Reversible reaction
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A reversible reaction is a chemical reaction that may be reversed. In other words, the reactant and product of one reaction may reverse roles, without adding chemicals.
Symbolically:
<math>aA + bB \leftrightarrow cC + dD</math> - products <math>C</math> and <math>D</math> are produced from reactants <math>A</math> and <math>B</math>, but <math>C</math> and <math>D</math> can react to form <math>A</math> and <math>B</math>.
In an irreversible reaction the equilibrium states are shifted so close to either the products or the reactants that the reaction effectively does not have an equilibrium between the products and the reactants. Hence, irreversible reactions can be viewed as an extreme, "special case" of reversible reactions. Irreversible reactions are often called "spontaneous" or "favorable". These reactions are usually entropically driven, as opposed to thermodynamically driven. In an irreversible reaction, there is generally a great increase in entropy.
Metabolic pathways are a series of biochemical reactions which are, as a whole, irreversible. If the biochemical reactions involved in this process were reversible, we would convert our own DNA back to food molecules if we stop eating even for a short period of time.