Particle number fluctuations in a membrane channel

Channel-facilitated transport of metabolites across biological membranes re sults in excess noise in the current carried by small ions. This noise orig inates from fluctuations of the number of metabolite molecules in the chann el due to their diffusion. We have carried out a theoretical study of parti cle number fluctuations in a cylindrical pore. First, we obtain the power s pectral density of these fluctuations as a function of pore length and radi us, as well as the diffusion constants of the particle in the pore and in t he bulk, in the absence of particle-pore interactions. We then perform thre e-dimensional Brownian dynamics simulations that show excellent agreement w ith the analytical result. Finally, we demonstrate that explicit expression s for the low-frequency limit of the spectral density can be found even whe n the particle interacts with the pore. (C) 2000 American Institute of Phys ics. [S0021-9606(00)70842-7].