We study the influence of gravity waves (GWs) on the distributions of minor chemical constituents in the mesosphere/lower thermosphere (MLT) on the basis of a 3-dimensional global chemistry-transport model called MECTM. The MECTM is driven by the dynamics of the Kuehlungsborn Mechanistic general Circulation Model (KMCM) which resolves mid-frequency GWs down to horizontal wave lengths of 350 km and describes their wave-mean flow interaction is a self-consistently by means of an advanced turbulent diffusion scheme. The effects of GWs on the transport and photo chemistry of minor constituents is assessed on the basis of a sensitivity experiment. In our control simulation, the MECTM is driven with the full dynamical fields from an annual cycle simulation with the KMCM, A perturbation simulation with just the MECTM is defined by eliminating all mesoscale variations with horizontal wavelengths shorter than 1000 km from the dynamical fields via spectral filtering. Hence, both MECTM-simulations are driven by the same large-scale dynamics. For reasons of consistency, we also apply the same numerical grid and time step in these simulations. The response of the MECTM to GW perturbations reveals strong effects on the trace-gas concentrations. In particular, constituents with large photochemical lifetime are strongly affected by vertical wave mixing, while constituents with short lifetime reflect the dependence of their reaction rates on mesoscale temperature perturbations and on the changed distribution of long-living constituents. The mean model response varies with latitude and season, but is strongest around the mesopause where also the GW amplitudes are largest. We present detailed discussions of the simulated GW effects on water vapor, atomic oxygen, carbon dioxide, and odd hydrogen and we highlight the consequences for our understanding of the general circulation of the MLT.