Figure 1.

A general diffusional model applicable to molecular sources and sinks. (A) Diffusion occurs in the steady-state between two concentric spheres, an inner sphere of radius a and an outer sphere of radius b. (B) Case I. The inner sphere acts as a source, determining a higher concentration of molecules locally. (C) Case II. The inner sphere acts as a sink, determining a lower concentration of molecules locally. Concentrations were calculated using Eq. 8, cases I and II, with a = 0.5 nm, b = 10 μm, D = 13 μm²/s, u = 6022 molecules/μm³ (10 μM), and q = 400 s⁻¹. The arrows indicate the direction of flux.