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

Respiration in Anaerobic Conditions

Objective: Outline the pathways of anaerobic respiration in mammals (lactate fermentation) and in yeast cells (ethanol fermentation), highlighting the biochemical steps, energy yield and physiological significance.

1. Why Anaerobic Fermentation Occurs

When oxygen supply is insufficient to meet the demand for oxidative phosphorylation, cells resort to fermentation to regenerate NAD⁺ from NADH, allowing glycolysis to continue and produce a small amount of ATP.

2. Lactate Fermentation in Mammals

Typical in skeletal muscle during intense exercise.

2.1 Overall Reaction

\$\text{Glucose} + 2\,\text{ADP} + 2\,\text{P}i \;\longrightarrow\; 2\,\text{Lactate} + 2\,\text{ATP} + 2\,\text{H}2\text{O}\$

2.2 Step‑by‑Step Pathway

1. Glycolysis converts one molecule of glucose to two molecules of pyruvate, producing a net gain of 2 ATP and 2 NADH.
2. In the absence of oxygen, pyruvate is reduced to lactate by lactate dehydrogenase (LDH):

   • Reaction: \$ \text{Pyruvate} + \text{NADH} + \text{H}^+ \rightarrow \text{Lactate} + \text{NAD}^+ \$
   • This regenerates NAD⁺ for glycolysis.

3. Lactate accumulates in the muscle and is transported to the liver (Cori cycle) where it can be converted back to glucose.

2.3 Energy Yield

• ATP from glycolysis: 2 ATP per glucose.
• No additional ATP from the fermentation step.
• Total yield: 2 ATP per glucose (compared with \overline{30} ATP in aerobic respiration).

2.4 Physiological Notes

• Lactate accumulation contributes to muscle fatigue and acidity.
• Rapid regeneration of NAD⁺ allows continued ATP production during short, high‑intensity bursts.

3. Ethanol Fermentation in Yeast

Commonly used in brewing, baking and bio‑fuel production.

3.1 Overall Reaction

\$\text{Glucose} + 2\,\text{ADP} + 2\,\text{P}i \;\longrightarrow\; 2\,\text{Ethanol} + 2\,\text{CO}2 + 2\,\text{ATP}\$

3.2 Step‑by‑Step Pathway

1. Glycolysis: glucose → 2 pyruvate + 2 ATP + 2 NADH.
2. Decarboxylation of pyruvate (pyruvate decarboxylase):

   • Reaction: \$ \text{Pyruvate} \rightarrow \text{Acetaldehyde} + \text{CO}_2 \$

3. Reduction of acetaldehyde to ethanol (alcohol dehydrogenase):

   • Reaction: \$ \text{Acetaldehyde} + \text{NADH} + \text{H}^+ \rightarrow \text{Ethanol} + \text{NAD}^+ \$
   • Regenerates NAD⁺ for glycolysis.

3.3 Energy Yield

• ATP from glycolysis: 2 ATP per glucose.
• No ATP is produced in the fermentation steps.
• Total yield: 2 ATP per glucose.

3.4 Industrial Relevance

• Ethanol is harvested as a bio‑fuel or alcoholic beverage.
• CO₂ is a valuable by‑product in baking (leavening) and carbonation.

4. Comparison of Lactate vs. Ethanol Fermentation

5. Summary

Both lactate and ethanol fermentations are anaerobic pathways that enable cells to continue glycolysis when oxygen is scarce. They differ in the final organic products, the enzymes involved, and their biological or industrial significance, but each yields only 2 ATP per glucose, highlighting the energetic advantage of aerobic respiration.