What happens in the absence of oxygen during respiration?

In the absence of oxygen, cells switch from aerobic respiration to anaerobic respiration, where glucose is partially broken down to release a small amount of energy.

This process occurs in the cytoplasm and does not involve the Krebs cycle or electron transport chain.

What happens step-by-step:

1. Glycolysis continues
Glucose is broken down into pyruvate, producing:

• 2 ATP (net gain)

• 2 NADH

2. Regeneration of NAD⁺
Since oxygen is not available to accept electrons, NADH must be converted back to NAD⁺ for glycolysis to continue.

3. Conversion of pyruvate (fermentation):

• In human muscles (lactic acid fermentation):
  Pyruvate → Lactic acid

• In yeast and some microorganisms (alcoholic fermentation):
  Pyruvate → Ethanol + CO₂

Consequences of anaerobic respiration:

• Low energy yield: Only 2 ATP per glucose

• Incomplete breakdown of glucose

• Formation of by-products: Lactic acid or alcohol

Effects in humans:

• Lactic acid accumulation causes muscle fatigue and cramps during intense exercise

• Oxygen debt occurs, requiring extra oxygen later to remove lactic acid

In simple terms: Without oxygen, cells produce less energy and form by-products like lactic acid or alcohol instead of fully breaking down glucose.