In the matrix reactions what happens to the original carbons in pyruvate?

In the matrix the carbon atoms of pyruvate are converted to acetyl CoA to enter into the TCA cycle.

Explanation:

Pyruvate is the end product of glycolysis. Pyruvate is is converted to acetyl CoA by the catalytic activity of pyruvate dehydrogenase enzyme.

Acetyl CoA then enters the the TCA cycle to to continue the rest of the steps of aerobic respiration. TCA cycle result in the formation of a NADH, FADH2, CO2 AND H2O.