Organism that use diffusion to transport nutrients...?

Organisms have methods for transporting nutrients other than diffusion for one main reason: it's ugly slow(except when involving CO2 and O2)! If humans relied on only diffusion to get nutrients from the intestines to the rest of the body, it would take months! With this information it is clear that if an organism used only diffusion to transport nutrients into cells, they would have to have a very painfully low metabolic rate. =D