Respiration

Abstract

Absorbed energy is partitioned between two major pathways: synthesis (processes involved with building and replacing the tissues) and respiratory metabolism (Figure 3.1). The latter transfers some of the chemical, potential energy in food to high-energy, phosphate bonds (~P), usually in adenosine triphosphate (ATP) molecules, and these, or at least the energy that they store, are then used to power mechanical work (e.g. muscular contraction), chemical work (e.g. active transport) and synthesis itself. Before proceeding, however, the reader should observe some caution on terminology. ‘High energy phosphate bond’ does not refer to the bond energy of the covalent linkage between the phosphorus atom and the rest of the molecule. The phosphate bond energy is not localised but is a reflection of the energy content of the whole triphosphate molecule before and after its conversion to a diphosphate. The phrase ‘high energy phosphate bond’ is, nevertheless, widespread and will be used in what follows as a convenient shorthand.