  The problem of transport through nanochannels is one of the major questions
in cell biology, with a wide range of applications. Brownian ratchets are
fundamental in various biochemical processes, and are roughly divided into two
categories: active (usually ATP-powered) molecular motors and passive
constructions with a directional bias, where the transport is driven by thermal
motion. In this paper we discuss the latter process, of spontaneous
translocation of molecules (Brownian particles) by ratcheted diffusion with no
external energy input: a problem relevant for protein translocation along
bacterial flagella or injectosome complex, or DNA translocation by
bacteriophages. We use molecular dynamics simulations and statistical theory to
identify two regimes of transport: at low rate of particles injection into the
channel the process is controlled by the individual diffusion towards the open
end (the first passage problem), while at a higher rate of injection the
crowded regime sets in. In this regime the particle density in the channel
reaches a constant saturation level and the resistance force increases
substantially, due to the osmotic pressure build-up. To achieve a steady-state
transport, the apparatus that injects new particles into a crowded channel has
to operate with an increasing power consumption, proportional to the length of
the channel and the required rate of transport. The analysis of resistance
force, and accordingly -- the power required to inject the particles into a
crowded channel to oversome its clogging, is also relevant for many
microfluidics applications.
