added to RuBP to form a six‑carbon compound that splits into two molecules of 3‑PGA. Using ATP and NADPH, most of these 3‑PGA molecules are converted back into RuBP, while a portion goes on to form sugars. This regeneration of RuBP is essential because without it there would be no substrate for CO2 fixation and the cycle would stop. NADP+ and ADP are produced when energy carriers are used, and glucose is produced later from the carbon that leaves the cycle, but neither of those is the molecule that is regenerated to keep CO2 fixation going.