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In three of the four processes, the natural tendency is to move substances or particles from an area of high concentration to one that is lower. There is no need to expend energy for those processes, those being diffusion, osmosis (diffusion by water), and facilitated diffusion, more commonly known as passive transport. It is the fourth process, active transport, which requires the expenditure of energy by the cell membrane, usually in the form of ATP. The cell membrane itself is composed of a phospholipid bilayer which has the quality of being semipermeable, meaning it will allow some substances, such as water, to pass, while denying others. Particles larger than water pass through special "gateway" cells set up in the cell membrane for that express purpose.
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