Name: _______________________________ Date: ________________ Period: _________
Anatomy and Physiology: LAB OSMOSIS
A shipwrecked sailor is stranded on a small desert island with no fresh water to
drink. He knows he could last without food for up to a month, but if he didn't
have water to drink he would be dead within a week. Hoping to postpone the
inevitable, his thirst drove him to drink the salty seawater. He 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!
Objectives
When you have successfully completed this lab, you should be able to:
1. Explain the principles of diffusion and osmosis.
2. Know that the concentration of a solute in a solution will affect the movement of water across a
cell membrane.
3. Write a lab report explaining your work.
Introduction:
The plasma membrane of the cell is selectively permeable, which means that it allows the passage of
some substances while it prevents the passage of others. Cells constantly move molecules across the
plasma membrane, into and out of the cytoplasm. Some molecules are actively transported across the
membrane, a process which requires transport proteins and energy input. Active transport occurs
against a concentration gradient. In contrast to this active transport, diffusion and osmosis describe
the movement of molecules along the concentration gradient in a passive process that does not
require cellular energy.
A gradient is simply the condition that exists when something is distributed unevenly. One example
of a gradient is a temperature gradient. It is usually warmer in the south and cooler in the north. The
temperature is distributed unevenly. Another example of a gradient is the difference in altitude
between a lake high in the mountains and the ocean below. This example is a useful way to think
about the potential energy inherent in a gradient. The water in the lake will tend to flow towards the
ocean, not the other way around. This tendency to flow from high to low is analogous to the tendency
of dissolved molecules to move from an area of high concentration to an area of low concentration.
We all know that energy in the form of hydroelectric power can be harvested as water flows
downhill.
Diffusion is the movement of dissolved molecules from a region of high concentration to a region of
low concentration. This process tends to spread the molecules out. Keep in mind that diffusion occurs
even in the absence of a membrane.
Osmosis is the diffusion of water molecules across a selectively permeable membrane. Selectively
permeable membranes allow some substances, including water, to pass freely through the membrane,
but block the passage of others. As in diffusion, the water molecules move from a solution with a
higher concentration of water to a solution with a lower concentration of water.
Procedure:
Like any molecule, water can be present in high or low concentrations. While it may at first seem
counter-intuitive to imagine one solution with a higher concentration of water than another solution,
this actually happens all the time. Think about a 5% salt solution. By weight this solution is 5% salt
and 95% water. Imagine that this solution was placed across a semipermeable membrane from a
distilled (0% salt and 100% water) water solution. Water molecules would move down their
concentration gradient from the high concentration to the low concentration, i.e., from the 100%
solution to the 95% solution, but the membrane would prevent the salt molecules from moving.
Materials: Potato, 3 plastic cups, markers, salt, sugar, distilled water, paper, pencil, electronic
balance, timer, metric ruler, small Ziploc plastic bag, foil, and corer.
Procedure:
1. Use a blade to square off the ends of your potato. Your potato’s cell will act like the sailor’s
cell.
2. Stand your potato on end and use your corer to bore 3 vertical holes.
3. Remove the potato cylinders from the borer and measure their length in cm.
4. Cut the 3 potato cylinders or cubes (to the same length), about 4-5 cm long.
5. Record the length and turgidity of the potato cylinders in your data table.
6. Place the potato cylinders in the Ziplock bags to prevent dehydration before use.
7. Take three cups and label them accordingly with sugar, salt, and distilled water.
8. Now fill each cup 2/3 with the correct solution.
9. Mass each of the potato cylinders and record this mass in your data table (blot off your
cylinders first).
10. Place one of your potato cylinders in each cup and cover the top with the foil. Leave the
potato cylinders n the cup for about 45 minutes.
11. Carefully remove the potato cylinder from the distilled water solution, pat it dry on a paper
towel.
12. Measure the length and record the length and the appearance in the data table.
13. Measure and record the mass as well.
14. Repeat step 12 and 13 for the remaining potato cylinders in the salt and sugar solution.
15. Clean up your area and return all materials to proper location.
Data:
Results of Osmosis in Potato Cells
Initial Final Change Initial
length length
in
Mass
Solution
cm
cm
length
g
(day1) (day2)
cm
(day1)
Tonicity of
Final
Initial
Final
Change
Solution
Mass
Turgidity Turgidity
in mass
(iso-, hypo-,
g
(flaccid (flaccid
g
or
(day2)
or crisp) or crisp)
hypertonic)
Distilled
water
Salt
Solution
Sugar
Solution
Results and Conclusion:
1. Did any of the potato cylinders change in their turgidity (flexibility), and if so, which ones
changed?
2. Explain why the flexibility of the potato slices changed.
3. Define isotonic, hypotonic, & hypertonic solutions.
4. If potato slices changed in length or turgidity, what process was responsible for this?
5. Make a sketch of your potato cylinder in the distilled water and use arrows to show the
direction of water movement across the potato cell membranes.
6. What type of solutions were the salt & sugar solutions. Explain how you know this.
7. Which solution served as the control for this experiment & why?
8. In which solutions was there a greater solute concentration outside of the cells?
9. In which direction did water move through these cell membranes?
10. Using the information you've discovered from this experiment, explain why the sailor died
that drank saltwater.