Bivariate functions of the molecular masses and the K-band luminosity, with the molecular mass derived with a luminosity-dependent conversion factor (left) and a constant (right), respectively. The colour code refers to the correlation between the two variables (i.e. larger for those lying in the pink region). The spread is larger for the luminosity-dependent conversion factor. In this latter case, galaxies with a larger K-band luminosity have a lower molecular mass content, while those of a lower K-band luminosity have a large molecular mass content (see text).