def get_molecules(reactions):
    """
    Get a list of all molecules used by reactions

    Args:
           reaction (dict): all reactions that will be used by transport
    Returns:
           self.molecule_ids (list): all molecules used by these reactions
    """
    molecule_ids = []
    for reaction_id, specs in reactions.iteritems():
        stoichiometry = specs['stoichiometry']
        substrates = stoichiometry.keys()
        enzymes = specs['catalyzed by']
        molecule_ids.extend(substrates)
        molecule_ids.extend(enzymes)
    return list(set(molecule_ids))
