Diffusion and Osmosis Lab
PART 1: SURFACE-AREA-TO-VOLUME RATIO AND CELL SIZE
Background
Cell size and shape are important factors in determining the rate of diffusion. Think about cells
with specialized functions, such as the epithelial cells that line the small intestine or plant root
hairs. What is the shape of these cells? What is the size of these cells? How do such cells obtain
nutrients?
Pre Lab Questions
1. Use the centimeter side of a ruler to measure each of these four cubes (to the nearest tenth).
After taking accurate measurements, calculate the surface area, volume, and surface-areato-volume ratio for each of the four cubes.
a. What trend do you notice as the cell size increases?
_________________________________________________________________________________________
Cube A
Surface Area (SA)
Volume (V)
SA : V ratio (reduced)
Cube B
Surface Area (SA)
Volume (V)
SA : V ratio (reduced)
Cube C
Surface Area (SA)
Volume (V)
SA : V ratio (reduced)
Cube D
Surface Area (SA)
Volume (V)
SA : V ratio (reduced)
Problem
 Are cells more efficient at moving materials across their surface when the surface area to
volume ratio is higher or lower?
Hypothesis
 Create your own hypothesis. What size/shape cell do you think is most efficient at
transport across the cell membrane? (Bigger? Smaller? Other?)
Materials
 Agarose, Water, 1% Phenolphthalein, 0.1 M NaOH, 0.1 M HCl or Vinegar
Demonstration Task Procedure
 Place phenolphthalein in two test tubes. Add 0.1 M HCl to one test tube. Add 0.1 M NaOH to
the other test tube. Swirl to mix the solutions and observe color change.
Data

In this section of your lab notebook, write what you observed during the demonstration
task.
Analysis/Conclusions
 In a couple of sentences, summarize the effect that phenolphthalein has when added to an
acid and added to a base. (Make sure you identify which solution was the acid, and which
was the base).
Independent Task Procedure
 To create agar cubes: Measure 20 g of agarose, and add to a beaker containing 1 L of water.
Place on hot plate and heat until agarose is fully dissolved and solution is clear. Remove
beaker from hot plate. Add 10 mL of 1% phenolphthalein. Add a few drops of 0.1 M NaOH,
until solution turns bright pink. Pour the agarose mixture in shallow pans and refrigerate
12 hours. (This step has already been done for you).
 Using a dull knife, cut three blocks of agar of different size. These three blocks will be three
different sized models of cells.
 Measure the surface area and volume for each of these “cells” and determine the reduced
surface-area-to-volume ratio for each of the three cells. (Create a data table)
 Using the materials listed, design an experiment to test the predictions you just made
regarding the relationship of surface area and volume to the efficiency of diffusion. Once
you have finished planning your experiment, have your teacher check your design. When
you have an approved design, write your procedure in your lab notebook, run your
experiment.
 After observing diffusion for equal amounts of time in each cell, immediately cut the blocks
in half and measure the depth to which the solution has penetrated each cell. This is the
diffusion depth. Record in millimeters. From that, you can calculate the rate of diffusion
(mm/min) by dividing the distance traveled, by the time it took to for that diffusion to
occur.
 Finally, calculate the percent penetrance. Do this by dividing the penetrated volume by the
total volume.
Data



This section should include a data table that shows the surface area, volume, and the
reduced surface-area-to-volume ratio for each of your three cubes. This data table should
also include the diffusion depth, diffusion rate, and percent penetrance.
You should also have written observations describing what you observed when you
conducted your experiment.
Finally, you may want to include a sketch of your cubes at the end of your experiment to
help show your final results.
Analysis/Conclusions
 In this section, analyze your results. How does the surface-area-to-volume ratio affect the
efficiency of diffusion in the cell? Do your experimental results support your predictions?
Draw conclusions from your data table.
PART 2: MODELING DIFFUSION AND OSMOSIS
Background
In this experiment you will create models of living cells using dialysis tubing. Like cell membranes,
dialysis tubing is made from a material that is selectively permeable to water and some solutes.
