Acetyl CoA formation, the Krebs cycle, and the respiratory chain follow glycolysis. Acetyl CoA enters the Krebs cycle. This cycle is a series of oxidation-reduction reactions that produces ATP, NADH, FADH2, and CO2. Substrate-level phosphorylation produces ATP in the Krebs cycle. Energy-rich electrons from NADH and FADH2 fuel the respiratory chain.
Electrons move through a series of carriers that release energy at each step.
The terminal electron acceptor, oxygen, is reduced to form water. Electron transport energy establishes a proton gradient that pumps protons from the mitochondrial matrix into the intermembrane compartment. Adding the ATPs produced in substrate-level phosphorylation and oxidative phosphorylation and subtracting those used to enter the mitochondrion predicts a theoretical maximum energy of 36 ATPs per glucose. Negative feedback coordinates the rates of glycolysis, acetyl CoA formation, and the Krebs cycle. In the absence of oxygen, alcoholic, lactic acid, or other fermentation pathways may run.