Thermodynamics in biology is all about energy flow and balance. Enthalpy, entropy, and free energy help us understand how cells function, from protein folding to ATP production. These concepts explain why some reactions happen spontaneously while others need a push.

Biological processes often involve a delicate balance between order and chaos. By applying thermodynamic principles, we can predict how cells will react to changes, whether it's during metabolism or when adapting to new environments. Understanding these energy rules is key to grasping life's complexity.

Thermodynamic Concepts in Biological Systems

Enthalpy, entropy, and free energy

• Enthalpy (H)
  • Quantifies heat content in a system measures total energy
  • Crucial for understanding biological processes involving heat transfer (cellular respiration, photosynthesis)
  • Expressed in units of energy (joules or calories)
• Entropy (S)
  • Gauges disorder or randomness in a system increases in spontaneous processes
  • Key to comprehending the direction of biological reactions (protein folding, DNA replication)
  • Measured in joules per kelvin (J/K)
• Free energy (G)
  • Gibbs free energy determines the spontaneity and direction of biological processes
  • Represents the useful work obtainable from a system (ATP hydrolysis, enzyme-catalyzed reactions)
  • Combines enthalpy and entropy: $G = H - TS$ where T is temperature in Kelvin

Calculations in biological reactions

• Change in enthalpy (ΔH)
  • Calculated using Hess's Law sums enthalpies of formation
  • $ΔH = ΣH_{products} - ΣH_{reactants}$ applied in metabolic pathways (glycolysis)
• Change in entropy (ΔS)
  • Determined using standard molar entropies considers molecular complexity
  • $ΔS = ΣS_{products} - ΣS_{reactants}$ relevant in protein denaturation
• Change in free energy (ΔG)
  • Computed using the Gibbs free energy equation incorporates temperature effects
  • $ΔG = ΔH - TΔS$ used to predict reaction spontaneity (ATP synthesis)
  • Standard free energy change: $ΔG° = -RT ln K_{eq}$ where R is gas constant, T is temperature, and $K_{eq}$ is equilibrium constant

Thermodynamics in Biological Processes

Spontaneity of biological processes

• Spontaneity criteria
  • Negative ΔG indicates a spontaneous process (exergonic reactions)
  • Positive ΔG indicates a non-spontaneous process (endergonic reactions)
  • ΔG = 0 signifies equilibrium no net change in the system
• Enthalpy-entropy compensation
  • Balances enthalpy and entropy changes in biological systems (protein-ligand binding)
• Le Chatelier's Principle
  • Explains system responses to condition changes (oxygen binding to hemoglobin)
• Coupled reactions
  • Non-spontaneous reactions driven by spontaneous reactions (ATP synthesis coupled to electron transport chain)

Thermodynamics at biological equilibrium

• Equilibrium state
  • ΔG = 0 at equilibrium no net change in the system
  • Crucial for understanding steady-state processes (enzyme kinetics)
• Relationship at equilibrium
  • $ΔG = ΔH - TΔS = 0$ balances enthalpy and entropy changes
  • $ΔH = TΔS$ at equilibrium energy is evenly distributed
• Factors affecting equilibrium
  • Temperature effects on the TΔS term influence reaction direction
  • Pressure effects on enthalpy impact volume-dependent processes (protein folding)
• Biological examples of equilibrium
  • Enzyme-catalyzed reactions reach steady-state (Michaelis-Menten kinetics)
  • Membrane transport processes balance concentration gradients (ion channels)