  Most models designed to study the bidirectional movement of cargos as they
are driven by molecular motors rely on the idea that motors of different
polarities can be coordinated by external agents if arranged into a motor-cargo
complex to perform the necessary work [gross04]. Although these models have
provided us with important insights into these phenomena, there are still many
unanswered questions regarding the mechanisms through which the movement of the
complex takes place on crowded microtubules. For example (i) how does
cargo-binding affect motor motility? and in connection with that - (ii) how
does the presence of other motors (and also other cargos) on the microtubule
affect the motility of the motor-cargo complex? We discuss these questions from
a different perspective. The movement of a cargo is conceived here as a hopping
process resulting from the transference of cargo between neighboring motors. In
the light of this, we examine the conditions under which cargo might display
bidirectional movement even if directed by motors of a single polarity. The
global properties of the model in the long-time regime are obtained by mapping
the dynamics of the collection of interacting motors and cargos into an
asymmetric simple exclusion process (ASEP) which can be resolved using the
matrix ansatz introduced by Derrida [Derrida et al].
