Bioenergetics, the topic of volume 5 of this Series, is concerned with the energetics, the kinetics, and the mechanisms of energy conversion in biological systems. This phenomenon can be investigated on diffeÂ- rent levels of complexity. On a global level the role of biological proÂ- cesses for the steady state of our enviroment is considered. At the physiological level, the relation between energy input and the physioloÂ- gical state of an organism is of interest, while at the cellular level the biochemical pathways for degradation and synthesis of all relevant substrates is investigated. At present the majority of bioenergetic stuÂ- dies pertain to the molecular level. The processes in a cell are catalyÂ- zed by a large number of proteins called enzymes. The enzymes inÂ- volved in energy transduction can be considered as molecular maÂ- chines which transform energy from one form into another, or transfer energy from one process to another. Living systems operate far from equilibrium and are open in the therÂ- modynamic sense, i. e. they exchange energy and matter with the surÂ- roundings. Chapter 1 presents the principles of non equilibrium thermoÂ- dynamics applied to biological systems. About 0. 05% of the energy from the sunlight which reaches the surface of the earth is used by plants and algae as well as some bacteria to synthesize organic comÂ- pounds, and thus supplies all organisms with the energy necessary for life.
