Asked • 07/10/19

What is an irreversible reaction?

There are reactions with large Delta G negative values. Why these reactions are irreversible? As in: out of 10 steps of Glycolysis, 3 are irreversible steps. I need an explanation for why they are irreversible.

Mikie H.

This goes back to equilibrium reaction chemistry. Those 3 reactions have extremely large K values. Because of this, when you perform the calculation dG = -RTlnK, if K is a large value, then dG is a large negative value. This means the free energy change favors the "forward" reaction so strongly, that the reverse reaction is extremely unlikely to occur. Hence, steps 1, 3, and 10 of glycolysis, for example, are termed "irreversible." This makes sense because they use ATP/ADP substrate level phosphorylation to either add to or remove from the sugar intermediates high energy phosphates. When equilibrium constant, K, is much greater than 1, the equilibrium strongly favors the forward reaction. Likewise, by applying these large K values to the free energy relation equation, strong negative free energy change confirms the forward reactions are very spontaneous and way more likely to occur than the reverse reactions.
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07/21/19

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Kush A. answered • 07/29/19

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Irreversible reactions are considered irreversible when they have such large delta G negative values because once the reactions have proceeded spontaneously, they will require the same amount positive free energy (+G), meaning the reverse reaction won't be able to take the same "path" that the forward reaction took place to go back. There will probably have to be an addition of energy via ATP hydrolysis or another outside factor to reverse the reaction. Thus, a spontaneous (-delta G) reaction will not be spontaneously reversible with the same path it took.

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