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Green River BIOL 211 - DIFFUSION AND OSMOSIS IN SELECTIVE PERMEABLE MEMBRANES

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Lab 4: Diffusion and Osmosis (Revised Fall 2009) Lab 4 - Biol 211 - Page 1 of 23 Lab 4. Diffusion and Osmosis in Selectively Permeable Membranes Prelab Assignment Before coming to lab, read carefully the introduction and the procedures for each part of the experiment, and then answer the prelab questions at the end of this lab handout. Hand in the prelab assignment just before the start of your scheduled lab period. Goals of this Lab After completing this lab exercise you should be able to..... • Distinguish between diffusion and osmosis. • Describe the mechanism of diffusion and osmosis at the molecular level. • Explain why diffusion and osmosis are important to cells. • Describe what is meant by selectively permeable and explain the role of a selectively permeable membrane in osmosis. • Determine the effects of concentration and temperature on diffusion and osmosis. • Describe the effects of hypertonic, isotonic, and hypotonic solutions on animal and plant cells. • Explain how the presence of a cell wall affects osmotic behavior of plant cells. Introduction All living things must obtain certain materials from their environment. For example, most animal cells obtain nutrients, oxygen, and water from their surroundings. In addition, they must get rid of waste materials. This exchange of substances between the cell and its environment is critical to survival and is dependent upon these materials being dissolved in water. Water is a great environment for life. Without it, life as we know it is impossible. If, as suspected, life in our solar system exists only on earth, it is probably because ours is the only planet known to possess liquid water on its surface. The cytoplasm and extracellular environment of living cells are aqueous. The cytoplasm of living cells is composed of water (~70% to 80% water), which is the solvent or dissolving agent, and numerous dissolved solutes (e.g. salts, sugars, amino acids, vitamins, etc.). Virtually all substances entering and leaving cells are dissolved in water. The combination of a solvent and dissolved solute(s) is a solution. To perform their functions, cells must maintain a steady state in the midst of an ever-changing environment. This constancy is maintained by the regulation of the movement of solutes into and out of the cell. To achieve this control, a delicate membrane composed of a phospholipid bilayer that contains embedded and surface proteins bound cells. Cell membranes (also called plasma membranes) can distinguish different substances, slowing or inhibiting the movement of some while allowing others to pass through. Because not all substances penetrate the membrane equally well, cell membranes are said to be selectively permeable (or differentially permeable), allowing some substances to pass through easily, while completely or partially excluding others. Although there are several methods by which solutes may enter or leave cells, the most common is diffusion. Diffusion is the movement of a substance (e.g. molecules and ions) from a region of higher concentration to one of lower concentration. Diffusion is a passive process, meaning that it occurs without the expenditure of energy. The energy driving diffusion comes only from the intrinsic kinetic energy found in all atoms, ions, and molecules. If nothing obstructs the movement, a solute will diffuse until it spreads itself out uniformly and reaches a dynamic equilibrium.Lab 4: Diffusion and Osmosis (Revised Fall 2009) Lab 4 - Biol 211 - Page 2 of 23 So far, we’ve only described the movement of solutes across membranes. However, water (the solvent) also moves across cell membranes. The movement of water across a selectively permeable membrane from a region where water is highly concentrated to a region where its concentration is lower is known as osmosis. Osmosis is a special kind of diffusion, the diffusion of water. In osmosis water always moves by diffusion across a selectively permeable membrane from a hypotonic solution (low solute concentration, high water conc.) to a hypertonic solution (high solute concentration, low water conc.). A hypertonic solution has a high solute concentration (and therefore a low concentration of water) relative to another solution. This is because when a solute is dissolved in water, its molecules (or ions) fill the space previously occupied by water molecules, thus reducing the concentration of water. On the other hand, a hypotonic solution has a low solute concentration (and thus a high concentration of water) relative to another solution because in the absence of large amounts of solute there is more space available for water molecules. Hence, osmosis can also be defined as the diffusion of water molecules across a selectively permeable membrane from a dilute solution (low solute conc. = hypotonic) to one that is concentrated (high solute conc. = hypertonic). A cell placed into a hypertonic solution will shrink due to the loss of water. If too much water is lost, cell death may result. On the other hand, if a cell is placed in a hypotonic solution, water will enter the cell, causing it to swell and possibly burst. A cell placed in an isotonic solution, one that has the same solute concentration as another solution, the cytoplasm in this case, will not gain or lose water because water molecules will enter and leave the cell at equal rates. A. More water enters the cell than leaves the cell: There is a net movement of water from the hypotonic extracellular environment (lower solute conc., higher water conc.) to the hypertonic cytoplasm (high solute conc., low water conc.). B. Crenation: More water leaves the cell than enters the cell: There is a net movement of water from the hypotonic cytoplasm to the hypertonic extracellular environment Figure 1. The two diagrams illustrate the process of osmosis, the diffusion of water across a selectively permeable membrane from a hypotonic to a hypertonic solution. The membrane is impermeable to the solute in these examples. .Lab 4: Diffusion and Osmosis (Revised Fall 2009) Lab 4 - Biol 211 - Page 3 of 23 The difference in concentration of like molecules in two regions (e.g. on each side of a cell membrane) is called a concentration gradient. Diffusion and osmosis take place down concentration gradients, that is, from an area of high concentration to an area of low concentration. Over time, the concentration of the solvent and solute


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Green River BIOL 211 - DIFFUSION AND OSMOSIS IN SELECTIVE PERMEABLE MEMBRANES

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