4.3 Thermodynamics of Biological Processes

Thermodynamics governs energy flow in living systems. From ATP hydrolysis to redox reactions, these principles drive metabolic pathways and cellular processes. Understanding how energy transforms and transfers is key to grasping the fundamentals of biological systems.

Biological molecules obey thermodynamic laws too. Hydrophobic effects, hydrogen bonding, and other interactions shape protein folding and DNA structure. These forces, along with coupled reactions and efficiency considerations, form the basis for complex cellular functions and metabolic cycles.

Thermodynamic Principles in Biological Systems

Energy flow in metabolic pathways

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• First Law of Thermodynamics in biological context
  • Conservation of energy applies to metabolic reactions ensuring total energy remains constant
  • Energy transforms between chemical, mechanical, and thermal forms during cellular processes
• Second Law of Thermodynamics in metabolic processes
  • Entropy increases in spontaneous reactions driving metabolic pathways forward
  • Irreversibility of metabolic pathways results from entropy production (glycolysis)
• Gibbs free energy in biochemical reactions
  • $\Delta G = \Delta H - T\Delta S$ quantifies energy available to do work
  • Negative $\Delta G$ indicates spontaneous reactions occur without energy input
• ATP as the energy currency of cells
  • Hydrolysis of ATP to ADP releases $\Delta G° = -30.5 \text{ kJ/mol}$
  • ATP hydrolysis couples to drive unfavorable reactions (biosynthesis)
• Redox reactions in metabolism
  • Electron transfer in energy production powers cellular processes
  • NAD+ and NADH serve as electron carriers in metabolic pathways

Thermodynamics of biological molecules

• Hydrophobic effect
  • Drives protein folding and membrane formation by minimizing water contact
  • Entropy-driven process increases overall system disorder
• Hydrogen bonding
  • Enables DNA base pairing through specific nucleotide interactions
  • Contributes to protein secondary structures (alpha helices, beta sheets)
• Van der Waals interactions
  • Weak but numerous interactions stabilize macromolecular structures
  • Contribute to protein folding and ligand binding
• Electrostatic interactions
  • Facilitate enzyme-substrate binding through charge complementarity
  • Stabilize protein-protein interactions in complexes and signaling
• Conformational entropy
  • Influences protein stability by favoring flexible structures
  • Affects ligand binding affinity through entropic penalties

Coupled Reactions and Biological Efficiency

Coupled reactions in biological systems

• Coupled reactions
  • Link favorable and unfavorable processes to drive cellular functions
  • Enable energy transfer between different metabolic pathways
• ATP hydrolysis as a driving force
  • Couples to biosynthesis reactions powering anabolic processes
  • Drives active transport across membranes against concentration gradients
• Electron transport chain
  • Couples electron flow to proton pumping creating an electrochemical gradient
  • Proton gradient powers ATP synthesis through ATP synthase
• Metabolic cycles
  • Citric acid cycle couples oxidation and reduction reactions for energy production
  • Urea cycle couples ammonia removal with ATP hydrolysis
• Allosteric regulation
  • Couples regulatory binding to enzyme activity changes controlling metabolism
  • Enables feedback inhibition in biosynthetic pathways

Efficiency of biological processes

• Photosynthesis efficiency
  • Light reactions convert light energy to chemical energy (ATP and NADPH)
  • Calvin cycle fixes carbon into glucose using ATP and NADPH
  • Limiting factors include light absorption efficiency and CO2 concentration
• Cellular respiration efficiency
  • Glycolysis produces ATP through substrate-level phosphorylation
  • Oxidative phosphorylation generates ATP via chemiosmotic coupling
  • Theoretical ATP yield (~38) exceeds actual yield (~30) due to proton leak
• Thermodynamic limits on biological processes
  • Carnot efficiency sets maximum theoretical efficiency for energy conversions
  • Entropy production in metabolic pathways reduces overall efficiency
• Comparison of artificial vs biological systems
  • Photovoltaic cells achieve ~20% efficiency vs ~1% for photosynthesis
  • Fuel cells reach ~60% efficiency compared to ~40% for cellular respiration
• Metabolic adaptations for efficiency
  • C4 and CAM photosynthesis enhance CO2 fixation in hot, dry environments
  • Brown fat thermogenesis increases metabolic efficiency in cold-adapted mammals