BIOLOGY Volume 1 - A Guide to General Biology - 2004

9. UTILIZATION OF ENERGY

9.3. Cellular Respiration

9.3.7. Energy Conversion Efficiency in Aerobic and Anaerobic Respiration

Aerobic Respiration

During aerobic respiration, 38 ATP molecules are produced for every molecule of glucose oxidized.

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The total amount of energy released during the Complete oxidation of glucose is 2880 kJ per 1 mol.

One mole of ATP contains 30.6 kJ.

38 moles of ATP contain 30.6 x 38 = 1162.8 kJ.

Thus, the energy conversion efficiency in aerobic respiration is: 1162.8/2880 = 40.4%.

Anaerobic respiration

1. Yeast (Alcoholic Fermentation). During alcoholic fermentation, two ATP molecules are produced per glucose molecule.

The total amount of energy released from glucose upon its conversion to ethanol is 210 kJ per 1 mol.

Two moles of ATP contain 2 x 30.6 = 61.2 kJ.

Consequently, the energy conversion efficiency in alcoholic fermentation is 61.2/210 = 29.1%.

2. Muscles (Lactic acid fermentation). During lactic acid fermentation, two ATP molecules are produced per glucose molecule.

The total amount of energy released from glucose upon its conversion to lactic acid is 150 kJ per 1 mol.

Thus, the energy conversion efficiency in lactic acid fermentation is 61.2/150 = 40.8%.

The figures given above show that the energy conversion efficiency in each of these systems is quite high compared to petrol (25–30%) or steam (8–12%) engines. Furthermore, The amount of energy stored as ATP during aerobic respiration is 19 times greater than during anaerobic respiration (38 ATP molecules per glucose molecule in the first case and 2 ATP molecules In the second). From this perspective, aerobic respiration is significantly more efficient than anaerobic respiration. This is because, during anaerobic respiration, a substantial portion of the energy remains "trapped" in ethanol or lactic acid. The energy contained in ethanol remains permanently inaccessible to yeast, making alcoholic fermentation a highly inefficient process in terms of energy yield. However, a considerable amount of energy can later be extracted from lactic acid if oxygen becomes available. In the presence of oxygen, lactic acid is converted into pyruvic acid in the Liver. The latter then enters the Krebs cycle and is fully oxidized to СО₂ and Н₂О, resulting in the additional production of A large number of ATP molecules. Alternatively, utilizing ATP energy, glucose can be regenerated from pyruvic acid through a process that essentially reverses Glycolysis.