  We estimate the impact on the stellar evolution of the uncertainties in the
$3\alpha$ and the $^{12}{\rm C}(\alpha,\gamma)^{16}{\rm O}$ reaction rates
[...]. We calculated models of low- and intermediate-mass stars for different
values of the rates. The $3\alpha$ reaction rate was varied up to $\pm 24\%$,
while the $^{12}{\rm C}(\alpha,\gamma)^{16}{\rm O}$ reaction rate was varied by
up to $\pm 35\%$. The models were calculated for two different initial chemical
compositions to represent different stellar populations. A $M = 0.67$
$M_{\odot}$ model was chosen as representative of the halo ancient stars, while
for disk stars, the $M=1.5$ $M_{\odot}$ and $M=2.5$ $M_{\odot}$ models were
considered. The impact of the two reaction rates on the central He-burning
lifetime and the asymptotic giant branch (AGB) lifetime, as well as the mass of
the C/O core at the central He exhaustion and the internal C and O abundances,
was investigated. A variation of the $^{12}{\rm C}(\alpha,\gamma)^{16}{\rm O}$
reaction rates resulted in marginal differences in the analysed features among
the three considered stellar masses, except for the C/O abundances. The central
He-burning lifetime changed by less than 4%, while the AGB lifetime was
affected only at the 1% level. The internal C and O abundances showed greater
variation, with a change of about 15%. The uncertainty in the $3\alpha$
reaction rate mainly influences the C and O central abundances (up to 10%) for
all the models considered, and the AGB lifetime for intermediate mass stars (up
to 5%). Most of the investigated features were affected by less than 2%. The
current uncertainty in the two reaction rates has a negligible effect on the
predicted evolutionary time scale with respect to other uncertainty sources.
The variability in the chemical profile at the end of the shell He-burning
phase is still relevant.
