Rucete ✏ Campbell Biology In a Nutshell
Unit 2 THE CELL — Concept 8.2 The Free-Energy Change of a Reaction Tells Us Whether or Not the Reaction Occurs Spontaneously
Understanding the energy changes in chemical reactions is crucial for grasping how cells perform work. The concept of free energy helps predict which reactions can occur spontaneously and which require an input of energy.
1. Free Energy (G) and Spontaneity
Free Energy (G): A measure of a system's capacity to perform work when temperature and pressure are uniform.
Change in Free Energy (ΔG): Calculated using the formula:
ΔG = ΔH - TΔS
ΔH: Change in enthalpy (total energy)
ΔS: Change in entropy (disorder)
T: Absolute temperature in Kelvin
Spontaneous Reactions: Occur without external energy input; characterized by a negative ΔG.
2. Free Energy, Stability, and Equilibrium
Higher G: Indicates instability; systems tend to move toward more stable states with lower G.
Equilibrium: The point at which G is at its lowest; systems at equilibrium cannot perform work.
Metabolic Disequilibrium: Cells maintain a state of disequilibrium to continue performing work; equilibrium equals death for a cell.
3. Exergonic and Endergonic Reactions
Exergonic Reactions:
Release free energy (negative ΔG)
Spontaneous
Example: Cellular respiration
Endergonic Reactions:
Absorb free energy (positive ΔG)
Non-spontaneous
Example: Photosynthesis
4. Energy Coupling in Cells
Cells couple exergonic processes to drive endergonic ones, ensuring that energy-requiring reactions proceed.
In a Nutshell
Free energy (G) is a key indicator of a system's capacity to perform work.
Reactions with a negative ΔG are spontaneous and release energy.
Reactions with a positive ΔG are non-spontaneous and require energy input.
Cells harness energy by coupling exergonic and endergonic reactions to maintain life's processes.