Anaerobic Respiration

When oxygen is scarce or absent, the electron transport chain cannot operate and oxidative phosphorylation stops. Cells must then rely on anaerobic respiration — pathways that regenerate NAD without oxygen and allow glycolysis to continue producing a small amount of ATP. OCR specification module 5.2.2(f) requires you to describe the two main anaerobic pathways — lactate fermentation in mammals and ethanol fermentation in plants and yeast — and to understand why the ATP yield is so much lower than in aerobic respiration.

Key Definitions:

• Anaerobic respiration — respiration that does not require oxygen; relies on glycolysis plus a fermentation reaction that regenerates NAD.
• Lactate fermentation — the reduction of pyruvate to lactate by lactate dehydrogenase, found in mammalian muscle and red blood cells.
• Ethanol fermentation — the decarboxylation of pyruvate to ethanal, then reduction to ethanol, found in yeast and plant root cells under flooding.
• Oxygen debt (EPOC) — the extra oxygen required after exercise to metabolise accumulated lactate.
• NAD regeneration — the key function of any fermentation pathway.

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Why Fermentation Exists

Glycolysis produces 2 reduced NAD per glucose. Under aerobic conditions, these are re-oxidised to NAD by the electron transport chain. But if the ETC is not running (no oxygen), reduced NAD accumulates and the pool of NAD gets used up. Without NAD, triose phosphate dehydrogenase (in phase 3 of glycolysis) can no longer operate, and glycolysis stops.

The whole purpose of fermentation pathways is to regenerate NAD from reduced NAD, so that glycolysis can continue. Fermentation is not about making more energy — it is about keeping glycolysis going. The small amount of ATP produced comes from glycolysis itself, not from the fermentation step.

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Mammalian Lactate Fermentation

flowchart LR
    GLU[Glucose] -->|Glycolysis| PYR[2 Pyruvate]
    GLU -->|Glycolysis| RNAD[2 Reduced NAD]
    RNAD -->|Reduces pyruvate| LAC[Lactate]
    PYR --> LAC
    LAC --> NAD[NAD regenerated]
    NAD --> GLU

The Reaction

Pyruvate is reduced to lactate by the enzyme lactate dehydrogenase (LDH), using reduced NAD as the hydrogen donor:

$\text{Pyruvate} + \text{reduced NAD} \xrightarrow{\text{LDH}} \text{Lactate} + \text{NAD}$Pyruvate+reduced NADLDH​Lactate+NAD

Key Features

• No CO₂ is released.
• No ATP is made in this step.
• The regenerated NAD returns to glycolysis, allowing further ATP production (2 ATP per glucose).
• Net yield: only 2 ATP per glucose (vs ~32 aerobically).

When Does Mammalian Lactate Fermentation Occur?

• Strenuous exercise — when muscle oxygen demand exceeds supply (sprinting, weightlifting).
• Red blood cells — have no mitochondria at all, so they rely on lactate fermentation permanently.
• Tissues with poor blood supply — such as the lens of the eye.
• Certain anaerobic bacteria in the gut.

Consequences

• Lactate accumulation makes muscles acidic (lower pH), which is painful and contributes to fatigue.
• Lactic acidosis: in extreme cases (cardiac arrest, shock), lactate accumulation can be life-threatening.
• Oxygen debt (also called excess post-exercise oxygen consumption, EPOC): after exercise, extra oxygen is needed to deal with the accumulated lactate. The lactate is transported in the blood to the liver, where it is converted back to pyruvate (using NAD, so ATP is available) and then either oxidised to CO₂ and H₂O or converted back to glucose via gluconeogenesis.
• Panting after exercise is the body repaying the oxygen debt.

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Plant and Yeast Ethanol Fermentation

In plants (e.g. waterlogged roots) and yeast (under anaerobic conditions), pyruvate is converted to ethanol by a different pathway:

flowchart LR
    PYR[Pyruvate] -->|Decarboxylation| ETH[Ethanal + CO2]
    ETH -->|Reduction by reduced NAD| ETOH[Ethanol]
    RNAD[Reduced NAD] --> ETH
    ETOH --> NAD[NAD regenerated]

The Two Steps

1. Decarboxylation: pyruvate loses a CO₂ to become ethanal (acetaldehyde, 2C). This is catalysed by pyruvate decarboxylase.
2. Reduction: ethanal is reduced to ethanol by reduced NAD, catalysed by alcohol dehydrogenase. NAD is regenerated.

Key Features

• CO₂ is produced (unlike in lactate fermentation).
• Ethanol is produced — a 2-carbon alcohol.
• Reaction is irreversible — ethanol cannot be converted back to pyruvate in these organisms. This means plants and yeast cannot "repay an oxygen debt" in the same way mammals do.
• Net ATP yield: only 2 ATP per glucose, the same as lactate fermentation.

When Does Ethanol Fermentation Occur?

• Yeast (e.g. Saccharomyces cerevisiae) when oxygen is limited — the basis of brewing and baking.
• Plant roots in waterlogged soil — when oxygen cannot diffuse through the flooded soil.
• Anaerobic sediment bacteria — not usually on the OCR spec, but biologically widespread.

Commercial Applications