Fig. 2
Metabolic water production and utilization in the resting, awake, non-stimulated subject. The glycolysis pathway consumes two ATP and produces four ATP per glucose (Glc) metabolized, resulting in a net gain of two ATP along with the formation of two H₂O by the enolase reaction. The TCA cycle in mitochondria (Mito) consumes (denoted by a negative sign) four water molecules per glucose oxidized, with the production of 12 and 28 water molecules, respectively, occurring in the electron transport chain (ETC) and oxidative phosphorylation (OxPhos) steps. Cation pumps hydrolyze ATP and consume H₂O. Given the high turnover of ATP, it is assumed that the water produced by OxPhos is consumed by cation pumping. In the absence of any other stimuli, biosynthetic and other "housekeeping" reactions consume ATP. The net amount of H₂O produced by glycolysis, the tricarboxylic acid (TCA) cycle, and the electron transport chain (ETC) is assumed to be available for secretion. The total metabolic water production is 38 H₂O/glucose of which 10 is available for secretion. The respective rates of metabolic water production are obtained by multiplying the measured rate of glucose oxidation (CMRglc-ox) in the astrocyte (-a) or neuron (-n) (see Table 1). The following abbreviations are used: Glc-6P represents glucose-6-phosphate, while Fru-1,6P2 denotes fructose-1,6-bisphosphate. For further details, please refer to Sects. "Reactions that consume and produce water during metabolism of glucose" and "Metabolic water during metabolism of Glc-6P via branch pathways", Fig. 8, and Table 2