Plots showing the fractional abundances (with respect to H₂ number density) of CH and OH as functions of time, calculated using a gas-phase chemistry only model a) and a gas-grain model b) for a TMC-1-like cloud (see Sect. 4.1). Also shown in this figure are the fractional C and CO abundances. The shaded regions represent the observed range of CH and OH fractional abundances derived by us along the TMC-1 ridge.