5.2.8 Anaerobic Respiration

Anaerobic Respiration in Animals

• In animals, pyruvate is converted into lactate (lactic acid) to regenerate $NAD⁺,$ allowing glycolysis to continue.
• This occurs in situations where oxygen is insufficient, such as during vigorous exercise.
• The equation for this process is: $Pyruvate + NADH → Lactate + NAD⁺$
• Lactate builds up in muscles, causing fatigue and cramp. It is transported to the liver where it is converted back into pyruvate or glucose when oxygen becomes available.

Anaerobic Respiration in Plants and Microorganisms

• In plants and microorganisms such as yeast, pyruvate is converted into ethanol and carbon dioxide during anaerobic respiration.
• This process is known as alcoholic fermentation and also regenerates $NAD⁺$.
• The reactions are as follows:
  1. Pyruvate → Ethanal + CO₂ (via decarboxylation by pyruvate decarboxylase).
  2. Ethanal + NADH → Ethanol + $NAD⁺$ (catalysed by ethanol dehydrogenase).
• This process is used in industries such as brewing and bread-making.

ATP Yield in Anaerobic Respiration

• Only 2 ATP molecules are produced per molecule of glucose, compared to 36-38 ATP in aerobic respiration.
• ATP is generated during glycolysis, where glucose is phosphorylated and split into two molecules of pyruvate.

Comparison of Anaerobic Respiration in Animals vs. Plants/Microorganisms

Feature	Animals	Plants/Microorganisms
End Products	Lactate	Ethanol + Carbon Dioxide
Regeneration of $NAD⁺$	Yes	Yes
CO₂ Produced	No	Yes