Which of the following processes can occur in the absence of oxygen?

Fermentation is a metabolic process that occurs in the absence of oxygen, allowing cells to produce energy anaerobically. In this process, glucose or other carbohydrates are partially broken down to generate ATP (adenosine triphosphate), the energy currency of cells. The end products of fermentation, such as lactic acid or ethanol, differ depending on the organism and conditions but allow for continued energy production when oxygen is not available.

In contrast, oxidative phosphorylation, the Krebs cycle, and the electron transport chain require oxygen to operate efficiently. Oxidative phosphorylation generates ATP using the proton gradient created by the electron transport chain, utilizing oxygen as the terminal electron acceptor. Similarly, the Krebs cycle (also known as the citric acid cycle) operates within aerobic respiration and relies on oxygen to help fully oxidize substrates, producing energy and electron carriers. The electron transport chain itself directly links to oxygen, as it is where the reduction of oxygen occurs, forming water as a byproduct. Thus, only fermentation can sustain energy production in an anaerobic environment.