  We predict how protoplanetary disks around low-mass young stars would appear
in molecular lines observed with the ALMA interferometer. Our goal is to
identify those molecules and transitions that can be used to probe and
distinguish between chemical and physical disk structure and to define
necessary requirements for ALMA observations. Disk models with and without
vertical temperature gradient as well as with uniform abundances and those from
a chemical network are considered. As an example, we show the channel maps of
HCO$^+$(4-3) synthesized with a non-LTE line radiative transfer code and used
as an input to the GILDAS ALMA simulator to produce noise-added realistic
images. The channel maps reveal complex asymmetric patterns even for the model
with uniform abundances and no vertical thermal gradient. We find that a
spatial resolution of $0.2-0.5\arcsec$ and 0.5--10 hours of integration time
will be needed to disentangle large-scale temperature gradients and the
chemical stratification in disks in lines of abundant molecules.
