BIOL 1201 Chapter : Chapter 7 Textbook Outline
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Diffusion of a substance across a membrane with no energy investment. Diffusion is caused by the thermal energy that molecules generate due to their constant motion. Diffusion: the movement of molecules of any substance so that they spread out evenly into the available space. A molecule has the tendency to diffuse from where it is highly concentrated to an area where it is less concentrated. This is called diffusing down the concentration gradient. Example of moving down the concentration gradient: during cellular respiration, Dissolved oxygen diffuses into the cell across a plasma membrane. As long as cellular respiration consumes the o2 as it enters, diffusion into the cell will continue because the concentration gradient favors movement in that direction. Osmosis: diffusion of free water across a selectively permeable membrane. Tonicity: the ability of a surrounding solution to cause a cell to gain or lose water.