Diffusion and Osmosis

Introduction: Osmosis Osmosis is the diffusion of water (or another solvent) through a semipermeable membrane from an area of high water concentration to an area of low water concentration. An area with a low water concentration has a high solute concentration and on the flip side, a solution with a high water concentration has a low solute concentration. The movement of water allows the concentration of the two solutions to reach equilibrium. See figure 1. This does not require the use of energy and is considered passive transport . Solutions can be described by their solute concentration relative to another solution. For example, a solution that has more solute relative to another, is called hypertonic . A solution that has less solute relative to another, is called hypotonic . A solution that has the same solute relative to another, is called isotonic . Every type of cell has a “normal” concentration of various solutes in its cytoplasm. When the outside environment of the cell changes concentration, osmosis will occur to return the cell to equilibrium. Depending on the outside solution concentration, water will move into or out of the cell. The direction the water moves – in or out of the cell – doesn’t change, but what the specific effect on animal and plant cells differs. See figure 2. This equilibrium is dynamic, meaning the water is always moving back and forth across the selectively permeable membrane. Today you will use eggplant cells to measure osmosis. If water moves into a cell it should increase the mass of the cell. If water moves out of a cell, its mass should decrease. If water is moving equally back and forth, then the mass should stay the same. Predictions of what will happen to the eggplant cells when placed in different solutions and record on the data and questions sheet. Activity 3: Determining the tonicity of an unknown solution Materials Four cubes of eggplant Four beakers wax pencil electronic balance Weigh boat 50 mL of each of the following solutions: A, B, C, and X ruler Procedure 1) Cut four equal size pieces of eggplant without skin and seeds. A good size is around 2 cm x 2 cm x 2 cm 2) Label the beakers A, B, C, and X. 3) Place each beaker on a piece of paper towel that has also been labeled A, B, C, and X. 4) On each paper towel, write the following: initial mass (g) and final mass (g). 5) Calibrate the electronic scale. Place the plastic weigh boat on the scale and push the “tare” button. The reading on the scale should be zero (“0”) before you continue. 6) Weigh each of the four cubes of eggplant, one at a time. Round to the nearest 100 th of a gram. Be sure to tare the scale after you weigh each piece of eggplant. 7) Once you have finished weighing an eggplant, place it on the appropriate paper towel . The set up will look similar to the diagram on the next page. 8) When all eggplant cubes have been weighed, lower each cube into its respective beaker AT EXACTLY THE SAME TIME. This is time 0 and the mass is the initial mass. Record this on your data and questions sheet. 9) The beaker labeled X will be treated differently. Remove the eggplant from Beaker X every five minutes and weigh it. You can pause your timer. After it has been weighed, return it to the beaker. Again, be sure to follow the exact same procedure for weighing this cube each time. Record the mass in the table on your data and question sheet. 10) After 30 minutes, all cubes of eggplant will be removed permanently from their beakers and a final mass will be determined and recorded. Credit: LadyofHats [public domain] Figure 2. Red blood cells Plant cells Figure 1. Credit: OpenStax [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)]