Chapter 5: Bioenergetics: Energy Flow & Metabolism in Cells

Every living cell requires a continuous energy supply to perform six major categories of biological work: the synthesis of new molecules (synthetic work), mechanical movement, the concentration of substances across membranes, the establishment of electrical potentials, heat generation to maintain body temperature, and the production of light through bioluminescence. Organisms are classified by their energy source into phototrophs, which capture solar radiation, and chemotrophs, which extract energy from chemical bonds. While energy flows through the biosphere in a unidirectional path from the sun and is eventually lost as heat, matter cycles continuously through various oxidation and reduction states. The rules governing these energy transformations are described by the laws of thermodynamics. The first law dictates that energy is always conserved, meaning it can change form but cannot be created or destroyed. The second law explains that spontaneous processes always lead to increased randomness in the universe, a concept known as entropy. To assess the feasibility of biological reactions, scientists utilize the thermodynamic function of free energy, which represents the capacity to do work under constant pressure and temperature. A negative change in free energy indicates an exergonic, or spontaneous, reaction that releases energy, whereas a positive change signals an endergonic, non-spontaneous process requiring energy input. To determine these values, biologists often look at the equilibrium constant (Keq), which is the ratio of products to reactants when a reaction reaches a state of no net change. Unlike closed systems in a laboratory, living cells function as open systems that maintain a dynamic steady state rather than reaching thermodynamic equilibrium. Because a reaction at equilibrium has a free energy change of zero and can do no work, reaching equilibrium is equivalent to cellular death. By keeping metabolic reactions far from equilibrium through the continuous intake of energy and matter, cells successfully drive essential processes and maintain their complex structural order.