BackFree Energy Change and Spontaneity in Biological Reactions
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Concept 8.2: The Free-Energy Change of a Reaction
Introduction to Free Energy and Spontaneity
The free-energy change of a reaction is a central concept in biology, as it determines whether a reaction can occur spontaneously. Understanding this concept helps explain how cells harness energy for life processes and maintain order in living systems.
Free Energy (G): The portion of a system's energy that can perform work when temperature and pressure are uniform throughout the system.
Spontaneous Process: A process that occurs without energy input; it increases the stability of a system.
Nonspontaneous Process: Requires energy input to occur; decreases system stability.
Additional info: Free energy is a measure of a system's instability—its tendency to change to a more stable state.
Free-Energy Change, ΔG
Calculating Free-Energy Change
The change in free energy (ΔG) of a chemical reaction can be calculated using the following equation:
Equation:
ΔG: Change in free energy
ΔH: Change in enthalpy (total energy)
T: Temperature in Kelvin
ΔS: Change in entropy (disorder)
If ΔG is negative, the process is spontaneous. If ΔG is positive or zero, the process is nonspontaneous.
Free Energy, Stability, and Equilibrium
Relationship Between Free Energy and Stability
Systems tend to move toward lower free energy, which increases stability. At equilibrium, free energy is at its lowest possible value for the system.
Spontaneous processes: Move systems toward equilibrium and lower free energy.
Equilibrium: State of maximum stability; no net change occurs.
Metabolism: Living systems are never at equilibrium; constant energy flow is required to maintain life.
Example: Water flowing downhill loses potential energy, moving toward equilibrium.
Exergonic and Endergonic Reactions in Metabolism
Classification of Chemical Reactions
Chemical reactions in metabolism are classified based on their free-energy changes:
Exergonic Reaction: Releases energy; ΔG is negative; spontaneous.
Endergonic Reaction: Absorbs energy; ΔG is positive; nonspontaneous.
Reaction Type | ΔG Value | Spontaneity | Energy Flow |
|---|---|---|---|
Exergonic | Negative | Spontaneous | Energy released |
Endergonic | Positive | Nonspontaneous | Energy absorbed |
Example: Cellular respiration is an exergonic reaction; photosynthesis is endergonic.
Equilibrium and Metabolism
Equilibrium in Biological Systems
Reactions in isolated systems reach equilibrium and can do no work. In contrast, living cells are open systems, constantly exchanging energy and matter with their surroundings, preventing equilibrium and allowing continuous work.
Open System: Allows exchange of energy and matter; metabolism remains active.
Closed System: No exchange; reactions reach equilibrium and stop.
Example: Water flowing through a turbine keeps the system from reaching equilibrium, allowing work to be performed.
Free Energy and Metabolism
Application to Cellular Processes
The concept of free energy is essential for understanding how cells manage energy to drive metabolic reactions. Cells couple exergonic and endergonic reactions to maintain life.
Energy Coupling: Use of exergonic processes to drive endergonic ones.
ATP: The primary energy currency in cells, mediating energy transfer.
Additional info: The breakdown of glucose in cellular respiration is a key example of energy coupling in metabolism.
