Thermodynamics and Bioenergetics in Living Organisms

Thermodynamics and Bioenergetics

• The equation ΔG = ΔH - TΔS relates changes in free energy, enthalpy, and entropy under constant temperature and pressure conditions.
• ΔH is negative for exothermic reactions, and ΔS is positive for reactions that increase system randomness.
• Spontaneous reactions have a negative ΔG and do not require energy from their surroundings.

Energy Sources in Living Organisms

• Living organisms maintain internal order by taking free energy from their surroundings (nutrients or sunlight) and returning energy as heat and entropy.
• Cellular reactions involve the oxidation of glucose to CO2 and H2O, with the products being returned to the surroundings, increasing entropy.

Energy Requirements in Cells

• Living systems are isothermal, functioning at constant temperature and pressure.
• Cells acquire free energy from nutrient molecules, converting it into ATP and energy-rich compounds.
• Equilibrium is reached when the composition of a reacting system (reactants and products) stops changing.

Energy Coupling

• Chemical coupling of exergonic and endergonic reactions allows unfavorable reactions to occur.
• ATP reacts directly with metabolites that need "activation" in exergonic reactions, making the overall process spontaneous (ΔG < 0).
• Energy-requiring reactions are coupled to reactions that release free energy (exergonic).

Metabolism and Coupled Reactions

• Metabolism involves catabolism (degradative phase, releasing energy) and anabolism (building phase, requiring energy).
• Reactions are coupled through common intermediates, with ATP as an energy carrier.
• Many coupled reactions use ATP to generate a common intermediate.

Chemical Coupling Example

• Two reactions can be coupled through common intermediates (Pi and H2O) to form a spontaneous reaction (ΔG < 0).
• The strategy works only if ATP is continuously available.

Redox Pairs and Oxidation

• Reducing agents or reductants are electron-donating molecules, while oxidizing agents or oxidants are electron-accepting molecules.
• Conjugate redox pairs consist of an electron donor and an electron acceptor.
• Oxidation is often synonymous with dehydrogenation, catalyzed by dehydrogenases (oxidoreductases).

Electron Carriers

• Universal electron carriers include coenzymes (NAD+, NADP+, FMN, FAD) and proteins (iron-sulfur, cytochromes).
• NAD and NADP can dissociate from enzymes after reactions and undergo reversible reduction of the nicotinamide ring.
• Flavin nucleotides are tightly bound in flavoproteins, allowing single electron transfers and permitting the use of molecular oxygen as an ultimate electron acceptor.