Q: Explain the small yield of ATP from anaerobic respiration

In the absence of oxygen, the conversion of pyruvate to acetyl CoA, the Krebs cycle, and oxidative phosphorylation are prevented as oxygen is not available as the final electron acceptor.
Hence, only glycolysis can occur, generating 2 molecules of ATP by substrate level phosphorylation by breaking down glucose into pyruvate. Glycolysis can occur even in the absence of oxygen as the oxidising agent of glycolysis is NAD, not oxygen. A sufficient supply of NAD is available as the NAD reduced during glycolysis is regenerated by transferring electrons from NADH to pyruvate during the fermentation process.