  The exploration of pathways and alternative pathways that have a specific
function is of interest in numerous chemical contexts. A framework for
specifying and searching for pathways has previously been developed, but a
focus on which of the many pathway solutions are realisable, or can be made
realisable, is missing. Realisable here means that there actually exists some
sequencing of the reactions of the pathway that will execute the pathway. We
present a method for analysing the realisability of pathways based on the
reachability question in Petri nets. For realisable pathways, our method also
provides a certificate encoding an order of the reactions which realises the
pathway. We present two extended notions of realisability of pathways, one of
which is related to the concept of network catalysts. We exemplify our findings
on the pentose phosphate pathway. Lastly, we discuss the relevance of our
concepts for elucidating the choices often implicitly made when depicting
pathways.
