Download Diffusion and Osmosis Lab

AP Biology Lab – Unit 2
Diffusion and Osmosis Lab
Purpose/Objectives
How do cell membranes help regulate internal cellular makeup?
In Part 1, you investigate the relationship between the volume and surface area of a model cell and the rate
of diffusion. In Part 2, you will measure the water potential in plant cells by measuring the percent change in
mass of a potato cube put into different concentrations of sucrose solutions.
Learning Objectives
! To investigate the relationship among surface area, volume, and the rate of diffusion
! To design experiments to measure the rate of osmosis in a model system
! To analyze the data collected in the experiments and make predictions about molecular movement
through cellular membranes
! To connect the concepts of diffusion and osmosis to the cell structure and function
Pre-lab questions or research - Remember to write complete sentences. Assume the reader doesn’t know the
questions.
1. Diffusion is the movement of a solute from an area of higher concentration to an area of lower
concentration. Explain why solutions with uniform distributions of molecules across a concentration
gradient are said to be in dynamic equilibrium.
2. Explain what environmental factors affect kinetic energy and diffusion and how they alter diffusion rates.
3. Explain the difference in both water concentration and solute concentration for hypotonic solutions,
hypertonic solutions, and isotonic solutions.
4. Research and describe the acid-base properties of phenolphthalein indicator, including its expected
color changes and the pH range for each color form.
5. The phenolphthalein-agar model cells used in Part 1 are initially pink. (a) Predict the observations when
+
these cells are placed in dilute hydrochloric acid, assuming H3O ions are able to diffuse into the agar.
(b) How can these observations be used to measure the rate of diffusion?
6. Calculate the volume and surface area of a 2-cm cubic unit cell used in the Baseline Activity in Part 1.
7. Consider the following scenario: An artificial membrane that is selectively permeable encloses an
aqueous solution. The solution surrounding the model cell contains a different aqueous solution.
Assume that monosaccharides such as glucose and fructose are able to cross the membrane but that
larger disaccharides, such as sucrose, do no. The exact concentrations of each solute in the cell and
surroundings are shown in the figure below.
a. Which solute(s) will exhibit a net diffusion into the cell?
b. Which solute(s) will exhibit net diffusion out of the cell?
c. With respect to glucose, is the surrounding environment hypertonic or hypotonic to the cell?
0.2Mfructose
0.8Mglucose
2Msucrose
Table 1.
No.
1
2
3
4
5
Model Cell
(inside dialysis tubing)
1 M sucrose
1 M glucose
1 M sucrose
Distilled Water
Water
0.5Mfructose
0.5Mglucose
1Msucrose
Surrounding Environment
(plastic cup)
1 M salt
1 M sucrose
Distilled water
1 M salt
Water
Net diffusion
(into or out of cell)
8. Using your knowledge of concentration gradients and the permeability of the membrane, predict
whether there will be net diffusion of water by osmosis into or out of each model cell. Enter your
predictions in the table above.
Materials
Part 1. The Rate of Diffusion and Cell Size
Hydrochloric acid, HCl, 0.1 M, 150 mL
Phenolphthalein agar block
Sodium hydroxide, NaOH, 0.1 M
Beaker, 150-mL
Metric ruler
Paper towel
Plastic knife
Plastic spoon
Timer or phone with timer app.
Part 2. Calculating Water Potential for Plant
Cell
Glucose solution, C6H12O6, 1 M, 0.8M, 0.6M,
0.4M, 0.2M
Water, distilled
Balance, 0.01-g precision
Cups, plastic, 9-oz (6 cups)
Weighing dish, large
Graduated Cylinder
Safety Precautions
Hydrochloric acid is toxic by ingestion and inhalation. Sodium hydroxide and hydrochloric acid solutions are
corrosive to skin and eyes. Phenolphthalein contains alcohol and is a flammable liquid. Avoid contact of all
chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves, and a chemicalresistant apron. Follow all normal laboratory safety guidelines and wash hands thoroughly with soap and water
before leaving the laboratory.
Part 1. The Rate of Diffusion and Cell Size
Baseline Activity
1. Using a plastic knife and a metric ruler, cut the phenolphthalein agar block into a 2-cm cube.
2. Pour 100 mL of 0.1 M hydrochloric acid into a 150-mL beaker.
3. Using a plastic spoon, carefully place the agar cube in the beaker of hydrochloric acid.
4. Gently agitate the solution and turn the cube with a spoon occasionally while soaking.
5. After 10 minutes, gently remove the agar cube using the plastic spoon. Blot the cube dry using a paper
towel.
6. Using a plastic knife, cut the cube in half and measure the depth to which acid penetrated the cube.
7. Record observations and results.
Analysis of Results
Calculate the rate of diffusion of hydrochloric acid into the agar-phenolphthalein cube and compare the
surface area-to-volume ratio of this agar “model cell”.
Analysis Questions
Answer the following questions while reflecting upon your knowledge of cell size, diffusion, and nutrient
transfer.
a. Why are most cells so small? Why aren’t cells larger?
b. How does the rate of diffusion influence the ability of a cell to obtain needed nutrients?
c. Calculate the expected surface area-to-volume ratios for smaller and larger model cells.
d. Predict how the surface are-to-volume ratios might affect the rate of diffusion into a cell.
e. Many cells or organelles that play a key role in nutrient absorption or energy transfer have
highly “convoluted” membranes with many folds. How does this affect the surface area of the
cell or organelle and the rate of diffusion?
Part 2. Calculating Water Potential for Plant Cell
Procedure
1. Obtain six plastic cups and label them 1-6 using a wax pencil.
2. Obtain six potato cubes.
3. Place a weighing dish on the balance and zero the balance.
4. Measure and record the mass each potato cube.
5. Fill each cup with approximately 50 mL of the different molar sugar solutions and one with distilled
water.
6. Put a potato cube in each cup.
7. After 30 minutes, remove the potato cubes.
8. Re-mass the potato cube and calculate the percent mass change for each potato.
PercentChange =
(FinalMass − InitialMass)
× 100%
InitalMass
9. Identify each solution as hypotonic, hypertonic, or isotonic.
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10. Graph the percent mass change as a function of the molarity of the solution. Draw a best-fit line. The
potato would be in an isotonic environment at the molarity where the best-fit line crosses the x-axis. Use
this molarity to calculate the water potential of the potato.
Analysis Questions
Living cell membranes are selectively permeable and contain protein channels that permit the passage
of water and various molecules. Dialysis tubing is similar to a cell membrane in many ways. Answer the
following questions while reflecting upon your knowledge of membrane permeability, osmosis, and
diffusion.
a. Is the rate of diffusion directly proportional to the solute concentration?
b. What other variables might influence the rate and direction of osmosis?
c. How would diffusion of a starch solution be different than that of a protein?
d. How could you prove or disprove that a specific solute, such as sucrose, was able to diffuse
through a semi-permeable membrane?
Conclusion
Follow the lab guidelines to write a conclusion to this lab.