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MOVEMENT OF MOLECULES THROUGH CELLS A cell membrane is usually permeable to substances such as oxygen, water, and carbon dioxide, but not large molecules such as salt. Substances that can move into and out of a cell do so by one of three ways: diffusion, osmosis, or active transport. PASSIVE TRANSPORT Diffusion and osmosis are forms of Passive Transport. They do NOT require energy from the cell. DIFFUSION Diffusion is the main way molecules move in and out of a cell. Process where molecules move from an area of higher concentration, to an area of lower concentration. Concentration is the amount of a substance in a given volume. [HIGH] [LOW] http://www.indiana.edu/~phys215/lecture/lecnotes/lecgraphics/diffusion2.gif What causes diffusion? Molecules are constantly moving and colliding. More molecules results in more collisions. When molecules collide they push away from one another. OSMOSIS Osmosis- the diffusion of water molecules through a semi-permeable membrane. This process is very important because cells need adequate water. Semi-permeable membrane When the concentration of water outside the cell is lower than the concentration of water inside the cell, the cell will lose water molecules. This may cause the cell to shrink. [LOW] [HIGH] When the concentration of water outside the cell is higher than the concentration of water inside the cell, the cell will gain water molecules. This may cause the cell to swell. [HIGH] [LOW] ACTIVE TRANSPORT Active Transport is the movement of materials through a cell membrane using energy. Special proteins called transport proteins “pick up” molecules and carry them inside or outside the cell, using energy. Active Transport is required when: – 1. The cell needs materials that are in lower concentration outside the cell. – 2. The materials needed are too large to pass through the membrane by diffusion. Active Transport Video: http://www.youtube.com/watch?v=STzOiRqzzL4&feature=related ENERGY