Most of the poor science on these pages comes from the MAJOR misconception that diffusion is dependent on concentration gradients, mostly from bad teaching in years 7-12 of high-school biology classes! Most of this argument was clarified in 1970's by the UK Nuffield Science Project, where diffusion was, correctly, defined as RANDOM MOVEMENT of MOLECULES (particles) owing to their kinetic energy. Hence diffusion occurs all the time unless the temperature reaches 0*K.
A concentration gradient has the effect of generating NET diffusion from the area/volume having a higher concentration of the molecules under consideration to the volume with fewer of them. However when all of the different molecular species (particles) are evenly distributed, diffusion continues with on average equal movements of all species in all directions, now however there is no NET diffusion. In living systems (and probably in some non-living artificial systems), various carriers may pick up and bind certain molecules, hence the system can maintain the concentration gradient, so NET diffusion continues "down", or as I prefer "along" the concentration gradient.
Why Oh why can't we teach proper science instead of a dumbed down version that leads to all of the half understood writings here? It leads to misconceptions of osmosis, and, since we now know that living membranes are much more complex than we first believed, we need to teach correct science not a convenient but wrongly conceptualised version.