Lab 3: Diffusion and Osmosis - University of Evansville

After leaving the laboratory, use Excel to help youexamine your osmosis data. Keep in mind that therate of osmosis means the amount of water that entersthe bag per time interval. This rate depends on theconcentration of sucrose; the higher the concentrationof sucrose, the faster water enters the bag. Youcan determine the rate by finding the trendline slopeexpressed in grams per minute.Then, use the rates from the known sucrose concentrationsto graphically determine the relationshipbetween concentration of sucrose and rate of osmosis.Use this relationship to construct a standard curvewhich can be used to determine the percentage foryour unknown sucrose concentration.Section D - DialysisDialysis involves the selective separation of materialsby differential diffusion across a membrane. Theartificial kidney machine, used for patients whose kidneyshave impaired function, works on this principle.The patient’s blood is passed outside of the body andinto a special dialysis chamber bounded by a selectivelypermeable membrane. The chamber is surroundedby a dialysis buffer. Small molecular wasteproducts pass out of the blood into the buffer, and theconcentration of waste products equilibrates acrossthe membrane. Large molecules and red blood cellsare retained in the dialysis chamber. The blood is thenreturned to the body with a reduced concentration ofwaste products. As an example of differential diffusion,observe the demonstration with a dialysis bagcontaining starch solution, and liquid outside the bagcontaining iodine. Iodine stains starch a dark blue.For your lab report: Note the location of the bluecolor - is it inside of the bag or outside? What gradientsare present? Which diffusion occured? Why?How did that lead to the results you saw?Section E - Reporting your ExperimentThis week, you are to write a lab report, whichwill be due at the beginning of class in two weeks.Remember, this is not creative writing. Your goal isto explain your work clearly and concisely.25There are many ways to write a scientific report,but all scientific writing has some specific rules.Reports should be organized in labeled sections: (a)Introduction, (b) Methods, (c) Results and (d) Discussion.At this point in time, you should begin writingyour Introduction and your Methods sections andbegin developing the graphs for your Results section.After next week’s lab discussion on the ScientificMethod, you’ll be ready to put the finishing toucheson the first parts of your lab report and also create theDiscussion. A complete discussion of how to write asuccessful lab report can be found in the Appendix ofyour laboratory manual.Your introduction should, in your own words,explain the background for your experiment. Then,near the end of the introduction, explain your questionor hypothesis. You should use your textbook,your lab manual and outside sources to help you writethis section. For the diffusion and osmosis lab report,the introduction should include an explanation of theprinciples of diffusion and osmosis and how theseinfluenced your experiment. Please cite any outsidereferences in the text of your report; use the author’slast name and the page number (Smith, p 612). Thecomplete reference should be found at the end of thereport in a bibliography section. (Example: Smith,Mary. “The Bacterial Cell Wall.” J. Bacteriology 118:612.)The methods section should explain, briefly, butcompletely, how you did your experiments. Tell yourreader about your experimental design (more on thisnext week).The results section is where you report your data.This is where you place tables and graphs, alongwith written explanations of what is in them. Forthis experiment, your results section should includeyour data, slope calculations and % sucrose of theunknown solution. Be sure to include the number ofthe unknown solution you used.A discussion section contains interpretation ofyour data, explaining how it fits into the greater bodyof knowledge. You will hear more about the discussionsection next week.