high-energy bonds. Reactive aspects The hydrolysis of ATP into ADP and inorganic phosphate releases 20.5 kJ/mol of enthalpy. The values of the free energy released by cleaving either a phosphate (Pi) or a pyrophosphate (PPi) unit from ATP at standard state concentrations of 1 mol/L at pH 7 are: ATP + → ADP + Pi ΔG°' = −30.5 kJ/mol (−7.3 kcal/mol) ATP + → AMP + PPi ΔG°' = −45.6 kJ/mol (−10.9 kcal/mol) These abbreviated equations at a pH near 7 can be written more explicitly (R = adenosyl): [RO-P(O)2-O-P(O)2-O-PO3]4− + → [RO-P(O)2-O-PO3]3− + [HPO4]2− + H+ [RO-P(O)2-O-P(O)2-O-PO3]4− + → [RO-PO3]2− + [HO3P-O-PO3]3− + H+ At cytoplasmic conditions, where the ADP/ATP ratio is 10 orders of magnitude from equilibrium, the ΔG is around −57 kJ/mol. Production from AMP and ADP Production, aerobic conditions A typical intracellular concentration of ATP is hard to pin down, however, reports have shown there to be 1–10 μmol per gram of tissue in a variety of eukaryotes. The dephosphorylation of ATP and rephosphorylation of ADP and AMP occur repeatedly in the course of aerobic metabolism. ATP can be produced by a number of distinct cellular processes; the three main pathways in eukaryotes are (1) glycolysis, (2) the citric acid cycle/oxidative phosphorylation, and (3) beta-oxidation. The overall process of oxidizing glucose to carbon dioxide, the combination of pathways 1 and 2, known as cellular respiration, produces about 30 equivalents of ATP from each molecule of glucose. ATP production by a non-photosynthetic aerobic eukaryote occurs mainly in the mitochondria, which comprise nearly 25% of the volume of a typical cell. Glycolysis In glycolysis, glucose and glycerol are metabolized to pyruvate. Glycolysis generates two equivalents of ATP through substrate phosphorylation catalyzed by two enzymes, phosphoglycerate kinase (PGK) and pyruvate kinase. Two equivalents of nicotinamide adenine dinucleotide (NADH) are also produced, which can be oxidized via the electron transport chain and result in the generation of additional ATP by ATP synthase. The pyruvate generated as an end-product of glycolysis is a substrate for the Krebs Cycle. Glycolysis is viewed as consisting of two phases with five steps each. In phase 1, "the preparatory phase", glucose is converted to 2 d-glyceraldehyde-3-phosphate (g3p). One ATP is invested in Step 1, and another ATP is invested in Step 3. Steps 1 and 3 of glycolysis are referred to as "Priming Steps". In Phase 2, two equivalents of g3p are converted to two