Filtration 和 diffusion 的分別在那裹？

Filtration is movement of water and solute molecules across the cell membrane due to hydrostatic pressure generated by the cardiovascular system. Depending on the size of the membrane pores, only solutes of a certain size may pass through it. For example, the membrane pores of the Bowman's capsule in the kidneys are very small, and only albumins, the smallest of the proteins, have any chance of being filtered through. On the other hand, the membrane pores of liver cells are extremely large, to allow a variety of solutes to pass through and be metabolized. In kidney, the protein-free filtrate in the glomerulus is forced to the bowman's space due to combination of starling forces, which include the hydrostatic pressure of the glomerulus. Note that filtration only occurs in renal corpuscle (not tubules). Tubules only function to modify the concentration of urine/ re-absorb and secrete substances.

Diffusion is simply the net movement of material from an area of high concentration of that material to an area with lower concentration. It doesn't require energy and is dependent on the concentration gradient.