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Difficulty: MediumGaseous Exchange and Cellular Respiration

During strenuous physical exertion, human skeletal muscle cells experience localized oxygen deficiency and temporarily undergo lactic acid fermentation. What is the net yield of ATP molecules produced per molecule of glucose metabolized in this anaerobic pathway?

  1. A
    4 ATP molecules
  2. 2 ATP moleculesAnswer
  3. C
    36 ATP molecules
  4. D
    38 ATP molecules

Answer

2 ATP molecules
During anaerobic respiration (lactic acid fermentation), glucose undergoes partial oxidation through glycolysis in the cytoplasm. Because 2 ATP molecules are consumed to phosphorylate glucose and 4 ATP molecules are subsequently synthesized, the net gain is precisely 2 ATP molecules per glucose molecule.

Step-by-Step Solution

1
Identify the metabolic pathway described in the scenario.
The scenario describes lactic acid fermentation, which is an anaerobic pathway.
In the absence of sufficient oxygen, cells cannot utilize the electron transport chain or Krebs cycle in mitochondria and rely solely on glycolysis.
2
Calculate the net energy yield of glycolysis during anaerobic respiration.
Glycolysis consumes 2 ATP2\text{ ATP} during phosphorylation and generates 4 ATP4\text{ ATP} via substrate-level phosphorylation, giving a net yield of 4−2=2 ATP4 - 2 = 2\text{ ATP}.
Pyruvate is reduced to lactate to regenerate NAD+\text{NAD}^+ for continuing glycolysis, without yielding any further ATP molecules.

Key Concept

Anaerobic Respiration Net ATP Yield
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Topics

Gaseous Exchange and Cellular Respiration

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