Fermentation

Fermentation is a form of anaerobic respiration in which glucose is incompletely oxidised to release a small amount of energy in the absence of oxygen. It commonly occurs in many prokaryotes, yeast, and in muscle cells of animals during oxygen deficiency.

Core Idea: Fermentation is not mainly for ATP production; its primary purpose is to regenerate NAD⁺ so that glycolysis can continue.

Key Features of Fermentation

  • Uses glycolysis (EMP pathway) as the common initial pathway
  • Occurs entirely in the cytoplasm
  • Does not involve mitochondria
  • Oxygen is not required
  • Very low energy yield

Why is NAD⁺ Regeneration Important?

During glycolysis, NAD⁺ is reduced to NADH. If NAD⁺ is not regenerated:

  • Glycolysis will stop
  • ATP production will completely halt under anaerobic conditions

Fermentation reactions re-oxidise NADH back to NAD⁺, allowing glycolysis to continue and produce 2 ATP per glucose.

Types of Fermentation

There are two main types of fermentation:

1. Alcoholic Fermentation

Occurs in: Yeast (Saccharomyces cerevisiae) and some plant tissues

End Products: Ethanol + CO₂

Steps:

  1. Decarboxylation:
  • Pyruvic acid (3C) → Acetaldehyde (2C)
  • Enzyme: Pyruvic acid decarboxylase
  • CO₂ released
  1. Reduction:
  • Acetaldehyde → Ethanol
  • Enzyme: Alcohol dehydrogenase
  • NADH → NAD⁺ (regenerated)

Important Point: Yeast cells die when alcohol concentration reaches ~13%, explaining limits of natural fermentation.

Lactic Acid Fermentation

Occurs in:

  • Certain bacteria (e.g., Lactobacillus)
  • Animal muscle cells during strenuous exercise

End Product: Lactic acid

Steps:

  • Pyruvic acid (3C) is directly reduced to lactic acid (3C)
  • Enzyme: Lactate dehydrogenase
  • NADH → NAD⁺ regenerated
  • No CO₂ released

Physiological Note: Accumulation of lactic acid in muscles causes muscle fatigue and cramps.

ATP Yield in Fermentation

  • No ATP is produced after glycolysis
  • Net ATP gain = 2 ATP per glucose (from glycolysis only)
  • Less than 7% of glucose energy is released

Hence, fermentation is energetically inefficient compared to aerobic respiration.

Significance of Fermentation

  • Allows organisms to survive without oxygen
  • Maintains ATP production via glycolysis
  • Widely used in industry:
  • Alcohol (beer, wine)
  • Bread making (CO₂ release)
  • Curd and yogurt (lactic acid)

Important Points

  • Fermentation occurs without mitochondria
  • CO₂ is not released in lactic acid fermentation
  • Energy yield is very low compared to aerobic respiration

Memory Capsules – Fermentation Quick Recall

  • Anaerobic process (no O₂)
  • Occurs in cytoplasm
  • Only ATP source → Glycolysis (2 ATP)
  • Main aim → Regenerate NAD⁺

Alcoholic Fermentation:

  • Pyruvate → Acetaldehyde → Ethanol
  • CO₂ released
  • Organism: Yeast

Lactic Acid Fermentation:

  • Pyruvate → Lactic acid
  • No CO₂
  • Occurs in muscles & bacteria

Exam Tip:

→ Fermentation ≠ ATP production after glycolysis

→ Fermentation = NAD⁺ regeneration

💡 Questions and Answers

Q1. What is fermentation?

A1: Fermentation is an anaerobic process in which glucose is partially broken down to release a small amount of energy. It mainly helps in regenerating NAD⁺ so that glycolysis can continue in the absence of oxygen.

Q2. Why is fermentation necessary even though it produces very little ATP?

A2: Because fermentation regenerates NAD⁺ from NADH. Without NAD⁺, glycolysis would stop and no ATP would be produced under anaerobic conditions.

Q3. Differentiate between alcoholic and lactic acid fermentation.

A3:

  • Alcoholic fermentation produces ethanol and CO₂ and occurs in yeast.
  • Lactic acid fermentation produces lactic acid only, without CO₂, and occurs in muscles and bacteria.

Q4. What is the net ATP gain during fermentation of one glucose molecule?

A4: The net gain is 2 ATP molecules, which are produced only during glycolysis. No ATP is formed during the conversion of pyruvate to ethanol or lactic acid.

Q5. Why do muscle cells switch to lactic acid fermentation during heavy exercise?

A5: During heavy exercise, oxygen supply becomes insufficient. Muscle cells then use lactic acid fermentation to regenerate NAD⁺ and continue glycolysis for ATP production.