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Cellular Diffusion Beginning of Paper All cells have an outer protective casing, the cell membrane. It protects the cell from the outside world, but is also vital in the transfer of particles in and out of the cell. The cell membrane controls which particles can enter, and which particles can not, because it is selective .... Middle of Paper .... osmosis, and active transport, all of which involve only small simple molecules. Diffusion is the movement of a substance from a region of high concentration, to a region of low concentration of that substance. In other words, a movement down the concentration gradient. Osmosis is very similar to diffusion, however it involves the concentration of water. Osmosis, like diffusion, is a passive process, and does not require energy to occur. Active transport does require energy though, as it involves movement up the concentration gradient. T .... 595 words | 3 pages Subscription Length Price Join 30 days membership (recurring) $19.95 90 days membership (recurring) $39.95 180 days membership (non-recurring) $69.95