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Scientists have calculated that the complete oxidation of a standard amount of glucose releases 686 kcal. The production of a standard amount of ATP from ADP absorbs a minimum of about 7 kcal, depending on the conditions inside the cell. Recall that two ATP molecules are produced from every glucose molecule that is broken down by glycolysis.

Efficiency of glycolysis

_Energyrequiredto makeATP_

Energyreleased byoxidationof glucose

You can see that the two ATP molecules produced during gly-colysis receive only a small percentage of the energy that could be released by the complete oxidation of each molecule of glucose. Much of the energy originally contained in glucose is still held in pyruvic acid. Even if pyruvic acid is converted into lactic acid or ethyl alcohol, no additional ATP is synthesized. It's clear that gly-colysis alone or as part of fermentation is not very efficient in transferring energy from glucose to ATP.

Organisms probably evolved to use glycolysis very early in the history of life on Earth. The first organisms were bacteria, and they produced all of their ATP through glycolysis. It took more than a billion years for the first photosynthetic organisms to appear. The oxygen they released as a byproduct of photosynthesis may have stimulated the evolution of organisms that make most of their ATP through aerobic respiration.

By themselves, the anaerobic pathways provide enough energy for many present-day organisms. However, most of these organisms are unicellular, and those that are multicellular are very small. All of them have limited energy requirements. Larger organisms have much greater energy requirements that cannot be satisfied by glycolysis alone. These larger organisms meet their energy requirements with the more efficient pathways of aerobic respiration.

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