Respiratory process that opposes carbon fixation Which process in cellular respiration is essentially the reverse, in redox terms, of carbon fixation that occurs in the Calvin cycle of photosynthesis?

Introduction / Context:
Carbon fixation in photosynthesis reduces carbon dioxide into organic molecules using ATP and NADPH. The opposing respiratory logic is the oxidative decarboxylation of organic carbon back to carbon dioxide, releasing energy that cells capture in ATP and reduced cofactors. Identifying the respiratory step that most directly undoes fixation is key to understanding global carbon flux.

Given Data / Assumptions:

• The Calvin cycle fixes CO2 into carbohydrate.
• The citric acid cycle oxidizes acetyl units to CO2.
• Oxidative phosphorylation captures energy from electron transfer but does not decarboxylate carbon.

Concept / Approach:

The citric acid cycle (TCA cycle) performs multiple oxidative decarboxylations, converting acetyl CoA carbon atoms to CO2 while generating NADH and FADH2. This is the redox opposite of the Calvin cycle, which reduces CO2 using NADPH. Glycolysis partially oxidizes glucose but does not fully return carbon to CO2 under aerobic conditions; oxidative phosphorylation deals with electrons rather than carbon; fermentation pathways are anaerobic and do not perform full oxidation to CO2.

Step-by-Step Solution:

Verification / Alternative check:

Tracer studies with labeled carbon show loss of label as CO2 during the TCA cycle, confirming its role in reversing carbon fixation in redox terms.

Why Other Options Are Wrong:

Glycolysis does not fully oxidize carbon to CO2. Oxidative phosphorylation uses electron flow to synthesize ATP without decarboxylation. Fermentation routes regenerate NAD+ and release little or no CO2 depending on the pathway, not the bulk of cellular CO2.

Common Pitfalls:

Equating electron transport with carbon oxidation, or assuming glycolysis alone accounts for CO2 production.

Final Answer:

Citric acid cycle