Outline respiration in anaerobic conditions in mammals (lactate fermentation) and in yeast cells (ethanol fermentation)

Sourav PanOctober 31, 2024

Answered

In anaerobic conditions, cells can still produce ATP through glycolysis followed by fermentation. Since the electron transport chain cannot operate without oxygen, the cell must regenerate NAD⁺ by other means. Mammals and yeast cells perform different types of fermentation to sustain glycolysis under these conditions:

1. Lactate Fermentation in Mammals

• Organisms: Mammals, particularly muscle cells under intense exercise or oxygen-deficient conditions.
• Process:
  • In the absence of oxygen, pyruvate (the end product of glycolysis) is reduced to lactate by the enzyme lactate dehydrogenase.
  • During this reduction, NADH is oxidized back to NAD⁺, which is essential to keep glycolysis running, allowing ATP to continue being produced.
• Products:
  • The main products are lactate and NAD⁺.
  • Lactate accumulates in the muscle cells, contributing to fatigue and soreness, and can later be transported to the liver to be converted back to glucose in the Cori cycle once oxygen is available.
• ATP Yield: Only 2 ATP molecules per glucose molecule (from glycolysis), as the Krebs cycle and electron transport chain are inactive without oxygen.

2. Ethanol Fermentation in Yeast Cells

• Organisms: Yeast cells and some bacteria.
• Process:
  • In anaerobic conditions, pyruvate from glycolysis is first converted into acetaldehyde and releases CO₂.
  • Acetaldehyde is then reduced to ethanol by the enzyme alcohol dehydrogenase, with NADH being oxidized to regenerate NAD⁺, allowing glycolysis to continue.
• Products:
  • The products are ethanol, CO₂, and NAD⁺.
  • Ethanol and CO₂ are excreted by yeast cells, which is the basis of alcohol production and bread-making (where CO₂ causes the dough to rise).
• ATP Yield: Like lactate fermentation, ethanol fermentation only yields 2 ATP per glucose (from glycolysis), as there is no Krebs cycle or electron transport chain activity.

Summary

• Lactate fermentation (in mammals): Pyruvate → Lactate + NAD⁺
• Ethanol fermentation (in yeast): Pyruvate → CO₂ + Acetaldehyde → Ethanol + NAD⁺

Both processes enable ATP production under anaerobic conditions by regenerating NAD⁺, though they yield significantly less ATP than aerobic respiration.