Fermentation
நொதித்தல்
Stop confusing anaerobic respiration with fermentation. Fermentation is incomplete oxidation; it is not merely the absence of oxygen. If you cannot trace the fate of Pyruvic acid, you are failing the basics of cellular metabolism.
The idea
Fermentation is the incomplete oxidation of glucose achieved under anaerobic conditions. Pyruvic acid is converted into CO2 and ethanol in yeast, or lactic acid in certain bacteria and muscle cells. This process yields less than seven percent of the energy released during aerobic respiration. It is the primary metabolic pathway in prokaryotes and unicellular eukaryotes.
Why it matters
It allows organisms to regenerate NAD+ in the absence of oxygen, ensuring that glycolysis can continue to produce ATP.
NEET lens
NEET focuses on the enzymes involved: Pyruvate decarboxylase and Alcohol dehydrogenase. Memorize the products for yeast versus bacteria. Do not confuse the net ATP yield; it is strictly low. Expect a question on the specific conditions that trigger this pathway.
Clinical significance
In humans, strenuous exercise leads to oxygen debt in muscle cells, forcing them to switch to lactic acid fermentation. The accumulation of lactic acid causes muscle fatigue and soreness. This is a physiological adaptation, not a disease state.
Introduction
Fermentation is anaerobic respiration where pyruvate (from glycolysis) is reduced by NADH to regenerate NAD⁺, allowing glycolysis to continue without oxygen. Products are ethanol + CO₂ (alcoholic) or lactate (lactic acid).
Core concept
Alcoholic fermentation (yeast, plants): Pyruvate → acetaldehyde (CO₂ released, enzyme: pyruvate decarboxylase) → ethanol (NADH oxidized → NAD⁺, enzyme: alcohol dehydrogenase). Net: glucose → 2 ethanol + 2 CO₂ + 2 ATP. Lactic acid fermentation (muscle, bacteria): Pyruvate + NADH → lactate + NAD⁺ (enzyme: lactate dehydrogenase). Net: glucose → 2 lactate + 2 ATP. Key point: fermentation regenerates NAD⁺ to allow continued glycolysis — the ethanol/lactate itself is not the goal.
Go deeper
Muscle fatigue: during intense exercise, O₂ delivery cannot keep up with demand → lactic acid fermentation kicks in → lactate accumulates → pH drops → enzyme activity decreases → fatigue. Lactate is not 'waste' — it is recycled to the liver via Cori cycle → converted back to glucose via gluconeogenesis.
From Class 10
When oxygen isn't available, cells can still get some energy from glucose through fermentation — making alcohol or lactic acid instead of going through the full aerobic pathway.