When oxygen is unavailable, cells still need a way to keep glycolysis running so they can obtain ATP from glucose. This lesson explains how cells regenerate NAD+ without aerobic respiration and compares fermentation with anaerobic cellular respiration.
During glycolysis, NAD+ accepts electrons and hydrogen, becoming NADH. For glycolysis to continue, NADH must be oxidized back to NAD+. In aerobic respiration, NADH transfers its high-energy electrons to the electron transport chain, where oxygen is the final electron acceptor. Without oxygen, that route cannot operate as usual.
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What is fermentation?
Fermentation regenerates NAD+ from NADH by transferring electrons to an organic molecule, without using an electron transport chain.
How does anaerobic cellular respiration differ from fermentation?
Anaerobic cellular respiration uses an electron transport chain with a final electron acceptor other than oxygen. Fermentation transfers electrons to an organic molecule without an electron transport chain.
What does lactate dehydrogenase do?
It catalyzes the reversible conversion between pyruvate and lactate while interconverting NADH and NAD+.
How is ethanol produced during alcohol fermentation?
Pyruvate first loses carbon dioxide to form acetaldehyde. Alcohol dehydrogenase then uses electrons from NADH to reduce acetaldehyde to ethanol, regenerating NAD+.
How do facultative anaerobes differ from obligate anaerobes?
Facultative anaerobes can switch between aerobic respiration and fermentation depending on oxygen availability. Obligate anaerobes grow without molecular oxygen, which may be toxic to them.
Quiz questions
Why must cells regenerate NAD+ when glycolysis continues without aerobic respiration?Answer: Glycolysis requires NAD+ as an electron acceptor, so its regeneration allows glycolysis to continue.
Which description correctly distinguishes fermentation from anaerobic cellular respiration?Answer: Fermentation transfers electrons to an organic molecule without an electron transport chain; anaerobic respiration uses a chain and a non-oxygen acceptor.
During lactic acid fermentation, what happens to NADH as pyruvate becomes lactate?Answer: NADH is oxidized to NAD+.
Which sequence describes alcohol fermentation?Answer: Pyruvate releases carbon dioxide to form acetaldehyde, which is reduced to ethanol as NADH is oxidized.
A bacterium uses oxygen when available but switches to fermentation when oxygen is absent. How is it classified?Answer: A facultative anaerobe.
Based on Biology 2e by Mary Ann Clark, Matthew Douglas, Jung Choi (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.