Lab 5 Information
Educational Objectives:
Essentially, the goal of this exercise is to understand some of the processes that are involved in transport of materials in cells. Specifically, you will perform an experiment in which you will observe diffusion and osmosis. The lab manual has several different demonstrations of diffusion that you will not be doing. There is nothing wrong with these demonstrations, I have found, however, that the results are not always correct (it is not clear that the demonstrations are have adequate controls – and for this reason, you will not be doing them. You should, however, be able to predict what should happen in these exercises. You are also to know how to test solutions for the presence of sugars and the presence of starch.
Experimental Objectives:
You are to determine what substances (water, glucose and glycogen) are permeable through the dialysis tubing.
You are to determine the relationship between water diffusion and solute concentration.
Before coming to lab:
Read and do the prelab that are in your lab manual. One cautionary note: there are several sections that do not agree with your lecture notes on diffusion and osmosis. You are responsible for the lecture notes only and you should be able to correct those instances when the lecture and the lab manual disagree. For example, pay special attention to the definitions of isotonic and isosmotic. There are others.
With regard to the procedures, you will be doing sections III, section IV. You will not be doing section I, section II or section 5 although you should be able predict the results for these exercises.
For section III, the data from all groups will be pooled to allow statistical analysis.
Be sure to prepare you lab notebook with Introduction and Methods.
After the Lab:
For each beaker and dialysis tubing combination, you need to record the weight before and after the experiment (remember, you should have data from each group!). Determine the net water gain or loss for each tube, and calculate the average loss or gain for each, be sure to calculate the standard deviation. Beaker #4 is the control – make sure you understand the significance of this. There should be no net water movement in the dialysis bag in beaker #4, however, most likely you will find there is a small increase. This increase is due to experimental error. You will need to perform a t test for tubes 1, 2 and 3 – all compared against the control to determine if the change in the weight ineach experiement is due to random error or can it be attributed to the different solute concentrations.
By this time, you will have to decide how to organize your data and what it means. There needs to be some effort to explain your data with regard what you know about diffusion and osmosis. Just simply stating the data is not sufficient.
Sample Data Analysis:
Table 1 shows data from a sample experiment.
|
Table 1: Water Movement Through Dialysis Tubes. |
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|
Control |
25 mM Glucose in Tube |
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| Pre Weight (g) | Post Weight (g) | Difference (g) | Pre Weight (g) | Post Weight (g) | Difference (g) |
|
17.8 |
18.8 |
1.0 |
16.7 |
16.8 |
0.1 |
|
18.5 |
20 |
1.5 |
15.6 |
15.8 |
0.2 |
|
16.9 |
18.1 |
1.2 |
18.9 |
18.8 |
-0.1 |
|
16.4 |
17.2 |
0.8 |
17.5 |
17.9 |
0.4 |
|
18.5 |
19.4 |
0.9 |
15.4 |
15.6 |
0.2 |
|
Average Weight Change |
1.1 |
|
|
0.2 |
|
|
Standard Deviation |
0.3 |
|
|
0.2 |
|
|
p value from t test comparing difference of 25 mM bag with control |
0.00 |
|
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The data shows that there was a gain in water in the dialysis bag containing 25 mM glucose. This increase was significant (that is, not due to random errors) as the p value was less than 0.05 compared to the control.