Anaerobic Respiration
Anaerobic respiration is a form of cellular energy production that occurs without molecular oxygen, using alternative inorganic electron acceptors (such as nitrate, sulphate, or fumarate in prokaryotes) or organic molecules as terminal electron acceptors. In a stricter biochemical sense used in prokaryotic microbiology it is distinguished from fermentation, which involves no electron transport chain at all; however, in human physiology the term "anaerobic respiration" is often used interchangeably with lactic acid fermentation to describe the lactate-producing pathway that activates during intense exercise when oxygen delivery is insufficient. Anaerobic pathways yield only 2 ATP per glucose but produce ATP far more rapidly than aerobic pathways, making them critical for explosive physical efforts and allowing survival in anoxic environments.
Key Formula
C6H12O6 → 2 C3H6O3 (lactic acid) + 2 ATP
LaTeX: C_6H_{12}O_6 \xrightarrow{\text{no O}_2} 2C_3H_6O_3 + 2\text{ ATP}
| Symbol | Meaning | Unit |
|---|---|---|
| C₆H₁₂O₆ | Glucose (substrate) | mol |
| C₃H₆O₃ | Lactic acid (lactate) product | mol |
| ATP | Adenosine triphosphate (net yield) | molecules |
Worked Example
Problem
During a 100-m sprint, a sprinter's leg muscles consume 0.05 mol of glucose via lactic acid fermentation over 10 seconds. How much lactic acid is produced and how many moles of ATP are generated?
Solution
Step 1: C₆H₁₂O₆ → 2 C₃H₆O₃ + 2 ATP. Step 2: Lactic acid produced = 0.05 mol glucose × 2 = 0.10 mol lactic acid. Step 3: ATP generated = 0.05 mol glucose × 2 = 0.10 mol ATP. Step 4: Rate of ATP = 0.10 mol ÷ 10 s = 0.01 mol ATP/s.
Answer
0.10 mol lactic acid and 0.10 mol ATP are produced; ATP production rate = 0.01 mol/s.
Anaerobic Respiration in Different Organisms
| Organism | Electron Acceptor | End Product | ATP Yield |
|---|---|---|---|
| Human muscle cell | Pyruvate (organic) | Lactate | 2 per glucose |
| Yeast | Acetaldehyde (organic) | Ethanol + CO₂ | 2 per glucose |
| Desulfovibrio (bacterium) | Sulphate (SO₄²⁻) | Hydrogen sulphide (H₂S) | ~3 per glucose |
| Nitrosomonas (bacterium) | Nitrate (NO₃⁻) | Nitrogen gas (N₂) | ~4 per glucose |
| Fusobacterium | Fumarate | Succinate | ~3 per glucose |
Interactive Tools
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Related Terms
Aerobic Respiration
Aerobic respiration is the form of cellular respiration that requires molecular oxygen (O₂) as the final electron acceptor in the electron transport chain, enabling the complete oxidation of glucose to carbon dioxide and water with maximum ATP yield (~30–32 ATP per glucose). It proceeds through glycolysis, the link reaction (pyruvate decarboxylation), the citric acid cycle, and oxidative phosphorylation, all of which are tightly coupled within the mitochondrion. Aerobic respiration is the predominant energy-yielding pathway in all eukaryotes and many prokaryotes under oxygen-sufficient conditions, underpinning sustained muscular activity, brain function, and virtually every energy-demanding cellular process.
Fermentation (biology)
Fermentation is an anaerobic metabolic process in which cells oxidise organic substrates — most commonly glucose — without using oxygen as the terminal electron acceptor, instead using organic molecules to regenerate NAD⁺ from NADH so that glycolysis can continue. The two most widespread forms are lactic acid fermentation (found in muscle cells and many bacteria, producing lactate) and alcoholic fermentation (carried out by yeast and some bacteria, producing ethanol and CO₂). Although fermentation is far less energy-efficient than aerobic respiration (yielding only 2 ATP per glucose), it enables rapid energy production when oxygen is limiting and is exploited industrially in bread-making, brewing, and the production of biofuels and pharmaceuticals.
ATP (Adenosine Triphosphate)
Adenosine triphosphate (ATP) is the primary energy currency of all living cells, consisting of an adenine base, a ribose sugar, and three phosphate groups linked by high-energy phosphoanhydride bonds. When the terminal phosphate group is hydrolysed by ATPase enzymes to yield ADP (adenosine diphosphate) and inorganic phosphate (Pᵢ), approximately 30.5 kJ/mol of free energy is released under standard conditions (and up to ~54 kJ/mol under physiological conditions), which drives endergonic cellular processes including muscle contraction, active transport, biosynthesis, and signal transduction. A typical human cell turns over its own body weight in ATP every day, with mitochondrial oxidative phosphorylation producing the vast majority of this ATP.
From Greek "an-" (without) + "aer" (air) + "bios" (life) + Latin "respirare" (to breathe). The term "anaerobic" was introduced by Louis Pasteur in 1861 when he discovered that certain microorganisms could grow in the complete absence of oxygen. The distinction between anaerobic respiration (with an inorganic electron acceptor) and fermentation (without an ETC) was later refined by microbiologists in the 20th century.