Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 9.5 Fermentation and Anaerobic Respiration Enable Cells to Produce ATP Without the Use of Oxygen
Cells can generate ATP without oxygen by using fermentation or anaerobic respiration. These processes differ from aerobic respiration primarily in the absence of oxygen as the final electron acceptor.
Anaerobic Respiration
Occurs in certain prokaryotes in oxygen-deprived environments
Uses an electron transport chain but with final electron acceptors other than O₂ (e.g., sulfate)
Generates ATP through oxidative phosphorylation using a proton gradient
Produces by-products like H₂S instead of water
Fermentation
Does not involve an electron transport chain
Relies solely on glycolysis for ATP (2 ATP per glucose)
Requires regeneration of NAD⁺ to sustain glycolysis
Electrons from NADH are transferred to pyruvate or derivatives
Types of Fermentation
Alcohol fermentation:
Pyruvate → acetaldehyde → ethanol
(CO₂ released; NAD⁺ regenerated)Lactic acid fermentation:
Pyruvate is directly reduced to lactate
(No CO₂ released; NAD⁺ regenerated)
Facultative and Obligate Anaerobes
Obligate anaerobes: survive only without oxygen
Facultative anaerobes: can switch between fermentation and respiration depending on O₂ availability
In facultative organisms, pyruvate becomes a key branching point between fermentation and respiration
Fermentation vs. Respiration
Both use glycolysis and NAD⁺ as an electron acceptor
Respiration uses an electron transport chain to regenerate NAD⁺; fermentation uses organic molecules
Fermentation yields 2 ATP; respiration yields up to 32 ATP
Fermentation is much less efficient in energy harvest
Special Notes on Lactate in Humans
Some muscle fibers produce lactate even under aerobic conditions
Lactate can be used by other cells or converted back to glucose
Lactate production is not necessarily a sign of oxygen deficiency or muscle fatigue
Evolutionary Perspective
Glycolysis predates oxygen in Earth's atmosphere
Its widespread presence and cytosolic location suggest it is an ancient and conserved pathway
Likely evolved in early prokaryotes and still functions in both fermentation and respiration
In a Nutshell
Fermentation and anaerobic respiration allow ATP production without oxygen. Fermentation uses glycolysis plus reactions that regenerate NAD⁺, while anaerobic respiration uses an electron transport chain with non-oxygen final electron acceptors. Both processes highlight metabolic versatility and evolutionary adaptation.