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In glycolysis, the conversion of glucose to pyruvate results in a net gain of 2 ATP molecules. In gluconeogenesis however, the conversion of pyruvate back to glucose requires 6 ATP molecules. Why does there seem to be an apparent loss of energy?

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up vote 3 down vote accepted

This energy is not lost, it is dissipated as heat.

And this must happen because of thermodynamics -- the direction of the reaction is simply determined by the difference of energy between substrates and products, and it must be positive for the reaction to happen spontaneously.

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