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.