  Low-mass protostars are the extrasolar analogues of the natal Solar System.
Sophisticated physicochemical models are used to simulate the formation of two
protoplanetary discs from the initial prestellar phase, one dominated by
viscous spreading and the other by pure infall. The results show that the
volatile prestellar fingerprint is modified by the chemistry en route into the
disc. This holds relatively independent of initial abundances and chemical
parameters: physical conditions are more important. The amount of CO2 increases
via the grain-surface reaction of OH with CO, which is enhanced by
photodissociation of H2O ice. Complex organic molecules are produced during
transport through the envelope at the expense of CH3OH ice. Their abundances
can be comparable to that of methanol ice (few % of water ice) at large disc
radii (R > 30 AU). Current Class II disc models may be underestimating the
complex organic content. Planet population synthesis models may underestimate
the amount of CO2 and overestimate CH3OH ices in planetesimals by disregarding
chemical processing between the cloud and disc phases. The overall C/O and C/N
ratios differ between the gas and solid phases. The two ice ratios show little
variation beyond the inner 10 AU and both are nearly solar in the case of pure
infall, but both are sub-solar when viscous spreading dominates. Chemistry in
the protostellar envelope en route to the protoplanetary disc sets the initial
volatile and prebiotically-significant content of icy planetesimals and
cometary bodies. Comets are thus potentially reflecting the provenances of the
midplane ices in the Solar Nebula.
