Exergonic and Endergonic Reactions Notes

Exergonic and Endergonic Reactions

• When chemical reactions occur, work is performed, and the free energy of reactants is not equivalent to the free energy of the products.
• Exergonic Reactions:
  • These reactions are spontaneous, meaning they release energy.
  • An example is cellular respiration.
• Endergonic Reactions:
  • These reactions are non-spontaneous, meaning they require energy input.
  • An example is photosynthesis.
• In a spontaneous, exergonic process:
  • The free energy of the product (B) is lower than the free energy of the reactant (A).
  • Energy is released during the process.

Energy Diagrams of Chemical Reactions

• Scientists use energy diagrams to illustrate energy changes during chemical reactions.
• X-axis: Represents the progress of the reaction (not time).
• Y-axis: Represents the relative amount of energy in the reactants and products as the reaction progresses.
• Key components of an energy diagram:
  • Reactants
  • Transition state
  • Products
  • Free Energy

Activation Energy

• Enzymes function by reducing the energy required for a reaction, known as activation energy ($E_a$).
• The free energy at the start and finish of the reaction remains unchanged by the enzyme.
• A lowered $E_a$ results in reactions starting more frequently.
• The active site of an enzyme:
  • Brings the reactant(s) into the correct orientation.
  • Provides an ideal microenvironment for the reaction.

Energy Release and Absorption

• $E<em>1$ = $E</em>a$ = energy of activation
• $E<em>2$ = $E</em>{\text{products}} - E_{\text{reactants}}$ = net energy released by the reaction

Reactions Requiring Energy Inputs

• In biological systems, reactions that require energy inputs do not occur spontaneously.
• $E<em>1$ = $E</em>a$ = energy of activation
• $E<em>2$ = $E</em>{\text{products}} - E_{\text{reactants}}$ = net energy absorbed by the reaction

Energy Inputs

• Photosynthesis:
  • Requires a constant input of energy in the form of sunlight.
  • Sunlight provides the activation energy for the initial reactions.
• ATP (Adenosine Triphosphate):
  • Provides energy inputs for non-spontaneous metabolic reactions.

Cellular Respiration

• During cellular respiration, enzymes increase the rate of reactions by decreasing the activation energy of the reactions.
• Enzymes do not alter the initial energy of reactants or the final energy of the products.

Enzymes in Metabolism

• Metabolic reactions would not occur at the speed of living systems without enzymes.
• Reactions that would take years can occur in milliseconds inside cells with enzymatic assistance.
• Endergonic enzymatic reactions are coupled with exergonic reactions to become energetically favorable.
• Net result: energy release, increased entropy.
• Example:
  • Enzymatic reactions breaking down glucose release energy.
  • Enzymatic reactions building proteins require energy.
  • These reactions are coupled by the ATP-ADP cycle in metabolism.

Gibbs Free Energy

• Cellular Respiration:
  • Reaction is spontaneous.
  • \Delta G < 0
  • Energy is released.
• Photosynthesis:
  • Reaction is not spontaneous.
  • \Delta G > 0
  • Energy is added.

Entropy and Metabolic Reactions

• Although enzymes decrease the free energy required for every metabolic reaction, entropy always increases as energy is released as heat.
• Catabolic Reactions (Decomposition, Exergonic):
  • Break larger molecules into smaller, more stable ones.
  • Release free energy for cell use (transport, building molecules).
  • Always result in heat energy release, increasing entropy.
  • Less free energy is available for work after the reaction.

Activation Energy and Entropy

• The difference in free energy between reactants and products is the same with or without an enzyme.
• Without an enzyme:
  • Higher activation energy is required.
  • Greater increase in entropy.

Anabolic Reactions and Entropy

• Anabolic Reactions (Synthesizing, Endergonic):
  • Free energy available is higher in products than in reactants.
  • Must be coupled with catabolic reactions to occur.
  • Heat released (increase in entropy) is the difference between free energy input and free energy available in products.