You will fill your model cells with different solutions and determine the rate of diffusion.
Pre Lab Questions
*Rewrite and answer these questions in your lab notebook.
1. What is kinetic energy and how does it differ from potential energy?
2. What environmental factors affect kinetic energy and diffusion?
3. How do these factors alter diffusion rates?
4. How are concentration gradients important in diffusion and osmosis?
5. What is the explanation for the fact that most cells are small and have cell membranes with
many convolutions (folds)?
Problem
 How can you use weights of the filled cell models to determine the rate and direction of
diffusion? What would be an appropriate control for the procedure you just described?
 Will protein diffuse? Will it affect the rate of diffusion of other molecules?
Materials
Distilled or tap water, 1 M sucrose, 1 M NaCl, 1 M glucose, 5% albumin (egg-white protein), 5 pieces
of 20 cm- long dialysis tubing, cups, balances
Hypothesis
After choosing the solutions you will test for your experiment, use your knowledge about solute
gradients to predict whether the water will diffuse into or out of the cell.
Procedure
1. Choose up to four pairs of different solutions. One solution from each pair will be in the
model cell of dialysis tubing, and the other will be outside the cell in the cup. Your fifth
model cell will have water inside and outside; this is your control. Knot the dialysis tubing
in one end, fill with 10mL of solution, and knot to close the tube. Make sure to leave enough
space for water to diffuse into the tube. Also, keep the dialysis tubing moist!
2. Before starting, use your knowledge about solute gradients to predict whether the water
will diffuse into or out of the cell. (This is the hypothesis section!!) Make sure you label the
cups to indicate what solution is inside the cell and inside the cup.
3. Weigh each cell, record the initial weight, and then place it into a cup filled with the second
solution for that pair. Weigh the cell after 30 minutes. Create a data table and record the
final weight.
Data/Observations
1. Construct a data table for your lab. The data table should represent each of your five
solutions. It should include original weight, final weight, and percent change in weight.
2. Calculate the percent change in weight using the following formula: ((final – initial)/initial)
x 100. Show your calculations in your lab notebook and record your results in your data
table.
3. Diagram the flow of water based upon the contents of your model cell and the surrounding
solution.
Analysis/Conclusions
1. In your analysis, consider the following:
a. What factors determine the rate and direction of osmosis?
b. Compare the solutions you used. Which were more/less hypertonic? What is your
evidence?
c. Which pair(s) that you tested did not have a change in weight? How can you explain
this?
Post Lab Questions
1. How is the dialysis tubing functionally different from a cell membrane?
2. When will the net osmosis rate equal zero in your cell model?
3. How could you test for the diffusion of glucose? (Hint: think back to McMush lab)
4. You are in the hospital and need intravenous (IV) fluids. You read the label on the IV bag,
which lists all of the solutes in the water.
a. Why is it important for an IV solution to have salts in it?
b. What would happen if you were given pure water in an IV?
c. How would you determine the best concentration of solutes to give a patient in need
of fluids before you introduced the fluids into the patient’s body?
PART 3: OBSERVING OSMOSIS IN LIVING CELLS
Background (does not need to be included in lab write-up)
The interactions between selectively permeable membranes, water, and solutes are important in
cellular and organismal functions. For example, water and nutrients move from plant roots to the
leaves and shoots because of differences in water potentials. Based upon what you know and what
you have learned about osmosis, diffusion, and water potential in the course of your investigations,
think about these questions.
 What would happen if you applied saltwater to the roots of a plant? Why?
 What are two different ways a plant could control turgor pressure, a name for internal
water potential within its cells? Is this a sufficient definition for turgor pressure?
 Will water move into or out of a plant cell if the cell has a higher water potential than its
surrounding environment?
Whole Class Task
Pre Lab Questions
1. What is turgor pressure?
Problem
What causes vegetables to be crisp?
Materials
Celery, 2 cups, water, 1 M NaCl
Procedure
1. Soak celery sticks in 2 cups- one containing water, and one containing 1 M NaCl.
2. Break the sticks in half.
Observations
Write you observations for the demonstration task here.
