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In respiration, Krebs cycle starts with acetyl coenzyme A which is made from pyruvate.

However, it is said that the cycle keeps repeat it self with oxaloacetate turning back to citrate and cycle starts again, so why is acetyl coenzyme A constantly supplied to the cycle?

According to that logic, shouldn't there be no need for acetyl-coA after the first round?

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Pyruvate transfers an acetyl group to coenzyme A.

The acetyl group from acetyl co-A is the molecule that provides the 2 carbon atoms that get added to oxaloacetate to form citrate, and as you said, citrate starts the cycle over again.

Without coenzyme A, pyruvate cannot transfer the acetyl group to oxaloacetate to form citrate so without coenzyme A/acetyl co-A, the process would stop.

This is a slightly busy version of the diagram, but it does let you see how molecules in the cycle are formed and where carrier molecules interact with the molecules in the cycle.

By Narayanese, WikiUserPedia, YassineMrabet, TotoBaggins [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
- By Narayanese, WikiUserPedia, YassineMrabet, TotoBaggins [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

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