Experiment 3 Exercise 1: Diffusion – Movement of Solutes across a Membrane
We will be using dialysis tubing to simulate a semipermeable membrane. This tubing allows small molecules (e.g., water, ions, glucose) to pass while preventing large molecules (e.g., macromolecules like proteins, starch, glycogen) from moving across. Be sure you have read over the suggested material before starting this exercise and that you have reviewed the following animations:
McGraw-Hill. 2006. How Diffusion Works https://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html
McGraw-Hill. 2006. How Osmosis Works https://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html
Experimental Design
A. The dialysis bag we will use is permeable to water and small molecules (e.g., less than 500 g/mol) and impermeable to large molecules (e.g., more than 500 g/mol).
B. The dialysis bag is filled with a mixture of glucose (molecular weight = 180 g/mol) and protein (molecular weight = 10,000 g/mol) dissolved in water. A small subsample of the dialysis bag contents is saved and will be used in Step 4.
C. The dialysis bag is then placed into a beaker of water. A small subsample of beaker water is also saved and is to be used in Step 4 as well.
The presence or absence of glucose and protein will be determined using indicators. Indicators change colors in the presence certain materials. The two tests that we’ll use are the Benedict’s test for simple sugars (e.g., glucose) and the Biuret test for the presence of proteins.
· If glucose is present, the Benedict’s indicator will turn green. If no glucose is present, the solution will be blue.
· If protein is present, the Biuret indicator will turn violet. If the solution remains clear, then no protein is present.
D. The subsample of dialysis bag solution and the beaker water are tested for the presence of glucose and protein. See Table 1 below for the results.
E. The dialysis bag is then left in the beaker of water for 60 minutes.
F. At the end of 60 minutes, the dialysis bag solution and the beaker water are again tested for the presence of glucose and protein. See Table 1 below for the results.
Table 1. Results of testing of the dialysis bag and beaker contents at the beginning and end of the Experiment.
| Test for Glucose | Test for Protein | |||
| Beginning | End | Beginning | End | |
| Dialysis Bag | Green | Green | Violet | Violet |
| Beaker | Blue | Green | Clear | Clear |
Questions
1. Summarize the results regarding the presence (+) or absence (-) of glucose and protein in the dialysis bag and beaker in Table 2 below (4 pts):
Table 2.
| Glucose | Protein | |||
| Beginning | End | Beginning | End | |
| Dialysis Bag | ||||
| Beaker |
2. Explain the movement or lack of movement of protein and glucose across the dialysis bag membrane (4 pts)
3. Which solution, that in the bag or that in the beaker, is hypotonic compared with the protein solution (2 pts)?
4. What factors affect the movement of molecules across a semipermeable membrane? Which factor plays the greatest role in biological systems (4 pts)?
5. Briefly explain what active transport is and how it differs from passive transport, especially in terms of concentration gradients (4 pts).