6.1 The metabolic map: an overview of the com plexity of metabolism 6.4 The relationship of free energy to stabil ity, spontaneous change, equilibrium, and work 6.5 How a free-energy gradient keeps metaboli sm away from equilibrium: a hydraulic a nalogy 6.6 ATP 6.7 Energy coupling by phosphate transfer 6.8 The ATP cycle 6.9 An energy profile of a reaction 6.10 Enzymes lower the barrier of activation en ergy 6.12 The catalytic cycle of an enzyme 6.13 Environmental factors affecting enzymes An Introduction to Metabolism 6.