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 Diffusion and Osmosis Diffusion: The net movement of particles (atoms, molecules, or ions) from an area of higher concentration to an area of lower concentration (i.e. along a concentration gradient), resulting from random motion. When particles of a solute are evenly distributed throughout a solution, the particles are said to be at dynamic equilibrium. Concentration Gradient: A difference in the concentration of a substance from one point to another, as for example, across a cell membrane. Simple Diffusion occurs when small, nonpolar (uncharged) solute molecules move directly through a biological membrane down their concentration gradient. The rate of simple diffusion is directly related to the concentration of the solute; the more concentrated the solute, the more rapid the diffusion. Osmosis: The net movement of water (the principal solvent in biological systems) by diffusion through a selectively permeable membrane from a region of higher concentration of water (called a hypotonic solution) to a region of lower concentration of water (called a hypertonic solution). Osmotic Pressure: The pressure that must be exerted on the hypertonic side of a selectively permeable membrane to prevent diffusion of water (by osmosis) from the side containing pure water. Isotonic: A term applied to solutions that have identical concentrations of solute molecules and hence the same osmotic pressure. Turgor Pressure: Hydrostatic pressure that develops within a walled cell (i.e. a plant cell) when the osmotic pressure of the cell’s contents is greater than the osmotic pressure of the surrounding fluid. Plant cells with high turgor pressure are said to be turgid. Plasmolysis: The shrinkage of cytoplasm and the pulling away of the plasma membrane from the cell wall when a plant cell (or other walled cell) loses water, usually in a hypertonic environment. Facilitated Diffusion: The passive transport of ions or molecules by a specific carrier protein in a membrane. As in simple diffusion, net transport is down a concentration gradient, and no additional energy has to be supplied. Diffusion Osmosis Facilitated Diffusion of Potassium Ions Response of Cells Based on Osmotic Pressure Turgor Pressure and Plasmolysis Work Questions: 1) What are facilitated and simple diffusion? How are they similar? How are they different? 2) What is osmotic pressure and how does it influence an a) plant cell or b) animal cell? 3) In what direction do particles move along their concentration gradient? Would your answers be different for facilitated diffusion compared with simple diffusion? 4) What is considered to be the universal biological solvent? 5) Describe what happens in simple diffusion. Answer Key 1) Facilitated Diffusion is the passive transport of ions or molecules by a specific carrier protein in a membrane. Whereas simple diffusion is when small, nonpolar solute molecules move directly through a biological membrane down their concentration gradient. Facilitated and simple diffusion are similar because net transport is down a concentration gradient and no additional energy is needed. Facilitated and simple diffusion are different because a specific carrier protein is needed for facilitated diffusion; a carrier protein is not needed in simple diffusion. 2) Osmotic Pressure is the pressure that must be exerted on the hypertonic side of a selectively permeable membrane to prevent diffusion of water (by osmosis) from the side containing pure water. In animal cells, osmotic pressure determines whether a cell lyses (in a hypotonic solution), undergoes plasmolysis (in a hypertonic solution), or does neither (in an isotonic solution). In plant cells, osmotic pressure determines whether a cell is turgid (in a hypotonic solution), becomes flaccid (in an isotonic solution), or undergoes plasmolysis (in a hypertonic solution). 3) In a concentration gradient, particles move from a region of high concentration to an area of low concentration until the particles are evenly dispersed. Both facilitated and simple diffusion particles follow this trend. 4) Water (H2O) 5) In simple diffusion, small, nonpolar solute molecules move directly through a biological membrane down their concentration gradient. The rate of simple diffusion is directly correlated to the concentration of the solute; the more concentrated the solute, the faster the diffusion. Document Source: Derived from pages 116-­‐120 in Biology: FCCJ volume 1 by Eldra Solomon, Linda Berg, and Diana W. Martin Edited by Sara Childers, June 6th, 2011 for FSCJ South Campus Library Learning Commons Science Lab. Permission to copy and use is granted for educational use provided this copyright label is displayed.