def uM_to_mgml(species_mws, species_concs, scale=1000000):
    """

    Args:
        species_mws: (dict) For example {'enzyme_1' : 33000, 'substrate_1' : 150}
        species_concs: (dict) For example {'enzyme_1' : 10, 'substrate_1' : 10000}
        scale: (int) The scale we are working on.  1=M, 1000=mM, 1000,000=uM.  The default is uM.

    Returns:
        A dictionary of species concentrations in mg/ml.
        For example {'enzyme_1': 0.33, 'substrate_1': 1.5}
    """

    dict_of_mgml = {}

    for specie, mw in species_mws.items():
        vol = 1 # 1L
        conc = species_concs[specie] / scale #returns conc in M

        moles = vol*conc #in moles

        mass = moles * mw #in grams, this is grams in 1L

        dict_of_mgml[specie] = mass

    return dict_of_mgml