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Oxaloacetate forms together with acetyl-CoA citric acid. Now oxaloacetate is used but also re-used again in this cycle. But where does the first oxaloacetate comes from? Is that from the mother or produced by DNA or....?

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  • $\begingroup$ Applicable for all biological cyclic reactions; and a molecular egg-or-chicken situation. As all cells respire continuously, even when it wasn't born (it was its precessor-cells) , so we can't find any 'starting-time' or 'starting-moment'. Rather we could think-out how the OAA for world's first TCA-cycle, came?. You've pointed out a right paradox. $\endgroup$ Sep 21, 2016 at 15:52
  • $\begingroup$ However, from another-view, there is no first-last in such a cyclic-reaction. We consider citric acid is the first-molecule because we've learned it that-way. But in some-other interpretation of reaction-networks; some-other compound is the first and some-another compound is the last. It highly depends upon how we view it. $\endgroup$ Sep 21, 2016 at 15:54

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There's not just one molecule of acetyl-CoA that's going through the TCA cycle. Every intermediate compound is present at a certain concentration, so there will be many oxaloacetate molecules per cell.

When the cell divides, it's split in two and the contents are also divided. So a new cell will also have many oxaloacetate molecules in it. This way the daughter cell can just keep running.

For the "first" oxaloacetate molecule we have to go back billions of years, and this would be a very complex question to answer. My guess would be that the first oxaloacetate molecule was produced by a non-TCA pathway, and the TCA cycle evolved later.

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  • $\begingroup$ Great answer. I can't think this question being answered in any better way. $\endgroup$
    – Tyto alba
    Sep 21, 2016 at 10:44
  • $\begingroup$ So each daughter cell would have less oxaloacetate? Less cycle input power? $\endgroup$
    – Nemexia
    Apr 23, 2018 at 6:50

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