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Project title or topic of activity

Awesome Osmosis

Author(s): Cliff Gross, Josh De Zeeuw, Ted Simpao

Date: 4-27-2001


Summary of Activity

In this station the students will learn about osmosis and salt regulation in marine organisms. The topics covered include the salinity differences between fresh and salt water; why marine organisms must be able to control salt levels; and finally, how various marine organisms control their salt levels. The station includes two hands-on activities. The first activity is an experiment where students place sections of a potato into solutions of differing levels of salinity. This introduces students to the concept of osmosis, along with hypertonic, hypotonic, and isotonic environments. The second activity involves looking at freshwater plant cells under a microscope. The students compare the cell’s original appearance with their appearance after saltwater has been added. This activity reinforces the concept of osmosis and hyper/hypo/iso tonicity. Finally, students discuss various marine organisms deal with the osmotic effects of their environment.


Grade levels

5 -8

Background information


  • Concentration: the amount of stuff dissolved in solution. (Seawater has a higher salt concentration than fresh water).
  • Diffusion: the dispersal of matter within an environment such that it becomes equally concentrated throughout the environment.
  • Hypertonic: a solution containing a greater amount of dissolved stuff than a creature or object in the solution.
  • Hypotonic: a solution containing a lesser amount of dissolved stuff than a creature or object in the solution.
  • Isotonic: a solution containing an equal amount of dissolved stuff than a creature or object in the solution.
  • Osmoregulation: the process of regulating the amount of salt and other dissolved substances to control the loss or gain of water from osmosis.
  • Osmosis: the diffusion of water across a semi-permeable membrane.
  • Salinity: the relative amount of salt dissolved in water. (Seawater has a higher salinity than freshwater).
  • Semi-permeable membrane: a membrane that permits the free passage of water but prevents the passage of a dissolved substance like salt.

The fundamental concepts of this station are salinity and osmoregualtion. Salinity refers to the amount of salt dissolved in water. Freshwater in lakes, rivers, and streams has a lower salt content and thus a lower salinity than seawater. The salinity of the surrounding environment is an important constraint that marine organisms must deal with in order to survive.

Diffusion refers to the "desire" of all matter to be equally concentrated in its environment. If a large concentration of something is put into a particular region of the environment, it will disperse until its concentration is uniform throughout the environment, provided it does not encounter any barriers through which it cannot pass. Salt exists in water as sodium ions (Na+) and chloride ions (Cl-). Charged ions like sodium ions and chloride ions are unable to pass through most biological membranes. However, water molecules are able to pass through most biological membranes so salinity imbalances within biological systems are naturally corrected via the diffusion of water across semi-permeable membranes to equalize salt concentrations on both sides of the membrane. This process is known as osmosis.

When the concentration of dissolved solids, such as salt, are equal on both sides a semi-permeable membrane, the solution is said to be isotonic and there is no net flow of water to either side of the membrane. As a result, there is no net change in salinity in an isotonic solution. When the concentration of dissolved solids (salt) is greater inside a semi-permeable membrane than outside the membrane, the solution outside the membrane is said to be hypotonic and water will diffuse across the membrane from outside to inside in an effort to decrease the salinity inside the membrane. When the concentration of dissolved solids (salt) is greater outside a semi-permeable membrane than inside the membrane, the solution outside the membrane is said to be hypertonic and water will diffuse across the membrane from inside to outside in an effort to decrease the salinity outside the membrane.

Because water will naturally diffuse across biological membranes from regions of lower salinity to regions of higher salinity, marine organisms and freshwater must take great care to maintain the proper balance of water and salt within their bodies to sustain life, through the process known as osmoregulation. The different salinities of freshwater and seawater present different challenges to the organisms that live in these habitats. Organisms living in seawater must have a means of preventing the loss of water from the body to the highly saline and potentially hypertonic environment. Freshwater organisms must deal with the opposite problem of preventing excessive amounts of water from the potentially hypotonic freshwater environment entering their highly saline bodies. And organisms such as salmon, who are capable of living in both freshwater and seawater, must have a means of dealing with the different salinities of these different habitats.

[Note: The seawater environment is not hypertonic to all marine organisms, just as the freshwater environment is not hypotonic to all freshwater organisms. Implications of this and reasons for this will be discussed later in the activity.]

Credit for the activity

The potato experiment utilized in this station was borrowed from the "Diffusion and Osmosis" activity found in Patricia Kennedy’s Human Biology workbook. Eric Brooks, a science teacher who has been helping our class throughout the semester, suggested the microscope activity. Additionally, both Professor Katrina Mangin and Patricia Kennedy have provided us with extensive assistance in the design of this station.

Estimated time to do the activity/activities

The station, including both activities and discussion, should last approximately 30 minutes per group of students. The station can be shortened to approximately 15 - 20 minutes by eliminating the microscope activity.

Goals of Activity/Activities:


By the end of this station students should be able to:

  1. Understand osmosis and be able explain the differences between isotonic, hypertonic, and hypotonic solutions.
  2. Describe some of the methods of osmoregualtion by freshwater and marine organisms


Materials Needed

Potato experiment:

  • Sectioned potato (see "Preparation and Teacher Heads-Up" below).
  • Cookie cutters of equal size (preferably in the shape of a fish).
  • 3 small beakers or clear dishes.
  • Table salt.
  • Water.
  • Data sheet (see attached).

