Potato Osmosis
A shipwrecked sailor is stranded on a small desert island with no fresh water to drink. She knows she could last without food for up to
a month, but if she didn't have water to drink she would be dead within a week. Hoping to postpone the inevitable, her thirst drove her
to drink the salty seawater. She was dead in two days. Why do you think drinking seawater killed the sailor faster than not drinking
any water at all? Today we explore the cause of the sailor's death. We'll prepare solutions of salt water to represent the sea, and we'll
cut up slices of potato to represent the sailor. Potatoes are made of cells, as is the sailor!
Research Question / Objective:
Why is it bad to drink salt water? How do concentrations of solutions affect the movement of water in/out of living cells? How do
sucrose concentrations affect osmosis in potatoes?
Understanding the effect of solution concentrations on the movement of water (osmosis).
Introduction
Prior to the experiment, write an introduction designed to answer the following questions:
1) Discussion of IV: What is concentration? How is it measured? What is a solvent? What is a solute? Why is water called
the universal solvent?
2) Discussion of DV: What is movement/diffusion of water or osmosis? What causes it? How can it be measured?
3) Predictable Relationship Between IV and DV: What happens to the rate of osmosis as concentration changes? Why
4) Discussion of Different Levels of IV: Why will we use different concentrations of sucrose? What will happen in each of
them? Why?
5) Discussion of Other variables that could affect results:What things will be kept constant across all groups on purpose?
Why? How will we increase validity and reliability of our measurements?
Hypothesis: Based on the introduction, create your own hypothesis for what will happen in this experiment:
Materials:
Eletronic Balance; Sucrose x 850g; Water (Distilled) x 5L; Graduated Cylinder (500ml); Cups x 5; Plastic Wrap; Potato Cutter; Knife;
Procedure:
Remember to write down any errors or deviations while you do the experiment
Day 1
1) Prepare molar solutions of sucrose (This step may be performed by your teacher ahead of time)
a. 1M = put 342 g of sucrose in a 1000ml beaker and fill it with water.
b. .75 M = put 240 g of sucrose in a 1000ml beaker and fill it with water.
c. .5 M = put 171 g of sucrose in a 1000ml beaker and fill it with water.
d. .25 M = put 86 g of sucrose in a 1000ml beaker and fill it with water.
e. 0M = Fill empty 1000ml beaker with distilled water.
f. No solution
2) Divide the cups into 5 groups. Label cups with the molar solutions they will contain.
3) Cut potato into slices:
Use a knife to square off the ends of your potato. Your potato's cells will act like the sailor's cells.
Stand your potato on end & use your cork borers to bore 3 vertical holes.
Remove the potato cylinders from the cork borer & measure their length in centimeters.
Cut the 3 potato cylinders to the same length (about 4 -5 centimeters long).
Alternative: Peel off potato. Make slices of equal width (1/4in), and then use a potato dicer to make
cores. Use 3 cores per sample.
4) Use a scale/balance to record the mass of each core or set of cores in your data table
5) Also note the turgidity (crispor flacid) of the potato cylinders in your data table (day 1).
Note: If you are not placing the potatos immidiately in the solution, place the potato samples in a small ziplock bag to
prevent them from dehydrating before they're used. If you are proceeding directly from cutting to the experiment, place
potatos directly on the cups (covered with foil or plastic)
6) Now fill 2/3 of each cup (enough to cover potatos)
7) Wait 3 minutes and write down any qualitative observations that differentiate solutions at this point
8) Leave the potato cylinders in the solution for 24 hours.
Day 2
13. Make qualitative observations that differentiate solutions at this point
14. Carefully remove the potato cylinder from the distilled water solution & pat it dry on a paper towel (Do not over do this. Do
not place potoes on paper towel or squeeze them. Just lightly pat the outside so they are not dripping wet)
15. Measure & record the mass of the potatoes from each solution
16. Calculate change in mass (Final – Initial)
17. Calculate % mass change (Final – Initial) / (Initial)
18. Note the turgidity of the pototoes from each solution
19. Clean up your equipment & area and return materials to their proper place.
Results:
Solution
0M
.25M
.50M
.75M
1.0M
Initial
Mass (g)
Final
Change
% Change
Turgidity
Initial
Final
Qualitative Obsevations
3 min
Next day
Create a line graph the % mass change over changes in sucrose concentration (title, units, and axis required)
Conclusion:
1. Did any of the potato cylinders change in their turgidity (flexibility), and if so, which ones changed?
2. What was the relationship between molarity of the solution and % mass change?
3. The results reject of failed to reject your hypothesis?
4. Explain why the mass and turgidy of the potato slices changed.
5. In which solutions was their a greater solute concentration outside of the cells?
6. Which solutions if any were hypertonic, hypotonic, or isotonic to the potato? How do you know?
7. Make a sketch of your potato cylinders in the 1.0M and destilled water. Use arrows to show the direction of water movement
across the potato cell membranes. Also sketch what would happen in an isotonic solution.
8. Use your graph to determine the sucrose molarity of the potatos you used.
9. Why was it important not to leave potatoes open to the air too long between cutting and putting them in solution?
10. Why was it important to be careful while “patting” down the potatoes on day 2?
11. What was done to introduce a “control” group to this experiment? Why was this done?
12. If you knew the molarity of the potato ahead of time, how could you add another control group to the experimental design?
