<p> Cell Transport</p><p>*A cell must exchange materials with its surroundings, a process controlled by the cell membrane</p><p>*Cell membranes are selectively permeable, regulating what enters/leaves the cell</p><p>Passive transport – substances can cross through a cell membrane without using any energy</p><p>I. Diffusion – movement of molecules from a high concentration to a </p><p> low concentration</p><p> a) a difference in concentration = concentration gradient</p><p> b) Diffusion will continue until the concentrations are equal = </p><p> dynamic equilibrium (movement continues, but equilibrium is </p><p> maintained)</p><p> c) Diffusion depends on the size, type, and chemical nature </p><p>(polarity) of molecule</p><p>Ex: Cell membrane is made of phospholipids, which are </p><p> nonpolar. Therefore, nonpolar substances will diffuse </p><p> through a cell membrane. II. Osmosis – diffusion of water across a cell membrane</p><p> a) Isotonic solution: Solute concentration is the same as that </p><p> inside the cell; no net water movement across the plasma </p><p> membrane (water will move in either direction.)</p><p> b) Hypertonic solution: Solute concentration is greater than that </p><p> inside the cell (water conc. less than that inside); cell loses </p><p> water</p><p>*In plant cells, results in a loss of turgor pressure.</p><p>1. Hypotonic solution: Solute concentration is less than that inside</p><p> the cell (water conc. is greater than that inside); cell gains water</p><p>*Cells may burst = cytolysis; </p><p>*In a plant cell, the cell membrane is push out against the </p><p> cell wall = turgor pressure III. Facilitated Diffusion – diffusion assisted by specific proteins </p><p> called carrier proteins</p><p>Steps in Facilitated Diffusion</p><p> a) Carrier protein binds to molecule</p><p> b) Carrier protein changes shape, protecting molecule from the </p><p> interior of cell membrane</p><p> c) Molecule is released on other side, protein returns to original </p><p> shape</p><p>*Proteins are specific (or picky) as to what molecule they will </p><p> bind to</p><p>IV. Ion Channels – proteins that act as small passageways through the</p><p> cell membrane for ions (charged atoms)</p><p> a) Ion channels are specific to one type of ion</p><p>* Sodium ion channels will only let sodium ions through</p><p> b) Some are always open, while others have “gates”</p>