(abridged) In this work we have a closer look at the gas content or fraction and the associated star formation rate in main sequence and starburst galaxies at z=0 and z~1-2 by applying an analytical model of galactic clumpy gas disks to samples of local spiral galaxies, ULIRGs, submillimeter (smm), and high-z starforming galaxies. The model gas and dust temperatures are determined by the heating and cooling equilibrium. Dense clouds are heated by turbulent mechanical and cosmic ray heating. The molecular abundances of individual gas clouds are determined by a detailed chemical network involving the cloud lifetime, density, and temperature. Molecular line emission is calculated with an escape probability formalism. The model calculates simultaneously the total gas mass, HI/H_2 mass, the gas velocity dispersion, IR luminosity, IR spectral energy distribution, CO spectral line energy distribution (SLED), HCN(1-0), and HCO+(1-0) emission of a galaxy given its size, integrated star formation rate, stellar mass radial profile, rotation curve, and Toomre Q parameter. The model reproduces the observed CO luminosities and SLEDs of all sample galaxies within the model uncertainties (~0.3 dex). Whereas the CO emission is robust against the variation of model parameters, the HCN and HCO+ emission is sensitive to the chemistry of the interstellar medium. The CO and HCN mass-to-light conversion factors including CO-dark H_2 are given and compared to the values found in the literature. Both, the HCN and HCO+ emission trace the dense molecular gas to a factor of ~2 for the local spiral galaxies, ULIRGs and smm-galaxies. About 80% of the molecular line emission of compact starburst galaxies originates in non-selfgravitating gas clouds. The integrated Kennicutt-Schmidt law has a slope of ~1 for the local spirals, ULIRGs, and smm-galaxies, whereas the slope is 1.7 for high-z starforming galaxies.
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