13. Were there any mistakes you made during the experiment? What do you think these caused?
14. List 1-2 limitations of the experimental design and suggest ways to fix them in the future.
15. In what type of solution do plant cells do best & why?
16. Give 5 situations in life in which the molarity of a solution elicits responses from organisms in it
17. Using the information you've discovered from this experiment, explain why the sailor died that drank saltwater.
18. Design another experiment about osmosis.
Alternatives & Extensions to Increase Inquiry
Compare salt/sugar/desitlled
Compare different types of potato
Ommit introduction text
Use dyalysis bags instead of potatoes
Ommit procedures and have students design their own experiments to answer research question
Sample Introduction:
Solutes are particles dissoved in a solvent. Water is the universal solvent and part of all life forms (60-90% of living material
is made of water). Thermodynamics explains that particles tend to naturely disperse evenly, or diffuse across systems. This requires no
energy as it happens thgrough molecules own kinetic energy. It happens because there is a greater probability that particles will move
move from where there are many to where there are few than the other way around, especially since this avoid collisions, disperse
heat, and increases entropy. However, barriers may impeed this motion. In life, semipermeable plasma membranes allow certain
molecules across, while blocking others.
Osmosis is the movement or diffusion of water across semipermeable membranes in response to differences in water
potential / solute concentration of solutions in each side of the membrane. The outline below summarizes the Effects of Solutions on
Cells by Osmosis & Diffusion:
 Hypotonic: Less concentrated than cell (Water potential greater outside cell)  Solute moves out and water moves in 
Overall mass increase in the cell  could lead to cytolysis (cell bursting)
 Hypertonic: More concentrated than cell  water potential greater inside cell  Solute moves in and water moves out 
Overall mass decrease in the cell  could lead to plasmolysis (cells whilting)
 Isotonic: Same concentration as cell (Water potential the same inside and outside cell)  No net movement of solute/solvent
 No net mass change  Cell in equilibrium
The solute will only move across if the membrane is permeable to it. In living cells, smaller molecules and polar molecules
have an easier time going in, while large and hydrophobic molecules are blocked.
The greater the difference between inside and outside the cell, the faster this movement will occur because the more likely it
is for particles to diffuse. Therefore, given equal period, cells will grow or shrink more (and gain/loose more mass) if they are in
solutions that are very different from their own internal concentration. This can be measured through the % mass change between
initial and final measurements. = (Final – Initial) / Initial x 100.
In this lab, we will be investigating our research question by studying osmosis of water in/out of potatoes (and
consequentially, its cells) in sucrose (disaccharide) solutions of different concentrations.
By exposing potatoes to solutions of different concentrations, the molarity of the cells in the potato can be determined and the
pattern of osmosis can be studied.
Plant cells (such as those in potatoes) have tough cell walls and large central vacules to resist and regulate turgor pressure
(osmotic pressure). This means they can tolerate fluctuations in concentration (or molarity) of solute better than many animal cells.
They tend to maintain their shapes and expand/shrink the vacuole to cope with pressure from osmosis that would otherwise cause
plasmolysis or cytolysis in animal cells. Regardeless, osmosis will still take place (even if slower and if cell shape is mostly
preserved). Therefore, the cell will gain or loose water in the end and in extreme conditions, even plant cells will undergo cytolysis
and plasmolysis. Nevertheless, because most types of plants need a constant supply of nutriends and depend on a vascular system that
uses water to carry them by capilary action (adhesion/cohesion) against gravity in their stems, plants do better in hypotonic
environments where water tends enter the cells constantly. To maintain this negative osmotic pressure, plants perform transpiration, or
loose water through their leaves.
In chemistry, the concentration of a solution is defined as how much solute is in the solvent. It is often refered to as molarity
and measured in moles (instead of simply mass/volume, which could be confused with density). For sucrose, a 1M solution will have
342g of sucrose for every liter (1000ml) of destilled water. The average molar concentration of sucrose in potato cells varies with the
type of potato used.
Most baking potatoes have sucrose concentrations between 0.30 and 0.40M. In concentrations lower than this, the potatoes
will be in a hypothonic solution and will gain mass and expand. In concentrations higher than that, the potatoes will be in hypertonic
solutions and will loose mass and shrivel. This effect should be greater the more the solution concentration deviates from the potato
concentration.
Because sucrose is a disaccharide (two-sugars linked: glucose + fructose), it is too large to go across the cell membrane by
diffusion. However, it does go through facilitated diffusion (assistance of protein channels). Since solute diffusion will be taking place
at the same, this could affect our study of the rate of osmosis through % mass change. Furthermore, if there is any loss of water to the
atmosphere, hands of person manipulating potatoes, paper towels, etc, this could affect the massing, as well. Finally, the use of the
balance should minimize instrumental error, but not human error in measuring mass. There may also be intrumental error if the
balance is not reliable (to increase reliability and validity, the experiment should be repeated, class data should be averaged, and mass
could be “double-checked” on a secondary balance). Amount of solutions in each cup used could vary (even if attempts at precise
measurements were made). Though it may not affect a measurement of % mass change, it could lead to slighly greater pressures if
there is too much or too litlte liquid for exchange with potato when compared to other samples.
Sample Hypotheses:
The smaller the molarity of the sucrose solution, the greater the percentage mass change will be.
While solutions with molarity above 0.3 will cause the potato to loose mass, others will cause the potato to gain mass.