Universal laws of energy exchange, the laws of thermodynamics, govern all interactions among organisms (and all matter). Two are especially important in explaining how organisms manage their energy needs.
First Law of Thermodynamics—the conservation of energy—simply states that while the form of energy can be changed, energy itself can neither be created nor destroyed. Energy exists in two forms: potential energy (stored energy available to do work) and kinetic energy (the energy used to do work).
Second Law of Thermodynamics—the law of entropy (disorder)—in brief, states that chemical reactions run downhill, i.e., the products of the reaction always have less potential energy than the original reactants. Entropy measures the randomness or disorder that forever increases in systems to which no energy is added.
In each energy exchange—from the first photosynthetic reaction to the last in the food web where carnivores dine on one another—energy escapes, primarily as heat. Although the heat energy remains in the system (fulfilling the First Law of conservation), the energy is no longer available to do work, hence it is “lost” to further metabolism. Each of the exchanges is exergonic (ex = out; energy out). The heat of the system rises in proportion to the loss of potential energy. To maintain the organized systems like organisms, therefore, energy is added constantly in a series of endergonic (end = in; energy in) processes.