Which pathway is the primary carbon dioxide fixation pathway in photoautotrophs and chemolithotrophs?

• A. Glycolysis
• B. Krebs cycle
• C. Calvin Cycle
• D. Electron transport chain

Answer: (C)

CO2 fixation in photoautotrophs and chemolithotrophs is accomplished mainly by the Calvin cycle. This pathway fixes inorganic carbon by combining CO2 with a five-carbon sugar (RuBP) via the enzyme RuBisCO to form 3-phosphoglycerate. The cycle then uses ATP and NADPH produced by light reactions to convert this carbon into glyceraldehyde-3-phosphate, which can be used to synthesize sugars like glucose. The RuBP is regenerated to continue the cycle, allowing ongoing fixation of CO2.

Glycolysis, by contrast, is a sugar-b breaking pathway that harvests energy from glucose rather than fixing carbon. The Krebs cycle mainly oxidizes acetyl-CoA and releases CO2, not fix CO2. The electron transport chain transfers electrons to generate ATP and does not fix carbon. Therefore, the Calvin cycle uniquely serves as the primary CO2 fixation mechanism in these organisms.