Phases serve to isolate species. To change phases, species have to transfer across a membrane. Transfer through biological membranes involves several mechanisms that accounts inter alia on the size and polarity of the species. Dedicated carriers activate the transfer of molecules for which simple diffusion is inefficient. Within a phase, species diffuse. In the simplest context, the diffusion of a species is to be traced to a main driving force. However, in general, several driving forces collaborate, or compete, to move particles in the diffusion process. Depending on the boundary conditions, these couplings may give rise to quite typical flows.
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