Metabolism Thermodynamics

By Dr Ananya Mandal, MD

Metabolic processes are chemical reactions and these often involve generation of heat. Living organisms, however, do not follow all the laws of thermodynamics. Organisms are open systems that exchange matter and energy with their surroundings. This means that living systems are not in equilibrium, but instead are dissipative systems that maintain their state of high complexity.

Cellular metabolism couples the spontaneous processes of catabolism with the non-spontaneous processes of anabolism. In thermodynamic terms, metabolism maintains the balance.

Laws of Thermodynamics and metabolism

Catabolism, in total, releases energy. Anabolism requires energy. The laws of thermodynamics apply to metabolism.

What is thermodynamics?

Thermodynamics is the study of energy transformations as applied to all physicochemical systems including biological. Bioenergetics is the study of energy conversion in biological systems.

First law of thermodynamics

The first law of thermodynamics is the Law of conservation of energy. For biological systems, this describes the transfer of energy from one form to another.

Second law of thermodynamics

The second law of thermodynamics is that Entropy must increase if a reaction is to be spontaneous. A process can only occur spontaneously if the sum of the entropies of the system and its surroundings increases.

However, it is difficult to measure the changes in entropy in a biological system. What can be used is the Gibbs Free Energy of a Reaction at constant pressure and temperature. ∆G is the difference between the energy contained in the products of a reaction and the reactants:

∆G = (energy of products) - (energy of reactants)

For a reaction in equilibrium the direction of an enzyme catalyzed reaction and for that matter a metabolic pathway, depends of the change in free energy.

Reactions occur only when the free energy (Gibbs) change is negative.

The equilibrium constant, Keq is determined as:  Keq = [products]/[reactants]. The standard state in biochemistry is at pH 7 and 1 molar concentration.

Exergenic and endergonic reactions

Chemical reactions are classified as being either exergonic or endergonic. That means that a reaction can either release energy useful for work (an exergonic reaction) or requires energy to proceed (an endergonic reaction). 

Reviewed by , BA Hons (Cantab)

Further Reading

Last Updated: Oct 31, 2012

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