2: Thermodynamics in biology

Thermodynamics in Biology

Thermodynamics governs energy transformations in biological systems, essential for understanding how cells harness energy for growth, maintenance, and reproduction. Its principles explain the spontaneity, direction, and efficiency of biochemical reactions.

First Law: Energy Conservation

Energy cannot be created or destroyed, only converted between forms. In biology:

• Photosynthesis converts solar energy to chemical energy (glucose).
• Cellular respiration transforms glucose into ATP, releasing heat.
  Biological systems are open, exchanging energy and matter with their environment.

Second Law: Entropy and Spontaneity

The universe’s entropy (disorder) always increases. While living cells maintain internal order (low entropy), they:

• Export entropy (e.g., heat waste) to surroundings.
• Drive processes by coupling energetically favorable (entropy-increasing) reactions to unfavorable ones.

Gibbs Free Energy ($\Delta G$)

$\Delta G$ determines reaction spontaneity at constant temperature and pressure:

• $\Delta G = \Delta H - T\Delta S$
  • $\Delta H$: Enthalpy change (heat absorbed/released).
  • $T\Delta S$: Temperature × entropy change.
• $\Delta G < 0$ (Exergonic): Spontaneous (e.g., ATP hydrolysis, $\Delta G \approx -30.5$ kJ/mol).
• $\Delta G > 0$ (Endergonic): Requires energy input (e.g., protein synthesis).

ATP: Energy Coupling

ATP hydrolysis (exergonic) fuels endergonic reactions via coupling:

• Phosphorylation: ATP donates phosphate to substrates (e.g., glucose → glucose-6-phosphate in glycolysis).
• Enzyme catalysis: Lowers activation energy, enabling $\Delta G$-driven reactions.

Equilibrium vs. Steady State

• Equilibrium ($\Delta G = 0$): No net reaction (rare in biology; implies no energy flow).
• Steady state: Open systems maintain constant metabolite concentrations via continuous energy input (e.g., glycolysis intermediates).

Thermodynamics in Metabolism

• Pathway regulation: Enzymes control flux to ensure $\Delta G < 0$ for pathway progression.
• Energy storage: Endergonic synthesis (e.g., glycogen) balances exergonic breakdown.

Thermodynamics underpins energy transfer across all biological scales, from enzyme kinetics to ecosystem energy flow.