In kreb cycle, pyruvic acid is break down in to carbon dioxide. This produces 2 ATP and 6 NADH , for every glucose molecule entering glycolysis. In electron transport chain most of the energy ( 34 ATP molecules, compared to only 2 ATP for glycolysis and 2 ATP for Krebs cycle). The electron transport chain takes place in the mitochondria. This stage converts the NADH into ATP. Glucose ( 6 carbon atoms) is split into 2 molecules of pyruvic acid (3 carbons each) in glycolysis. This produces 2 ATP and 2 NADH.