Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
The Citric Acid Cycle
The oxidation of glucose to CO2 and H2O releases significantly more energy than glycolysis does.

Above (Supplement 15-1), we noted that The breakdown of glucose to lactate via Glycolysis releases only a very small fraction of the chemical energy that can potentially be extracted from a glucose molecule. Much more energy is recovered when a glucose molecule is completely oxidized to СО₂ and Н₂О, as can be seen by comparing the standard free-energy changes for these two reactions:

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Fig. 16-2. Successive stages in the oxidation of methane to СО₂. As the four hydrogen atoms are removed from the carbon one by one, The amount of available Free energy drops stepwise. In simple Organic compounds, The ratio of carbon-bound hydrogen atoms to carbon atoms is roughly proportional to the Standard Free Energy of oxidation of that compound to СО₂.

When glucose is broken down anaerobically in Cells, the end product, lactate, retains approximately 93% of the energy originally contained in the glucose molecule. This is because lactic acid is a compound almost as complex as glucose, and also because no true oxidation takes place during glycolysis. The amount of free energy released upon the complete combustion of any organic compound is roughly proportional to the ratio of carbon-bound hydrogen atoms to the total number of carbon atoms, as illustrated by the simple example in Fig. 16-2. All biologically available free energy is released from glucose or other organic fuels only when all hydrogen atoms bonded to the carbon atoms of the molecule are removed and replaced by oxygen to form СО₂.