Summary
Highlights
This lesson introduces Energy and Metabolism in biological systems, recalling thermodynamics as the study of heat or energy and power, and how energy is used for biological work. The first law of thermodynamics states energy is conserved, only converted from one form to another, all living organisms obey these laws.
The Gibbs free energy expression (ΔG = ΔH - TΔS) is introduced as the most common form. Delta symbols indicate changes in components. G represents Gibbs free energy (Joules per mole), the portion of energy available to do work. H is enthalpy (Joules per mole), measuring total energy or heat content. S is entropy (Joules per Kelvin moles), measuring disorder or randomness, and is modified by temperature (T) in Kelvin.
Entropy is explained as a measure of disorder. Examples include a messy vs. organized desk, and billiard balls before and after being struck. A negative change in entropy means creating more order, while a positive change indicates greater disorder and freedom of movement.
Delta values represent changes in conditions. A positive Delta G means energy was put into the system (endergonic, non-spontaneous), like rolling a ball uphill. A negative Delta G means energy was released (exergonic, spontaneous), like a ball rolling downhill. Delta G relates to equilibrium, not the speed of a reaction (kinetics). If Delta G is zero, the system is at equilibrium; living systems are never at equilibrium.
Life creates order, representing a large negative change in entropy, which is energetically unfavorable. To make life spontaneous, energy must be released (metabolic processes). More complex organisms require more energy. Organisms strive for homeostasis (stable non-equilibrium) rather than equilibrium, maintaining stable conditions like pH and temperature where net Delta G is less than zero.
Unfavorable reactions can occur by coupling them with favorable ones. The example shows an endergonic reaction coupled with a highly exergonic one, resulting in an overall spontaneous process. Coupled reactions happen simultaneously or in rapid succession, common in metabolic pathways which often involve oxidation-reduction (redox) reactions.
Metabolic pathways often involve redox reactions. "OIL RIG" (Oxidation Is Loss, Reduction Is Gain) refers to electrons. Electrons are transferred from an oxidized molecule to a reduced molecule. Metabolism is fundamentally about moving electrons. Plants reduce carbon dioxide using sunlight to create glucose, which we then oxidize back to CO2 to release energy for cellular processes. Metabolic reactions are enzyme-catalyzed.