Q.34 The ΔG°′ for homolactic fermentation converting glucose to lactate is −196 kJ mol⁻¹. If ΔG°′ for the formation of ATP is +30.5 kJ mol⁻¹, then
(A) homolactic fermentation is 31% energy efficient
(B) the efficiency of energy conservation is 69%
(C) the energy stored in the form of ATP is 31%
(D) the process results in the loss of 31% of energy
The correct answer is (C) the energy stored in the form of ATP is 31%.
Homolactic fermentation converts one glucose molecule to two lactate molecules, releasing 196 kJ/mol of free energy (ΔG°′ = −196 kJ/mol). In this process, glycolysis yields a net of 2 ATP molecules per glucose.
Energy Calculation
Total energy from fermentation: 196 kJ/mol (available as |ΔG°′|).
Energy for 2 ATP formations: 2 × 30.5 kJ/mol = 61 kJ/mol.
Percentage stored in ATP: (61 / 196) × 100 ≈ 31%.
Option Analysis
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(A) homolactic fermentation is 31% energy efficient: Correct in describing ATP energy capture as 31% efficient, but less precise than (C).
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(B) the efficiency of energy conservation is 69%: Incorrect; this reflects lost energy (196 − 61 = 135 kJ/mol, or 69%), not conserved energy.
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(C) the energy stored in the form of ATP is 31%: Correct; exactly matches (61/196) × 100 = 31%.
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(D) the process results in the loss of 31% of energy: Incorrect; loss is 69% (135/196).
Introduction to Homolactic Fermentation Energy Efficiency
Homolactic fermentation energy efficiency is key for CSIR NET Life Sciences, where glucose converts to lactate with ΔG°′ = −196 kJ/mol, yielding 2 ATP at +30.5 kJ/mol each. This process captures only a fraction of available energy, making precise calculations essential for exam success.
Detailed ΔG°′ Calculation
In homolactic fermentation, glycolysis nets 2 ATP while releasing 196 kJ/mol total energy. ATP synthesis requires 61 kJ/mol (2 × 30.5), so efficiency = (61/196) × 100 = 31% stored in ATP. The remaining 69% dissipates as heat.
Why 31% Energy Stored in ATP?
This matches option (C), as energy conservation focuses on usable ATP form. Unlike aerobic respiration (∼32 ATP, higher efficiency), fermentation prioritizes NAD⁺ regeneration over maximum yield.
Common Exam Mistakes
Students confuse efficiency (31% conserved) with loss (69%), picking (B) or (D). Always use |ΔG°′| for total available energy in fermentation ATP yield questions.
