November 25, 2014 Period 3
Diffusion and Osmosis
This laboratory experiment demonstrates the processes of diffusion, osmosis, and the effect of solute concentration on water potential in a model membrane system. In part A of the experiment, the diffusion of small molecules through dialysis tubing was being tested. First, a dialysis bag, with the intention of serving as a selectively permeable membrane, was filled with a glucose and starch solution and then placed in a beaker of distilled water. Shortly after, IKI solution was added to the distilled water, and the entire beaker sat untouched for 30 minutes. The IKI solution was added into the beaker for the sole purpose of identification. If the starch or glucose were to react with the IKI solution, a color change would occur, which would allow for conclusions based on the size of different molecules and selective permeability to be drawn. Before the experiment, it was predicted that glucose, starch, water, and IKI molecules would diffuse across the semi-permeable membrane. In part B of the experiment, the relationship between solute concentration and the movement of water through a selectively permeable membrane by the process of osmosis was being investigated. Six dialysis bags were filled with different molarities of sucrose solution (0M, 0.2M, 0.4M, 0.6M, 0.8M, and 1M) and placed into distilled water for 30 minutes. The dialysis bags were weighed initially and after the 30 minute period. If the bag had a different mass after the 30 minute period, not only could it be concluded that osmosis took place, but the tonicity of the water in the beaker and the water in the dialysis bag could be inferred. Originally it was predicted that water would flow into the bag at every molarity of sucrose solution, increasing the mass of every bag after 30 minutes. In part C of the experiment, the water potential of potato cells was being tested. To do this, potato cores were placed into beakers filled with different molar concentrations of sucrose (0M, 0.2M, 0.4M, 0.6M, 0.8M, and 1M) and let sit overnight. The initial mass and the mass after 24 hours was carefully measured. Since water always travels from a high concentration to a low concentration, a positive change in mass would indicate lower water potential and a negative change would indicate a higher water potential. After sitting untouched for 24 hours, it was predicted that the percent change in mass would decrease as the molar concentration of sucrose increased. In part E of the experiment, onion epidermis cells were viewed under a microscope covered in distilled water and a 15% NaCl solution. The expected results were that the cell covered in distilled water would be turgid. On the other hand, it was expected that the cells covered in the 15% NaCl solution would plasmolysis, and the plasma membrane would become visible. Then, the cell was flooded with water to remove the NaCl solution, and it was expected that the cells would regain their original shape.
The life of a cell heavily depends on the fact that atoms and molecules have kinetic energy and are constantly moving. This movement comes in two forms, transport that requires no energy, and transport that requires energy in the form of Adenosine Triphosphate. Passive transport, the main theme of this lab, is the movement of a chemical substance across a cell membrane without expenditure of energy by the cell. Two types of passive transport that are heavily focused on in this lab are diffusion and osmosis. Diffusion is the random movement of molecules from an area of higher concentration of those molecules to an area of lower concentration. Osmosis can be simply described as the diffusion of water through a selectively permeable membrane from high water potential to low water potential. The net movement of water, considering both solute concentration and membrane permeability, is expressed through the concept of