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
McGraw-Hill. 2006. How Osmosis Works
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.
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|
Experiment 3 Exercise 2: Osmosis – The Movement of Water across a Membrane
Before starting, let’s see what you know about the terms hypotonic, isotonic and hypertonic. Examine the diagrams below. Note that the small green circles represent dissolved solutes like salt, glucose, and amino acids. You can assume that the additional space surrounding the solutes is water and that the tan area is INSIDE the cell.
1. Define each term below in terms of solute concentration outside compared to the inside of the cell. You do not need to explain which direction water will move (3 pts).
a. Hypotonic –
b. Isotonic –
c. Hypertonic –
The Biology Place. No Date. Osmosis: Movement of Water across Membranes
B. Read over the information presented and then Click on
C. Then, Click on . Read through the information presented and be sure to click on Animate beneath the illustration.
2. What concentration of salt is isotonic to animal cells (1 pts)?
3. When cells are in isotonic solution, is there movement of water into or out of the cell? If so, describe this movement (3 pts).
E. Read through the information presented and be sure to click on Animate beneath the illustration. When ready, answer the following question.
4. Describe the net movement of water molecules when cells are placed in a hypotonic solution. Explain why water moves this way (3 pts).
G. Read through the information presented and be sure to click on Animate beneath each of the illustrations. Answer the following questions. Your answers should incorporate the terminology used in the animations.
5. What happens to an animal cell when placed in a hypotonic solution (2 pts)?
6. What happens to plant cells when placed in a hypotonic solution? What accounts for the difference in outcomes between animal cells and plant cells (3 pts)?
I. Then, Click on . Read through the information presented and be sure to click on Animate beneath the illustration. Answer the following question.
7. Describe the net movement of water molecules when cells are placed in a hypertonic solution. Explain why water moves this way (3 pts).
K. Read through the information presented and be sure to click on Animate beneath the illustration. Answer the following questions.
8. Compare and contrast what happens to plant and animal cells when placed in a hypertonic solution. Be sure to use proper terminology (4 pts).
9. Based on what you learned in this exercise, explain why salt might make a good weed killer (3 pts).
Week 3 Experiment Grading Rubric
|Experiment 3 Exercise 1||Interpretation of results and demonstrated understanding of diffusion (Table 2 and Questions 1-5).||18 pts|
|Experiment 3 Exercise 2||Demonstrates understanding of isotonic solutions and no net movement of water (Questions 1-3).||7 pts|
|Demonstrates understanding of hypotonic solutions and the movement of water (Questions 4-6).||8 pts|
|Demonstrates understanding of hypertonic solutions and the movement of water (Questions 7-9).||10 pts|