Anaerobic Respiration

When oxygen is unavailable or in limited supply, cells cannot carry out oxidative phosphorylation. However, glycolysis can still proceed provided that NAD⁺ is regenerated from the NADH produced during glycolysis. Anaerobic respiration is the set of metabolic pathways that allow this regeneration to occur without oxygen, enabling a small but vital amount of ATP to be produced.

Key Definition: Anaerobic respiration is the partial oxidation of glucose to release energy in the form of ATP without the use of oxygen as the terminal electron acceptor.

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Why Anaerobic Respiration is Necessary

During glycolysis, NAD⁺ is reduced to NADH when triose phosphate is oxidised. If NADH cannot be reoxidised (because the electron transport chain requires oxygen), the supply of NAD⁺ in the cell would quickly be exhausted and glycolysis would stop.

Anaerobic pathways solve this problem by using the NADH from glycolysis to reduce pyruvate (or a derivative of pyruvate), thereby regenerating NAD⁺ and allowing glycolysis to continue producing ATP.

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Lactate Fermentation (in Animals and Some Bacteria)

Process

• Occurs in the cytoplasm of animal cells (e.g., muscle cells during intense exercise) and some bacteria (e.g., Lactobacillus).
• Pyruvate (3C) is reduced to lactate (3C) by the enzyme lactate dehydrogenase.
• NADH + H⁺ is the hydrogen donor — it is oxidised back to NAD⁺ in the process.

Summary Equation

Pyruvate + NADH + H⁺ → Lactate + NAD⁺

Overall Equation from Glucose

Glucose → 2 Lactate + 2 ATP

Key Features

• Lactate fermentation is reversible — when oxygen becomes available again, lactate can be converted back to pyruvate.
• The lactate produced is transported in the blood to the liver, where it is either:
  • Oxidised back to pyruvate (which enters the Krebs cycle), or
  • Converted back to glucose via gluconeogenesis (the Cori cycle).
• The accumulation of lactate lowers pH, which can inhibit enzyme activity and contribute to muscle fatigue.
• No carbon dioxide is produced during lactate fermentation.

Exam Tip: Lactate fermentation does NOT produce CO₂. This distinguishes it from ethanol fermentation. In lactate fermentation, the carbon skeleton of pyruvate remains intact.

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Ethanol Fermentation (in Yeast and Some Plant Cells)

Process

• Occurs in the cytoplasm of yeast cells and some plant cells under anaerobic conditions.
• This is a two-step process:

1. Decarboxylation — Pyruvate (3C) is decarboxylated to form ethanal (acetaldehyde, 2C) and CO₂. This is catalysed by the enzyme pyruvate decarboxylase.

2. Reduction of ethanal — Ethanal (2C) is reduced to ethanol (2C) by the enzyme alcohol dehydrogenase. NADH + H⁺ is the hydrogen donor, and it is oxidised back to NAD⁺.

Summary Equations

Step 1: Pyruvate → Ethanal + CO₂ Step 2: Ethanal + NADH + H⁺ → Ethanol + NAD⁺

Overall Equation from Glucose

Glucose → 2 Ethanol + 2 CO₂ + 2 ATP

Key Features

• Ethanol fermentation is irreversible in yeast — ethanol cannot be converted back to pyruvate.
• CO₂ is produced (unlike lactate fermentation).
• Ethanol is toxic to yeast cells at high concentrations (typically above ~14-15%), which limits fermentation.
• This process is exploited in brewing (beer and wine production) and baking (CO₂ causes bread to rise).

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Comparison of Aerobic and Anaerobic Respiration