That’s the lab : BIO 101 Lab: Osmosis and Diffusion Objectives: ● Determine the effect of temperature and molecular weight on the process of diffusion
● Observe plant and animal cells placed in hypotonic, isotonic, and hypertonic solutions and observe the effect of each of the solutions on the cells.
● Observe turgid and flaccid plant cells.
● Observe healthy red blood cells, crenated and hemolyzed blood cells. Background: Cells and Cell Membrane structure Living cells are filled with an aqueous solution, the cytosol. Many small solutes or particles, are dissolved in this watery cytosol, including ions, nutrients, waste and enzymes. Larger organelles (such as mitochondria and chloroplasts) and large complexes of molecules (e.g. ribosomes) are also located in this cytosol; organelles and complexes combine with the cytosol to form the cytoplasm of cells. The exterior barrier of all cells, is a cell membrane (plasma membrane). Because the cell membrane is comprised of both phospholipids and proteins it is semi-permeable: select molecules can easily pass through the phospholipid bilayer, while other molecules cannot. Phospholipids are comprised of hydrophobic fatty acid tails and polar phospho- heads. In the bilayer, the fatty layers are sandwiched in between the hydrophilic polar heads; one layer of polar heads lines the inside (cytosolic face) of the cell membrane and a second polar head, lines the outside of the cell membrane or surface. Diffusion, Facilitated Diffusion and Active Transport through membranes Diffusion is the process by which substances move from an area of high concentration to an area of lower concentration, until (under ideal circumstances) the area contains equal amounts of the substance. Molecules that can easily pass through a cell membrane are small and uncharged, such as gases: oxygen, carbon dioxide, and nitrogen and water. Some small molecules (e.g. glucose) and ions (e.g. sodium or chloride) can diffuse across a membrane if there is a transport protein or ion channel. These small solutes will move from areas of high concentration to low concentration through that select protein channel which is part of the membrane. Active transport occurs when a solute, such as sugar, moves against its gradient. Both energy and protein transporters are required for active transport to occur. Osmosis is a type of diffusion Osmosis occurs when water moves down its concentration gradient; that is, when water moves by diffusion across a selectively permeable membrane from high water concentration to low water concentration. In this case, the solutes are not diffusing because they have positive or negative charge or are too large, and/or their protein channels (transporters) are closed. Tonicity. Biologists describe all solutions as either isotonic, hypotonic or hypertonic to cells. Solutions that are isotonic are ones that have equal amounts of dissolved substances, and thus equal concentrations of water. Water movement is then equal across the cell membrane. A solution is hypotonic if it contains less dissolved substance than the solution inside the cell. Hypotonic solutions have relatively more water, and thus water will move from the solution across the membrane (into the cell). A solution outside a cell is hypertonic if it contains more dissolved substance than the solution inside the cell. Hypertonic solutions have relatively less water, and thus water will move out of the cell into the solution. Summary: Three terms refer to the concentrations of solutes in which a cell is placed:
● Isotonic solutions have an equal concentration of solutes outside a cell as the cytosol in the cell
● Hypotonic solutions have a lower solute concentration outside a cell than the cytosol in the cell
● Hypertonic solutions have a higher solute concentration inside a cell than the cytosol in the cell. Historic methods of food preservation have relied on the principle of osmosis. By placing foods (vegetables, fruits and meats) in a salty solution, called brine, or a sucrose solution, water is drawn out of the cells by osmosis. The result is a preserved or dried food which is much less likely to spoil; some examples are sauerkraut, beets, salted cod and ham. Materials: A. DIFFUSION
• Two plastic Petri dishes containing nutrient agarose with two wells • One bottle of green dye (202 g/mol)
• One bottle of blue dye (319 g/mol) • One wax pencil
• Ruler B. OSMOSIS (DETERMINE TONICITY) • Four 50-mL beakers • Potato cubes • Four solutions • Paper towels • Weighing dish • Balance C. ANIMAL CELLS • Three glass microscope slides • Three cover slips • Three test tubes • 0.9% NaCl, 10% NaCl, deionized water • Three droppers • Cow red blood cells D. PLANT CELLS • Two glass microscope slides • Two cover slips • Elodea leaves