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7.3: Passive transport is diffusion of a substance across a membrane with no energy investment diffusion: movement of molecules of any substance so that they spread out evenly into the available space In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated. Basically, a substance moves down its concentration gradient. Passive transport: diffusion of a substance across a biological membrane is called passive transport because a cell does not energy to make this happen. Effects of Osmosis on Water Balance Water diffuses across the membrane from the region of lower solute concentration to that of higher solute concentration until the solute concentrations on both sides of the membrane are equal. Osmosis: diffusion of water across a selectively permeable membrane Water Balance of Cells Without Walls tonicity: ability of a solution to cause a cell to gain or lose water isotonic: when a cell’s environment has no effect on the passage of water into and out of the cell because it’s the same as inside of the cell; no net movement hypertonic: a solution that, when surrounding a cell, will cause a cell to lose water; the cell will most likely shrivel up and die high solute, low solvent hypotonic: a solution that causes water to enter the cell faster than it leaves; the cell will most likely swell and burst low solute, high solvent A cell without rigid walls cannot tolerate excessive intake or excessive loss of water osmoregulation: control of water balance Water Balance of Cells with Walls The cell’s wall helps maintain the cell’s water balance. turgid: a cell can become very firm because the cell wall doesn’t allow the cell to expand too much when water goes in flaccid: a cell can become very limp if the cell and its environment are isotonic and no water enters the cell plasmolysis: in a hypertonic environment, the cell shrivels and causes the plant to dies Facilitated Diffusion: Passive Transport Aided by Proteins facilitated diffusion: the spontaneous passage of molecules or ions across a biological membrane with the assistance of specific transmembrane transfer proteins most transport proteins are very specific, allowing only certain substances in Two types of transport proteins: 1. channel proteins: provide corridors that allow specific molecules or ions to cross the membrane ex: aquaporins (water channels) and ion channels (open or close in response to a stimulus) 2. carrier proteins: alternates between two shapes, moving a solute across a membrane during the shape change