  Intracellular transport of large cargoes, such as organelles, vesicles or
large proteins, is a complex dynamical process that involves the interplay of
ATP-consuming molecular motors, cytoskeleton filaments and the viscoelastic
cytoplasm. The displacements of particles or probes in the cell cytoplasm as a
function of time are characterized by different (anomalous) diffusion regimes.
We investigate here the motion of pigment organelles (melanosomes) driven by
myosin-V motors in \emph{Xenopus laevis} melanocytes using a high
spatio-temporal resolution tracking technique. By analyzing the mean square
displacement (MSD) of the obtained trajectories as a function of the time lag,
we show that the melanosomes display a transition between subdiffusive to
superdiffusive behavior. A stochastic theoretical model is introduced to
generalize the interpretation of our data. Starting from a generalized Langevin
equation that explicitly considers the collective action of the molecular
motors we derive an analytical expression for the MSD as a function of the time
lag, which also takes into account the experimental noise. By fitting our model
to the experimental data we were able to discriminate the exponents that
characterize the passive and active contributions to melanosome dynamics. The
model also estimates the "global" motor forces correctly. In this sense, our
model gives a quantitative description of active transport in living cells with
a reduced number of parameters.
