Classical thermodynamics applies pri- marily to reversible processes in isolated systems (which cannot exchange mat- ter or energy with their surroundings) or in closed systems (which can only exchange energy). An isolated system inevitably reaches equilibrium. For exam- ple, if its reactants are in excess, the forward reaction will proceed faster than the reverse reaction until equilibrium is attained (∆G = 0), at which point the forward and reverse reactions exactly balance each other. In contrast, open sys- tems, which exchange both matter and energy with their surroundings, can reach equilibrium only after the flow of matter and energy has stopped. Living organisms, which take up nutrients, release waste products, and gener- ate work and heat, are open systems and therefore can never be at equilibrium. They continuously ingest high-enthalpy, low-entropy nutrients, which they convert to low-enthalpy, high-entropy waste products. The free energy released in this process powers the cellular activities that produce the high degree of organiza- tion characteristic of life. If this process is interrupted, the system ultimately reaches equilibrium, which for living things is synonymous with death.