Examining Diffusion and Osmosis
1. To simulate and observe the diffusion of solutes and the osmosis of water through a semipermeable membrane through color change and sugar tests. 2. To speculate osmosis occurring in dialysis bags and potato cores by comparing percentage change in masses.
Molecules are always in random, constant movement due to their kinetic energy. This causes the molecules of a cell to move around and bump into each other. The random movement of molecules from an area of high concentration to low is called diffusion. Osmosis, a specific kind of diffusion with respect to water takes place in the cells. This kind of diffusion is where water moves through a selectively permeable membrane, that is, a membrane that only allows specific molecules to pass through. Diffusion and osmosis stops when a dynamic equilibrium is reached, that is, the concentration on both sides are equal to each other. To be noted, that at this point, molecules are still in constant motion and random movement; however, the rates at which molecules moving from one area to another are equal and there is no net movement occurring between the two areas. If two solutions have reached this equilibrium state, they are said to be isotonic. If they differ, the solution with a higher concentration of solutes is hypertonic with respect to the other; the solution with lower concentration is hypotonic. Water potential describes the tendency of water moving to another area. Since solutions always tend to reach a state of equilibrium, water usually moves from a hypotonic solution to a hypertonic solution in order to dilute the higher concentration. Therefore, we say that hypotonic solutions have a lower solute concentration and a higher water potential. The net movement of water in a plant cell can be described using the two terms, turgor pressure and plasmolysis. Turgor pressure refers to the pressure of water exerting against its plasma membrane, which helps maintain the shape of the cell. When a plant cell is placed in a hypotonic environment, water moves into the cell, causing higher turgor pressure. However, when placed in a hypertonic solution, water moves out of the plant cell, therefore causing the cell to lose its turgor pressure. The loss of water and turgor pressure while a cell is in a hypertonic solution is called plasmolysis. In conducting this lab, we will be simulating diffusion and osmosis that occurs in the cells by using dialysis bags potato cores. In comparing the percentage changes in mass of these test objects after an isotonic state has been reached, we will be able to speculate if water has moved into or out of the bag.
Part A: dialysis tubing, Iodine Potassium Iodide (IKI) solution, 15% glucose/ 1% starch solution, glucose Testape or Lugol’s solution, distilled water, and a 250-mL beaker. Part B: six presoaked dialysis tubing strips, distilled water; 0.2M, 0.4M, 0.6M, 0.8M, and a 1.0M sucrose solution; six 250-mL beakers or cups, and a scale. Part C: a potato, knife, potato core borers, distilled water; 0.2M, 0.4M, 0.6M, 0.8M, and a 1.0M sucrose solution, and a scale.
Keep the potato core pointing away from you and other students when cutting the potato cores out Be careful cutting the potato slices, ask an adult or get a responsible person.
Refer to Lab One Diffusion and Osmosis Handout
Part A: Lab 1A Questions:
1. Glucose is leaving the bag, we know this because initially there was no change in colour of the Testape in the beaker, but our final data shows, there was glucose present in the beaker. The Iodine-Potassium-Iodide is entering the bag because initially we tested the beaker for starch using the Iodine-Potassium-Iodide solution and at that time, the solution color in the bag was clear, by the end of the experiment, it was a dark blue color, which is...
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