Electron micrographs of mitochondrial inner membranes indicate a regular, organized protein distribution across the inside surface. Protein digestion will remove these projections. The regularity of the protein structures hints at the requirement for physical position to insure function. The diffusion rate varies inversely with distance , and directly with temperature , concentration gradient , area through which diffusion takes place, and a fudge factor called the diffusion coefficient . The diffusion coefficient is empirically determined and varies with molecular size along with other attributes and solvent characteristics.
Membrane flux is diffusion across a membrane.
A membrane can act as a barrier or enhancer to diffusion. Certainly diffusion across a membrane will be different from that in the solvent. Osmosis is always and only the diffusion of water through a selectively permeable membrane . So, there must always be water present and a membrane that allows some water through it, but some of the solutes in the water permeate the membrane at a different rate than water creating a concentration difference on the two sides of the membrane. Remember that all chemicals in solution prefer to be equally distributed if all other things are equal.