Diffusion and Osmosis

PRE-LAB QUESTIONS
1. Compare and contrast diffusion and osmosis.
2. Draw a picture of a cell in isotonic, hypotonic, and hypertonic states.
3. Why don’t red blood cells swell or shrink in blood?
4. How do osmotic power plants work?
5. Research the structures that protect plant and animal cells from damage resulting from osmotic pressure. Write a few paragraphs explaining what they are, how they work, and where they are located.
 
EXPERIMENT 1: DIFFUSION THROUGH A LIQUID
Data Tables
Table 1: Rate of Diffusion in Corn Syrup
Time (sec) Blue Dye Red Dye
10
20
30
40
50
60
70
80
90
100
110
120
 
Table 2: Speed of Diffusion of Different Molecular Weight Dyes
Structure Molecular Weight Total Distance
Traveled (mm) Speed of Diffusion (mm/hr)*
Blue Dye
Red Dye
*To get the hourly diffusion rate, multiply the total distance diffused by 30.

Post-Lab Questions
1. Create a graph of your data from Table 1. Using a graphing program, such as Microsoft Excel®. If you do not have access to Microsoft Excel, use a free graphing program available online.
2. Examine the plot below. How well does it match the data you recorded in Table 1?

3. Which dye diffused faster?
4. Does the rate of diffusion correspond with the molecular weight of the dye?
5. Does the rate of diffusion change over time? Why or why not?
6. Do you think your results would change if the corn syrup was replaced with water? Why or why not?
 
EXPERIMENT 2: DIFFUSION – CONCENTRATION GRADIENTS AND MEMBRANE PERMEABILITY
Data Tables
Table 3: Indicator Reagent Data
Indicator Starch Positive
Control (Color) Starch Negative
Control (Color) Glucose Positive
Control (Color) Glucose Negative
Control (Color)
Glucose Test Strip n/a n/a
IKI n/a n/a

Table 4: Diffusion of Starch and Glucose Over Time
Indicator Dialysis Bag After 1 Hour Beaker Water After 1 Hour
Glucose Test Strip
IKI

Post-Lab Questions
1. Why is it necessary to have positive and negative controls in this experiment?
2. Draw a diagram of the experimental setup. Use arrows to depict the movement of each substance in the dialysis bag and the beaker.
3. Which substance(s) crossed the dialysis membrane? Support your response with data-based evidence.
4. Which molecules remained inside of the dialysis bag?
5. Did all of the molecules diffuse out of the bag into the beaker? Why or why not?
 
EXPERIMENT 3: OSMOSIS – DIRECTION AND CONCENTRATION GRADIENTS
Hypothesis:

Scientific Reasoning:

Data Tables
Table 6: Sucrose Concentration vs. Tubing Permeability
Band Color Sucrose % Initial Volume (mL) Final Volume (mL) Net Displacement (mL)
Yellow
Red
Blue
Green

Post-Lab Questions
1. For each dialysis bag, identify whether the solution inside was hypotonic, hypertonic, or isotonic in comparison to the beaker it was placed in.
2. Which dialysis bag increased the most in volume? Why?
3. What does this tell you about the relative tonicity between the contents of the tubing and the solution in the beaker?
4. What would happen if the tubing with the yellow rubber band was placed in a beaker of distilled water?
5. Suppose you had a 90% sucrose solution in one of the beakers instead of a 3% sucrose solution. How might this change your results?
6. Describe the similarities and differences between the dialysis bag and a cell membrane. Be specific.
7. If you want water to flow out of a dialysis bag filled with a 50% sucrose solution, what would the minimum concentration of the beaker solution need to be?
 
EXPERIMENT 4: OSMOSIS – TONICITY AND THE PLANT CELL
Data Tables
Table 7: Water Displacement per Potato Sample
Potato Potato Type and
Observations Sample Initial Displacement (mL) Final Displacement (mL) Net Displacement (mL)
1 1A
1 1B
2 2A
2 2B

Post-Lab Questions
1. How did the physical characteristics of the potatoes vary before and after the experiment? Did it vary by potato type?
2. What does the net displacement change in the potato sample indicate?
3. Different types of potatoes have varying natural sugar concentrations. Explain how this may influence the water potential of the potato cells.
4. Based on the data from this experiment, hypothesize which potato has the highest natural sugar concentration. Explain your reasoning.
5. Did water flow in or out of the plant (potato) cells? How do you know this?
6. Would this experiment work with other plant cells? What about animal cells? Why or why not?
7. From what you know of tonicity, what can you say about the plant cells and the solutions in the test tubes?
8. What do your results indicate about the concentration of the cytoplasm in the potato cells at the start of the experiment?
9. If the potato is allowed to dehydrate by sitting in open air, would the cells absorb more or less water? Explain.