15.1 Energy Is Required to Meet Three Fundamental Needs
All organisms, from simple bacteria to humans, require energy for three major purposes: (1) the mechanical work of movement, (2) the active transport of molecules across membranes, and (3) the biosynthesis of biomolecules and, ultimately, the synthesis of new cells.
15.2 Metabolism Is Composed of Many Interconnecting Reactions
The process of energy transduction takes place through metabolism, a highly integrated network of chemical reactions. Metabolism can be subdivided into catabolism (reactions employed to extract energy from fuels) and anabolism (reactions that use this energy for biosynthesis). The most valuable thermodynamic concept for understanding bioenergetics is free energy. A reaction can take place spontaneously only if the change in free energy (ΔG) is negative. A thermodynamically unfavorable reaction can be driven by a thermodynamically favorable one, which is the hydrolysis of ATP in many cases.
15.3 ATP Is the Universal Currency of Free Energy
The energy derived from catabolism is transformed into adenosine triphosphate. ATP hydrolysis is exergonic, and the energy released can be used to power cellular processes, including motion, active transport, and biosynthesis. Under cellular conditions, the hydrolysis of ATP shifts the equilibrium of a coupled reaction by a factor of 108. ATP, the universal currency of energy in biological systems, is an energy-
15.4 The Oxidation of Carbon Fuels Is an Important Source of Cellular Energy
ATP formation is coupled to the oxidation of carbon fuels. Electrons are removed from carbon atoms and passed to O2 in a series of oxidation–
15.5 Metabolic Pathways Contain Many Recurring Motifs
Metabolism is characterized by common motifs. A small number of recurring activated carriers, such as ATP, NADH, and acetyl CoA, transfer activated groups in many metabolic pathways. NADPH, which carries two electrons at a high potential, provides reducing power in the biosynthesis of cell components from more oxidized precursors. Many activated carriers are derived from vitamins, small organic molecules required in the diets of many animals.
15.6 Metabolic Processes Are Regulated in Three Principal Ways
Metabolism is regulated in a variety of ways, the principal ways among them being as follows. First, the amounts of some enzymes are controlled by regulation of the rate of synthesis and degradation. Second, the catalytic activities of many enzymes are regulated by allosteric interactions (as in feedback inhibition) and by covalent modification. Third, the movement of many substrates into cells and subcellular compartments also is controlled.
The energy charge, which depends on the relative amounts of ATP, ADP, and AMP, plays a role in metabolic regulation. A high-