Analysis/Conclusions
Why did the celery sticks break differently? What can you infer about the individual cells when
placed in the different solutions? Discuss this concept in this section of your lab notebook.
Microscope Task
Problem
What does osmosis look like in a living cell? How does changing the environmental conditions
affect the individual cells?
Hypothesis
Before making your hypothesis, read through the procedure. What changes do you expect to see
when the cells are exposed to the different solutions? Write this as your hypothesis using the
appropriate format.
Materials
Elodea, 2 microscope slides, 2 coverslips, dropper bottles containing: water, 1 M sucrose, 1 M NaCl
Procedure
1. Obtain a single leaf blade of Elodea (a water plant). Prepare a wet mount and view under
the light microscope. The Elodea only has two layers of cells, so it is thin enough to transmit
light through. (Make a drawing of your plant cells in the data section of your lab notebook)
2. Remove your cover slip, and place 1-2 drops of 1 M sucrose on your Elodea leaf. Replace
coverslip and view under the microscope. (Record your observations as a drawing in the
data section of your lab notebook).
3. Repeat step 1 and step 2- except this time use 1 M NaCl. Record all observations in your lab
notebook.
Data/Observations
In this section of your lab report, draw pictures of what you observed under the microscope.
(There should be four!) Are these cells turgid, flaccid, or plasmolyzed?
Analysis/Conclusions
Based on your observations, what can you infer about the tonicity of the environment the cells are
in? Have they taken up water? Released it? How can you tell? Describe the overall osmosis
process in these plant cells.
Post Lab Questions
1. Where is the cell membrane in relation to the cell wall? Can you see the two structures
easily? Why or why not?
2. What parts of the cell that you can see help control the water concentration inside the cell?
3. How did each treatment (NaCl and sucrose) affect the turgor pressure of the cell?
4. How would you measure the water potential of different types of plants?
Independent Investigation
Problem
How can you identify the concentrations of the sucrose solutions and use the solutions to determine
the water potential of potato tissue?
Pre Lab (does not need to be included in lab write-up)
Remember to look at The Biology Place: Lab Bench- Lab 1- Diffusion and Osmosis. (Reference email
or see Resources tab on www.brownbiology.com). Also, read through the bulleted questions in the
procedure
Materials
Potato/sweet potato, cork borer, electric balance, ruler, cups, sucrose solutions of different, but
unlabeled concentrations, prepared by your teacher (0, 0.2, 0.4, 0.6, 0.8, 1.0 M)
Procedure
Using the materials above, design an experiment to identify the concentrations of the sucrose
solutions and use the solutions to determine the water potential of the plant tissues. Be sure to
discuss your experimental design with your teacher before you begin. Your lab procedures as well
as explaining how to calculate water potential should be VERY thorough.
Use the following questions to guide your investigation:
 How can you measure the plant pieces to determine the rate of osmosis?
 How would you calculate the water potential of the solution?
 How would you calculate the water potential in the cells?
 Which solution had a water potential equal to that of the plant cells? How do you know?
 Is the water potential in the different plants the same?
 How does this compare to your previous determinations in the Elodea cells?
 What would your results be if the potato were placed in a dry area several days before your
experiment?
 When potatoes are in the ground, do they swell with water when it rains? If not, how do
you explain that, and if so, what would be the advantage or disadvantage?
Data/Observations
In your lab, you had to measure the potatoes in some way before and after to determine the rate of
osmosis. Record these measurements, as well as the percent change in a data table in your lab
notebook. Also, include any calculations you made to determine water potential. You will also need
to make a graph to determine the exact solute potential of the cells.
Analysis/Conclusions
Discuss your findings in the lab. What were some things that weren’t controlled as well as they
should have been? Is there anything that could have contributed to error in the lab? Explain why
you observed the differences that you did when the potatoes were placed in the different solutions.
Also, list what molarity each colored solution was, and present the calculated water potential of the
potato cells in this lab.