The network of chemical reactions that maintains a living system, supplies energy and builds or processes its material. Gas emissions can arise as primary metabolic byproducts or secondary metabolic byproducts, with different implications for an atmospheric biosignature gas.
Enzymes make this network operate on biologically useful timescales. Following material through successive substrates and reaction products explains why a single fast enzyme need not make the entire network fast: another step may constrain the flow. Changing enzyme amounts or activity therefore changes which routes can carry material, connecting molecular chemical kinetics to the maintenance and growth of a biological cell. See Alberts et al., Molecular Biology of the Cell: multienzyme complexes and cell metabolism.
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