Which pathway is primarily responsible for carbon dioxide fixation in photoautotrophs and chemolithotrophs?

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

Answer: (A)

Carbon dioxide fixation in autotrophs is carried out primarily by the Calvin cycle. This pathway uses ATP and NADPH produced by light reactions to convert inorganic CO2 into organic sugars, starting with the carboxylation of ribulose-1,5-bisphosphate by the enzyme Rubisco to form 3-phosphoglycerate, which is then processed through a series of steps to yield glyceraldehyde-3-phosphate and ultimately sugars and biomass. This is the main route by which photoautotrophs (plants, algae) fix CO2, and many chemolithoautotrophs rely on the same cycle for incorporating inorganic carbon.

Glycolysis is a breakdown pathway that converts glucose into smaller molecules for energy and biosynthesis, not a mechanism for fixing CO2. The electron transport chain focuses on transferring electrons to generate ATP via oxidative phosphorylation, without incorporating CO2. The Krebs cycle oxidizes acetyl-CoA to CO2 and generates energy and reducing equivalents, but it does not fix inorganic carbon into organic matter.