Fermantation
Fermentation is cellular respiration without oxygen which is the final electron acceptor in Oxidative Phosphorylation. In the absence of oxygen glycolysis can still produce some ATP. Cells can regenerate some NAD+ by taking electrons from two NADH to form pyruvate and two NAD+. This process will the create two ATP and one pyruvate. The process has create energy and pyruvate but it has also created the byproduct of alcohol or lactate.