Cellular Diffusion

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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. This involves protein carrier molecules binding to the substance and moving it into or out of the cell, against the concentration gradient.
If a large molecule needs to be moved in or out of the cell, then two other processes, endocytosis and exocytosis are used. Endocytosis involves the cell membrane engulfing a foreign molecule and enclosing it in a vesicle inside the cell. Exocytosis involves the golgi body enclosing the substance to be exported into a vesicle, which then fuses with the cell membrane, and releases the substance outside the cell. Both endocytosis and exocytosis require energy.

Inside cells, inside the ...

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