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.
Key Formula
C6H12O6 → 2 C2H5OH + 2 CO2 + 2 ATP (alcoholic fermentation)
LaTeX: C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + 2\text{ ATP}
| Symbol | Meaning | Unit |
|---|---|---|
| C₆H₁₂O₆ | Glucose (substrate) | mol |
| C₂H₅OH | Ethanol (product) | mol |
| CO₂ | Carbon dioxide (by-product) | mol |
| ATP | Adenosine triphosphate (energy yield) | molecules |
Worked Example
Problem
A yeast culture ferments 1 mol of glucose completely via alcoholic fermentation. How many moles of ethanol and CO₂ are produced, and how many ATP molecules are generated?
Solution
Step 1: The balanced equation is C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂. Step 2: 1 mol glucose → 2 mol ethanol and 2 mol CO₂. Step 3: ATP yield = 2 mol ATP per mol glucose (from glycolysis only). Step 4: In molecules: 2 × 6.022 × 10²³ = 1.204 × 10²⁴ ATP molecules.
Answer
2 mol ethanol, 2 mol CO₂, and 2 mol (≈ 1.204 × 10²⁴ molecules) of ATP are produced.
Lactic Acid vs Alcoholic Fermentation
| Feature | Lactic Acid Fermentation | Alcoholic Fermentation |
|---|---|---|
| Organisms | Muscle cells, Lactobacillus | Saccharomyces (yeast), some bacteria |
| Products | Lactate, NAD⁺ | Ethanol, CO₂, NAD⁺ |
| ATP yield | 2 per glucose | 2 per glucose |
| Industrial use | Yoghurt, cheese, sauerkraut | Beer, wine, bread, bioethanol |
| Reversibility | Lactate → pyruvate (in liver) | Irreversible under normal conditions |
Interactive Tools
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Related Terms
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.
Cellular Respiration
Cellular respiration is the set of metabolic reactions by which cells break down organic molecules — primarily glucose — in the presence or absence of oxygen to generate ATP, the universal energy currency of life. The complete aerobic pathway yields approximately 30–32 ATP molecules per glucose molecule through three sequential stages: glycolysis (cytoplasm), the citric acid (Krebs) cycle (mitochondrial matrix), and oxidative phosphorylation via the electron transport chain (inner mitochondrial membrane). Understanding cellular respiration is fundamental to nutrition science, exercise physiology, and the treatment of metabolic diseases.
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 Latin "fermentum" meaning "leaven" or "yeast", derived from "fervere" (to boil/bubble), describing the bubbling of CO₂ during yeast fermentation. The word entered scientific use through medieval alchemy. Louis Pasteur demonstrated in 1857 that fermentation is caused by living microorganisms, coining the phrase "la fermentation, c'est la vie sans air" (fermentation is life without air).