Tuesday, October 11, 2005

On reversibility of reactions and life

Irreversible reactions are those that either liberate energy as they happen or require energy in order to happen (this depends on the amount of Gibbs free energy difference of substrates and products).

An organism would try and do most of its work by using reversible reactions because these not only do not consume energy but also will not waste energy that could be used for more important tasks. In fact most of the irreversible energy freeing reactions happen in the glycolytic and cytric acid cycle where the free energy can readily be stored in the form of nucleotide triphosphates and other well known carriers of life. The reactions that are irreversible and consume energy are coupled to reactions that are irreversible and free energy (ATP dephosphorilation into ADP) and happen in situations where they are needed for the construction of polimers required for the cell's survival (DNA,RNA,etc.) or for the early steps of pathways that will later return the energy consumed (glycolysis for instance).

An important point to keep in mind however is that the control exerced by the organism in within the irreversible reactions and if all the reactions were reversible the organism would not be able to decrease its entropy and increase its environment's entropy and it could be no more complex than any thermodinamically closed system.


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