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Structural Biochemistry/Endergonic reaction

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General information[ edit ]

endergonic reaction

An endergonic reaction refers to a chemical reaction in which energy is being used in the overall reaction, making the reaction non-spontaneous and thermodynamiacally unfavorable. Energy is being absorbed as the reaction proceeds, and there is a net loss of energy in the surrounding system. Due to this consumption of energy, standard change in Gibbs free energy (ΔG) is a positive value under constant pressure and temperature: ΔG° > 0.

The magnitude of ΔG also represents the quantity of energy required to drive the reaction. If a chemical process is exergonic in one direction, then the reverse process must be endergonic. Plants get the required energy to make sugar from the environment by capturing light and converting it into chemical energy that can be used for other processes.

Some examples of endergonic reactions are muscle contractions and protein synthesis.

The equilibrium constant of endergonic reaction where ΔG° > 0 is less than 1: K < 1.

Endergonic reactions require an input of energy, usually larger than those of non spontaneous exergonic reactions , from an outside source to disturb the chemical equilibrium to cause changes, such as bond formation. This input of energy is called the activation energy. In certain reactions, a catalyst is available to speed up endergonic reactions. A catalyst can lower the activation energy barrier for the reaction. Thus, it speeds up the reaction process. The energy for an endergonic reaction is obtained by coupling the reaction with an exergonic reaction.

A familiar example of coupling exergonic reactions to endergonic reactions to promote spontaneity comes from ATP. ATP powers cellular work by coupling exergonic reactions to endergonic reactions. It is responsible for mediating most energy coupling in cells, and in most cases, acts as the immediate source of energy that powers cellular work. The synthesis of the amino acid glutamine from glutamic acid and ammonia is naturally endergonic and non spontaneous with a ΔG value of +3.4 kcal/mol but coupling this reaction with the exergonic process of ATP hydrolysis, -7.3 kcal/mol, will drive the reaction forward, making it spontaneous. In the same sense, a exergonic reaction must be coupled with the formation of ATP from ADP in order to make the reaction spontaneous and in most cases, cellular respiration provides the energy for the endergonic process of making ATP and plants use light energy, instead, to produce ATP.

An endergonic reaction can simply be understood by studying the following situation.
In a chemical reaction, the reactants make products and an equilibrium is reached. An endergonic reaction is one where more products are made from the equilibrium amount by disturbing the equilibrium with a form of energy. For example, heat will be absorbed into the system and the equilibrium will shift to the right (towards product side). Consequently, more products will be formed.

Summary[ edit ]

An endergonic reaction:

  • The free energy of initial state < free energy of final state
  • Energy needs to be put into the system in order to go from the initial start to the final state
  • +ΔG

References[ edit ]

Zumdahl, Chemistry Seventh Edition

Neil A. Campbell, Jan B Reece. Biology Seventh Edition, 2005 Pearson Education, Inc.

http://academic.brooklyn.cuny.edu/biology/bio4fv/page/exergon.htm

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        sciencing

        What Are the Differences Between Exergonic and Endergonic Reactions?

        By Eric Novinson; Updated April 25, 2017

        Burning charcoal is an exergonic reaction.

        Jupiterimages/Polka Dot/Getty Images

        Some chemical reactions consume energy, and others release energy, usually as heat or light. Exergonic reactions include the combustion of gasoline, because a molecule in the gasoline, such as octane, contains more energy than the water and carbon dioxide molecules that are released after burning the gasoline. A tree’s use of photosynthesis to assemble its bark from carbon dioxide and water is endergonic.

        Biological Reactions

        Endergonic reactions are frequently found in biological organisms, because the organism needs to assemble complex molecules such as fats and amino acids, according to Johnson County Community College. Although these reactions use up energy, the organism has the ability to use other types of molecules, such as sugars, as fuel. Endergonic reactions can never occur without a power source.

        Activation Energy

        Exergonic reactions usually still require some energy to start, even though the reaction will release energy once it is complete. This extra energy is the activation energy , which a molecule temporarily stores before releasing the activation energy and some additional energy. Charcoal requires a source of energy, such as a match, before it ignites, even though the charcoal releases much more energy once it starts burning.

        Sciencing Video Vault

        Reversible Reaction

        An endergonic reaction is also known as a reversible reaction. Burning a log reverses the reaction that was used to produce the log, breaking the carbohydrates in the log apart and releasing carbon and water, with the addition of a small amount of heat. It’s more difficult to reverse the exergonic reaction, burning the log, because the tree needs to collect much more energy from the sun to assemble the log. According to the University of Nebraska, Lincoln, reversibility depends on how much additional energy it would take to perform the reverse reaction, not whether the reverse reaction is possible.

        Energy Hill Diagram

        An energy hill diagram provides a visual display that shows whether a reaction is exergonic or endergonic. The diagram includes two axes, time at the bottom and the total energy of the chemical solution on the side. For an exergonic reaction, the amount of energy rises until the solution has enough activation energy, and then it falls. For an exergonic reaction, once the solution has enough activation energy, it may either continue to rise, or drop to a lower level that is still higher than the initial energy of the original molecules.

        References

        • University of Nebraska, Lincoln: Enzymes and Chemical Reactions
        • University of Arizona: Endergonic Reaction
        • Johnson County Community College: Energy Hill Diagram

        About the Author

        Eric Novinson has written articles on Daily Kos, his own blog and various other websites since 2006. He holds a Bachelor of Science in business administration from Humboldt State University.

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        Novinson, Eric. “What Are the Differences Between Exergonic and Endergonic Reactions?” Sciencing, https://sciencing.com/differences-between-exergonic-endergonic-reactions-2591.html. 25 April 2017.
        Novinson, Eric. (2017, April 25). What Are the Differences Between Exergonic and Endergonic Reactions? Sciencing. Retrieved from https://sciencing.com/differences-between-exergonic-endergonic-reactions-2591.html
        Novinson, Eric. “What Are the Differences Between Exergonic and Endergonic Reactions?” last modified April 25, 2017. https://sciencing.com/differences-between-exergonic-endergonic-reactions-2591.html

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