Following the citric acid cycle but before the electron transport chain and oxidative phosphorylation, most of the energy from the original glucose molecule is found in

mitochondrion

Explanation:

Overall, the process produces the 2 pyruvate plus 2 molecules of water, 2 ATP, 2 molecules of NADH, and 2 hydrogen ions (H+). The NADH carries electrons to the oxidative phosphorylation step of cellular respiration, which occurs inside of the mitochondrion