Biological processes are generally unidirectional and are very good examples of "kinetic" isotope reactions.
For example, photosynthesis preferentially takes up the light isotope of carbon 12C during assimilation of atmospheric CO2.
In this case, lighter water molecules (i.e., those with 16O) evaporate slightly more easily than heavier water molecules with 18O; this difference will be greater than it would be if the evaporation was taking place under equilibrium conditions (with bidirectional transport).
Condensation occurs almost exclusively by equilibrium processes, and so it enriches cloud droplets somewhat less than evaporation depletes the vapor.
The value of this "deuterium excess", as it is called, is about +10 per mil (1%) in most meteoric waters and its non-zero value is a direct manifestation of kinetic isotope fractionation.