Anaerobic Respiration (AQA A-Level Biology): Revision Notes

Anaerobic Respiration

Anaerobic respiration occurs when oxygen becomes temporarily or permanently unavailable to tissues or entire organisms. Understanding this process is essential because it allows organisms to continue producing ATP when aerobic respiration cannot function effectively.

What happens when oxygen is unavailable

When oxygen is absent, both the Krebs cycle and electron transfer chain stop functioning because they require oxygen as the final electron acceptor. This creates a major problem: reduced NAD accumulates rapidly and cannot be reoxidised back to NAD.

Without available NAD to accept hydrogen atoms from glycolysis, this vital process would halt completely. Since glycolysis provides the pyruvate needed for energy production, organisms must find alternative ways to regenerate NAD.

Two main types of anaerobic respiration

In eukaryotic organisms, anaerobic respiration follows two distinct pathways depending on the organism type:

1. Plants and microorganisms (such as yeast): Convert pyruvate to ethanol and carbon dioxide
2. Animals: Convert pyruvate to lactate

Ethanol production in plants and microorganisms

This pathway occurs in organisms like bacteria, fungi (especially yeast), and plant cells under waterlogged conditions. The process involves pyruvate losing a carbon dioxide molecule and accepting hydrogen from reduced NAD.

Chemical equation: $\text{pyruvate} + \text{reduced NAD} \rightarrow \text{ethanol} + \text{carbon dioxide} + \text{oxidised NAD}$

Lactate production in animals

Animal cells convert pyruvate directly to lactate without releasing carbon dioxide. This pathway becomes particularly important during intense physical activity when oxygen demand exceeds supply.

Chemical equation: $\text{pyruvate} + \text{reduced NAD} \rightarrow \text{lactate} + \text{oxidised NAD}$

Lactate fermentation occurs most commonly in muscle cells during strenuous exercise. When muscles work intensively, they consume oxygen faster than the circulatory system can deliver it, creating an oxygen debt.

However, lactate accumulation causes several problems: it increases acidity (lowers pH), affects enzyme function, and contributes to muscle fatigue and cramping.

The body eventually removes lactate by transporting it via the bloodstream to the liver, where it can be converted back to pyruvate or transformed into glycogen when oxygen becomes available again.

NAD regeneration - the key to continued energy production

The critical function of both anaerobic pathways is NAD regeneration. Without this process, the small pool of NAD molecules in cells would become entirely converted to reduced NAD, bringing glycolysis to a complete halt.

By providing alternative electron acceptors (pyruvate converted to ethanol or lactate), these pathways ensure that:

• Glycolysis can continue operating
• ATP production continues, albeit at reduced levels
• Cells maintain essential metabolic functions during oxygen shortage

Energy yields from anaerobic respiration

Anaerobic respiration produces significantly less ATP than aerobic respiration. The energy yield comes entirely from substrate-level phosphorylation during glycolysis, as neither the Krebs cycle nor electron transfer chain can operate without oxygen.

Energy comparison:

• Anaerobic respiration: Only 2 ATP molecules per glucose (from glycolysis alone)
• Aerobic respiration: Approximately 32-38 ATP molecules per glucose (from glycolysis, Krebs cycle, and electron transfer chain combined)

The reduced energy yield also explains why intense exercise cannot be sustained indefinitely - muscles operating anaerobically produce insufficient ATP for prolonged activity and accumulate lactate that impairs function.