Impact of changes in areas of cross section on equilibration kinetics: predictions from the diffusion model. (A) Idealized cells with the geometry of frog or mouse rods and for an idealized ciliated cell for which equilibration time courses were modeled by solving Eqs. 13–15. The rod ISs or the cell body (CB) were initially uniformly filled with diffusing substance (black), and the OS and cilia were empty. Equilibration after some period of time is indicated by uniform gray in all compartments. Arrow thickness denotes relative speed of equilibration (see B–D). (B) Time courses of equilibration into rod OS compartments. The lengths of each compartment were: IS, 5 µm; CC, 0.8 µm; OS, 25 µm. The CC had the same diameter in all cases, 0.4 µm (average diameter from Table I). A range of rod IS and OS diameters were modeled, including 7 µm, representative of frog rods, and 1.4 µm, based on the most recent measurements of mouse rod dimensions (Daniele et al., 2005). In all cases, D_IS = 5 µm² s⁻¹, D_CC = 2 µm² s⁻¹, and D_OS = 0.1 µm² s⁻¹. The mass in the OS normalized to the equilibrated OS mass is plotted. (C) Dependence of the T_1/2 of equilibration on the ratio of CC and IS–OS radii. Line is drawn through the points. (D) Time course of cilium equilibration. Ciliated cells (10-µm long cell body and 5 µm in diameter, possessing a cilium 0.4 µm in diameter and varied length) were modeled. The mass of the diffusing substance in the cilium normalized to the equilibrated mass is plotted for cilia of indicated length.