Many chemical and biological reactions occur inside and outside the human body continuously. Some of them are spontaneous and some are non-spontaneous. Spontaneous reactions are called as exergonic reactions whereas non spontaneous reactions are called as endergonic reactions.

Endergonic reactions

There are many reactions in nature that can occur only when sufficient energy from the surroundings is supplied. On its own these reactions cannot occur as they require high amount of energy to break the chemical bonds. External energy helps to break these bonds. The energy released from breaking of bonds then keeps the reaction going. At times the energy released during breaking of chemical bonds is too less to sustain the reaction. In such cases external energy is required to keep the reaction going.  Such reactions are called as endergonic reactions.

In chemical thermodynamics these reactions are also called as unfavourable or non-spontaneous reactions. The Gibbs free energy is positive under constant temperature and pressure which means that more energy is absorbed rather than released.

Examples of endergonic reactions include protein synthesis, Sodium – potassium pump on the cell membrane, nerve conduction and muscle contraction. Protein synthesis is an anabolic reaction which requires small amino acid molecules to come together to form a protein molecule. It involves lot of energy to form the peptide bonds. Sodium potassium pump on the cell membrane is concerned with pumping out of sodium ions and movement of potassium ions against the concentration gradient to allow cell depolarisation and nerve conduction. This movement against concentration gradient requires a lot of energy which comes from the breakdown of Adenosine tri phosphate molecule (ATP). Similarly muscle contraction can occur only when existing bonds between actin and myosin fibres (muscle proteins) break to form new bonds. This also requires a tremendous amount of energy that comes from ATP breakdown. It is for this reason that ATP is known as universal energy molecule. Photosynthesis in plants is another example of endergonic reaction. The leaf has water and glucose, yet it cannot generate its own food unless it gets sunlight. Sunlight is the external source of energy in this case.

For a sustained endothermic reaction to occur, the products the reaction must get eliminated through a subsequent exergonic reaction so the product concentration remains low always. Another example is melting of ice which requires latent heat to reach the melting point. The process of reaching to the level of activation energy barrier of the transition state is endergonic. Once the transition stage is reached the reaction can proceed to produce more stable products.

Exergonic reactions

These reactions are irreversible reactions which occur spontaneously in nature. By spontaneous it means ready or eager to happen with very little external stimuli. Example is combustion of sodium when exposed to oxygen present in the atmosphere. Burning of a log is another example of exergonic reactions. Such reactions liberate more heat and are called as favourable reactions in the field of chemical thermodynamics. The Gibbs free energy is negative under constant temperature and pressure which means that more energy is released rather than absorbed. These are irreversible reactions.

Cellular respiration is a classic example of exergonic reaction. Around 3012 kJ of energy is released when one molecule of glucose is converted to carbon dioxide. This eneegy is utilised by the organisms for other cellular activities. All catabolic reactions i.e. break down of large molecule into smaller molecules is an exergonic reaction. For example – carbohydrate, fat and protein breakdown released energy for the living organisms to do work.

Some exergonic reactions do not occur spontaneously and require a small input of energy to start the reaction. This input of energy is called activation energy. Once the activation energy requirement is fulfilled by an outside source, the reaction proceeds to break bonds and form new bonds and energy is released as the reaction takes place. This results in a net gain in energy in the surrounding system and a net loss in energy from the reaction system.

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