• Bioenergetics or biochemical thermodynamics deal with the study of energy changes (transfer and utilization) in biochemical reactions.
  • The reactions are broadly classified as exergonic (energy releasing) and endergonic (energy consuming).

Free energy

  • The energy actually available to do work (utilizable) is known as free energy [G].
  • The energy change as the system moves from its initial state to equilibrium, with no change in temperature and pressure is known as free energy change [∆G].
  • Standard free energy is the free energy change when the reactants and products are at a concentration of 1 mol/L at pH 7.0. It is denoted by the sign ∆Go.

Exergonic reactions

  • Each compound involved in a chemical reaction contains a certain amount of potential energy, related to the kind and numbers of its bonds.
  • In reaction, that occur spontaneously the products have less free energy than the reactants, thus the reaction releases free energy, which is then available to do work. Such reactions are called exergonic reactions.
  • The decline in free energy from reactants to products is expressed as a negative value.
  • Catabolic reactions are exergonic in nature.

Endergonic reactions

  • Endergonic reactions on the other hand require an input of energy, and therefore the value of ∆G is positive.
  • Anabolic reactions are endergonic in nature.



Endergonic processes processed by coupling to exergonic processes.

  • The vital processes i.e synthetic reactions, muscular contraction, nerve impulse contraction and active transport obtain energy by chemical linkage or coupling to oxidative reactions.
  • The conversion of metabolite A to metabolite B occurs with the release of free energy. It is coupled to another reaction in which the free energy is required to convert metabolite C to metabolite D.


  • As some of the energy liberated in the degradative reaction is transferred to the synthetic reaction in a form other than heat.
  • Example:

Metabolism is a highly coordinated cellular activity in which many multienzyme system (metabolic pthways) co-operate to accomplish four functions:

  • Obtain chemical energy by capturing solar energy or degrading energy rich nutrients from the environment.
  • Convert nutrient molecules into the cells own characteristic molecules including precursors of macromolecules.
  • Polymerize monomeric precursors into macromolecules: proteins, nucleic acids, and polysaccharides.
  • Synthesize and degrade biomolecules required in specialized cellular functions, such as membrane lipids, intracellular messengers and pigments.

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