Anaerobic Respiration and Respiratory Substrates

When oxygen is unavailable or in short supply, cells must use anaerobic pathways to continue producing ATP. This lesson covers the two main types of anaerobic respiration (in animals and in yeast/plants) and the use of different respiratory substrates. These topics are required for the Edexcel A-Level Biology (9BI0) specification.

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Why Anaerobic Respiration is Necessary

In the absence of oxygen:

• The electron transport chain (ETC) cannot function (no final electron acceptor).
• Reduced NAD and reduced FAD cannot be re-oxidised.
• NAD⁺ and FAD are not regenerated.
• Without NAD⁺, the oxidation reactions in glycolysis, the link reaction and the Krebs cycle cannot proceed.

The solution: anaerobic pathways that regenerate NAD⁺ from reduced NAD without using the ETC, allowing glycolysis to continue producing a small amount of ATP.

Exam Tip: The purpose of anaerobic respiration is NOT primarily to produce ATP — it is to regenerate NAD⁺ so that glycolysis can continue to produce ATP by substrate-level phosphorylation.

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Anaerobic Respiration in Animals: Lactate Fermentation

In mammalian muscle cells during vigorous exercise, oxygen supply cannot meet demand. The anaerobic pathway used is: