During anaerobic respiration in plants and yeast, pyruvate loses a molecule of carbon dioxide and accepts hydrogen from reduced NAD.

This produces an alcohol called ethanol and regenerates NAD to enter glycolysis.

The process is sometimes referred to as alcohol fermentation or ethanol fermentation.

Yeast is grown in anaerobic conditions where it ferments natural carbohydrates to produce ethanol, which can be used for wine and beer production.

During anaerobic respiration in animals, pyruvate accepts hydrogen from the reduced NAD produced in glycolysis.

This forms lactate and regenerates NAD to enter glycolysis.

This process is sometimes referred to as lactate fermentation.

Lactate is removed by the blood and taken to the liver to be converted to glycogen when oxygen is once again available.

When more oxygen is available, lactate can be oxidised to regenerate pyruvate.

The reduced quantity of ATP produced is insufficient to maintain vital processes for extended time periods.

Lactate (lactic acid) accumulates, causing cramp and muscle fatigue, and it also reduces the pH affecting enzymes.

Anaerobic respiration is less efficient than aerobic respiration and produces less ATP.

Cells can produce a small yield of ATP through glycolysis if the reduced NAD that is produced can be oxidised again.

This can be achieved through ethanol fermentation or lactate (or lactic acid) fermentation.

Without oxygen, only glycolysis can be used to generate ATP.

All the FAD and NAD will be reduced, so they cannot accept the protons and electrons released during the Krebs cycle and link reaction. The electron transfer chain stops and no further ATP is formed by oxidative phosphorylation without oxygen as the final electron acceptor.