  The transition form factors describing the semileptonic decays of heavy
pseudoscalar mesons are investigated within a relativistic constituent quark
model formulated on the light-front. For the first time, the form factors are
calculated in the whole accessible kinematical region, adopting meson wave
functions derived from various effective $q \bar{q}$ interactions able to
describe the meson mass spectra. It is shown that the decay rates of the $B \to
D \ell \nu_{\ell}$, $D \to K \ell \nu_{\ell}$ and $D \to \pi \ell \nu_{\ell}$
weak decays are mainly governed by the effects of the confinement scale,
whereas the $B \to \pi \ell \nu_{\ell}$ decay rate is sensitive to the
high-momentum components generated in the meson wave functions by the effective
one-gluon-exchange interaction. Our results are consistent with available
experimental data and predictions of lattice $QCD$ calculations and $QCD$ sum
rules.
