Question

Although O2 does not participate directly in TCA cycle, the cycle operates only when O2 IS present, why?

Answer 1

As a result 9 ATP are generated by electron transport-oxidative phosphorylation per turn of the cycle. Molecular oxygen does not participate in the citric acid cycle. However, the cycle operates only under aerobic conditions because NAD+ and FAD cannot be regenerated in the absence of oxygen.

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Answer 2

Although oxygen does not participate in TCA cycle, it operates only when it is present. It is because in TCA cycle, NAD+ acts as co-enzyme.  

Explanation:

It must be derived from NADH in TCA cycle which takes place in electron transport chain with the help or in presence of oxygen. NAD+ is a co-protein and goes about as an electron transporter in oxidizing responses at different positions in Krebs cycle.