Chapter 6 – Bioenergetics

Publisher Summary This chapter discusses the production and the use of various energy currencies in biological systems. The two energy “currencies” produced in chloroplasts following the trapping of radiant energy are ATP and NADPH. These substances represent the two main classes of energy-storage compounds associated with the electron transfer pathways of photosynthesis and respiration. This chapter examines energy storage in terms of the chemical potential changes accompanying the conversion of a set of reactants into their products. This consideration of the Gibbs free energy helps determine the amount of chemical energy that a particular reaction can store or release. The energy-carrying capacity of ATP in terms of the energetics of its formation and hydrolysis is evaluated. NADPH can be regarded as possessing electrical energy, with the particular amount depending on the oxidation–reduction potential of the system with which it interacts. After considering ATP and NADPH as individual molecules, the chapter places them in their biological context—namely, as part of the bioenergetic scheme of chloroplasts and mitochondria. Such organelle reactions are part of the overall flow of energy from the sun that goes through the biosphere, leading to the nonequilibrium condition that is characteristic of life.