exergonic reaction

Examples of exergonic reaction in the following topics:

• Activation Energy

  • Activation energy must be considered when analyzing both endergonic and exergonic reactions.
  • Exergonic reactions have a net release of energy, but they still require a small amount of energy input before they can proceed with their energy-releasing steps.
  • Cells will at times couple an exergonic reaction $(\Delta G<0)$ with endergonic reactions $(\Delta G>0)$, allowing them to proceed.
  • The free energy released from the exergonic reaction is absorbed by the endergonic reaction.
  • Although the image above discusses the concept of activation energy within the context of the exergonic forward reaction, the same principles apply to the reverse reaction, which must be endergonic.

• Free Energy

  • Reactions that have a negative ∆G and, consequently, release free energy, are called exergonic reactions.
  • Exergonic means energy is exiting the system.
  • On the other hand, the catabolic process of breaking sugar down into simpler molecules releases energy in a series of exergonic reactions.
  • Exergonic and endergonic reactions result in changes in Gibbs free energy.
  • Exergonic reactions release energy; endergonic reactions require energy to proceed.

• ATP: Adenosine Triphosphate

  • Cells couple the exergonic reaction of ATP hydrolysis with endergonic reactions to harness the energy within the bonds of ATP.
  • ATP provides the energy for both energy-consuming endergonic reactions and energy-releasing exergonic reactions, which require a small input of activation energy.
  • Cells couple the exergonic reaction of ATP hydrolysis with the endergonic reactions of cellular processes.
  • In this example, the exergonic reaction of ATP hydrolysis is coupled with the endergonic reaction of converting glucose for use in the metabolic pathway.
  • Sodium-potassium pumps use the energy derived from exergonic ATP hydrolysis to pump sodium and potassium ions across the cell membrane.

• The Second Law of Thermodynamics

  • Likewise, some energy is lost in the form of heat during cellular metabolic reactions.
  • Entropy changes also occur in chemical reactions.
  • In an exergonic chemical reaction where energy is released, entropy increases because the final products have less energy inside them holding their chemical bonds together.
  • As living systems take in energy-storing molecules and transform them through chemical reactions, they lose some amount of usable energy in the process because no reaction is completely efficient.

• Spontaneous and Nonspontaneous Processes

  • There are two types of processes (or reactions): spontaneous and non-spontaneous.
  • Spontaneity does not imply that the reaction proceeds with great speed.
  • The rate of a reaction is independent of its spontaneity, and instead depends on the chemical kinetics of the reaction.
  • An endergonic reaction (also called a nonspontaneous reaction or an unfavorable reaction) is a chemical reaction in which the standard change in free energy is positive, and energy is absorbed.
  • Endergonic reactions can also be pushed by coupling them to another reaction, which is strongly exergonic, through a shared intermediate.Saul Steinberg from The New Yorker illustrates a nonspontaneous process here.

• Citric Acid Cycle

  • The citric acid cycle is a series of reactions that produces two carbon dioxide molecules, one GTP/ATP, and reduced forms of NADH and FADH2.
  • The eight steps of the cycle are a series of redox, dehydration, hydration, and decarboxylation reactions that produce two carbon dioxide molecules, one GTP/ATP, and reduced forms of NADH and FADH2 .
  • This step is irreversible because it is highly exergonic.
  • The rate of this reaction is controlled by negative feedback and the amount of ATP available.
  • If ATP levels increase, the rate of this reaction decreases.

• ATP in Metabolism

  • In this way, ATP is a direct link between the limited set of exergonic pathways of glucose catabolism and the multitude of endergonic pathways that power living cells.
  • The energy from ATP can also be used to drive chemical reactions by coupling ATP hydrolysis with another reaction process in an enzyme.
  • During an endergonic chemical reaction, ATP forms an intermediate complex with the substrate and enzyme in the reaction.
  • This is illustrated by the following generic reaction:
  • In phosphorylation reactions, the gamma phosphate of ATP is attached to a protein.

• Solvent Effects

  • Reactions which involve the formation of charged atoms and molecules are usually extremely endothermic in the gas phase, but may become spontaneous in certain solvents.
  • These different entropy changes are incorporated in the free energy of solution, which is exergonic for NaCl, but endergonic for CaF2.

• Reaction Quotients

  • The reaction quotient is a measure of the relative amounts of reactants and products during a chemical reaction at a given point in time.
  • The reaction quotient, Q, is a measure of the relative amounts of reactants and products during a chemical reaction at a given point in time.
  • By comparing the value of Q to the equilibrium constant, Keq, for the reaction, we can determine whether the forward reaction or reverse reaction will be favored.
  • As the reaction proceeds, assuming that there is no energy barrier, the species' concentrations, and hence the reaction quotient, change.
  • Calculate the reaction quotient, Q, and use it to predict whether a reaction will proceed in the forward or reverse direction

• Changes in Temperature

  • Changes in temperature can affect the equilibrium state of a reversible chemical reaction.
  • Reactions can be classified by their enthalpies of reaction.
  • A diagram of the reaction coordinate for an exothermic reaction is shown in .
  • Exothermic reactions will be shifted toward the reactants.
  • Endothermic reactions, on the other hand, will be shifted towards product formation as heat is removed from the reaction's surrounding environment.