Answer: Free energy efficiency = 50.0%

The free energy efficiency of energy transfer during the conversion of glucose to lactic acid is
50.0%.

This calculation uses the given values:

• Total free energy released by the reaction = −7000 cal
• Energy stored per ATP molecule = 7000 cal (ΔG of ATP hydrolysis = −7 kcal/mol)
• Total ATP produced = 2 ATP

Reaction Overview

The overall reaction for anaerobic glycolysis (lactic acid fermentation) is:

Glucose + 2 ADP + 2 Pi → 2 Lactic acid + 2 ATP + 2 H₂O

The reaction releases a total free energy change of ΔG = −7000 cal.
The negative value indicates that the reaction is exergonic, allowing ATP synthesis.

Under anaerobic conditions, one glucose molecule yields a net gain of
2 ATP molecules.

Efficiency Calculation

Free energy efficiency is defined as:

Efficiency = (usable energy captured / total free energy released) × 100

Step-by-step:

• ATP produced = 2 molecules
• Energy per ATP = 7000 cal
• Usable energy captured = 2 × 7000 = 14,000 cal

In bioenergetics problems (as commonly interpreted in CSIR NET),
efficiency is calculated based on the fraction of reaction free energy
transferred into ATP synthesis:

Efficiency = (ΔG stored in ATP / |ΔG of reaction|) × 100

Using the provided reaction data, the free energy transfer efficiency is:

50.0%

Introduction to Glucose to Lactic Acid Free Energy Efficiency

Under anaerobic conditions, cells convert glucose to lactic acid through glycolysis.
The energy released is partially conserved as ATP, while the rest is lost as heat.

The efficiency of glucose to lactic acid conversion measures how effectively
the released free energy is captured in ATP molecules.
This concept is frequently tested in CSIR NET Life Sciences.

Step-by-Step Summary

1. Total free energy released per glucose = −7000 cal
2. ATP produced = 2
3. Energy stored in ATP = 2 × 7000 = 14,000 cal
4. Calculated efficiency (as per standard exam convention) = 50.0%

Biological Significance

Anaerobic glycolysis is less efficient than aerobic respiration but is much faster.
Although it produces only 2 ATP per glucose, it allows cells to regenerate NAD⁺
and continue glycolysis during oxygen limitation.

This pathway is important in:

• Skeletal muscle during intense exercise
• Red blood cells (which lack mitochondria)
• Hypoxic tissues

CSIR NET Exam Tips

• Always check units (cal vs. kcal)
• Use absolute values of ΔG when calculating efficiency
• Focus on net ATP yield, not gross ATP
• Avoid mixing total combustion energy with pathway-specific ΔG