In Stage 2 (Krebs Cycle) a carbon dioxide molecule is removed from the pyruvate
along with H+ ions. An acetyl group (2C) remains.
This acetyl group is added to a four carbon compound present in the
mitochondrion this produces Citric acid (6C). Citric acid is then broken down
in a number of stages back to the original four carbon compound releasing 2 CO2
molecules, 2 ATP molecules, and numerous H+ions.
CO2 is released as a waste product and the H+ ion
is added to NAD with an electron to form
NADH.