Microscope activity:

  • Microscope.
  • Prepared microscope slide with cover slip (see "Preparation and Teacher Heads-Up" below).
  • Leaves of Elodea/Anachris plant (available at any aquarium store).
  • Eyedropper.
  • Water.
  • Table salt.
  • Paper towels

Preparation & teacher "heads up"

Potato experiment:

Prior to the potato experiment, the activity leader should cut the potato into sections of approximately equal size. This cuts down on the activity time and prevents the students from having access to dangerous sharp objects.

Microscope activity:

Prior to the microscope activity, the activity leader should prepare the microscope slide by placing a single leaf from the Elodea/Anachris plant on a microscope slide, placing a few drops of water on the leaf, and covering it with a cover slip. The activity leader should also focus the microscope on a group of cells within the plant so that younger students do not actually handle and thus potentially damage the microscope.

Step-by-Step Procedure

Procedure for the Potato Activity

  1. Divide students into groups. Depending on class size, groups should range from 3 — 6 students per group.
  2. Give each group 3 pre-cut potato slices, a cookie cutter, salt, and 3 beakers to mix their solutions.
  3. Have students cut one shape out of each potato slice using the cookie cutter.
  4. Give students six possible salt concentrations to add to the water in their solution. Make sure some are very small and some are large.
  5. Have students prepare their 3 chosen solutions in the beakers using salt and water.
  6. Have students place one potato shape in each of the 3 solutions.
  7. Wait 10 - 20 minutes (the length of time depends on the total time allotted for the station and whether or not students will participate in the microscope activity) for experiment to osmosis to occur.
  8. During the 10 — 20 wait period, the potato-containing solutions will be allowed to sit undisturbed so osmosis can occur. The students will be introduced to the concept of osmosis by the activity leader through interactive discussion, and (if time allows) participate in the microscope activity.
  9. Try to lead students through discussion of salinity and osmoregulation using their previous knowledge to help them understand the concepts.
  10. Once the wait period has ended, students will remove potato the slices from solutions and try to reinsert the potato slices back into the potato section from which they were cut.
  11. Based on the "feel" of reinsertion, students will complete the data sheet (see attached) and discuss the results of the experiment with the activity leader.

Items for Discussion and Conclusion (Potato Activity):

  1. What happened to the potato in each of the solutions? Why?
  2. How could the student have prepared an isotonic solution? Hypertonic? Hypotonic?


Procedure for the Microscope Activity:

  1. Have each student look at the Elodea/Anachris cells under the microscope.
  2. The activity leader will prepare a solution of high salinity and place a few drops of this solution on the microscope slide at one edge of the cover slip. At the same time, the activity leader will place a piece of paper towel at the adjacent edge of the cover slip. This will create a system of flow, such that the salt water flows through the cells of the Elodea/Anachris plant and get soaked up by the paper towel.
  3. Have students look at the Elodea/Anachris cells after 1 — 2 minutes in the salt-water environment and note any changes that they observe.
  4. Note: The plant cells will adapt will adapt to the salty environment by losing water to environment via osmosis. As a result, the cell membranes will appear shrunken and the green chloroplasts will congregate in the center of each cell.

  5. The activity leader will repeat steps 2 and 3 using regular tap water instead of salt solution.
  6. Have students look at the Elodea/Anachris cells after 1 — 2 minutes in the fresh water environment note any changes that they observe from the previous salt solution condition.

Note: The plant cells will again adapt to their new less-salty environment via osmosis. As a result, the cell membranes will expand to fill the entire volume of each cell and the green chloroplasts will spread out throughout the membrane.

Items for Discussion or Conclusion (Microscope Activity):

  1. Why did the cells shrink when given the salt-water solution?
  2. Why did the cells expand when given the fresh water solution?

Images, work sheets, additional web pages

Make a data sheet that looks like the one below to collect data for the Potato Experiment.

Potato Experiment Data Sheet

Concentration of Solution
"Feel" of Reinsertion
Osmotic Character of Solution
Solution A




Solution B




Solution C






Items for discussion or conclusion


The concluding discussion is an important component part of the station. The experiment and intervening discussion should have introduced students to the idea of concepts of osmosis. Now it is time for the students to apply what they have learned to marine and freshwater organisms and discover how certain organisms deal with the process of osmoregulation. While the direction of the discussion is ultimately up to the activity leader and the students, we have included both an outline for activity leaders to follow as a guideline and a description of how selected marine and freshwater organisms osmoregulate.


  1. Have students discuss the osmoregulatory problems associating with living in fresh water and living in salt water.
  2. Introduce students to the primary organs of the excretory system, the organ system that salt-water and fresh-water organisms use for osmoregulation. These are the kidneys, the bladder, the gills, and the salt gland (in certain species).
  3. Use the information contained in the list below to explain how certain fresh-water and salt-water organisms osmoregulate.

Osmoregulation in various fresh-water and salt-water organisms:

Freshwater Snails: Fresh water snails have an outer shell that protects a large part of their surface from the osmotic inflow of water. The kidneys and excretory system provide additional osmoregulation.

Marine invertebrates: Marine invertebrates have body fluids that are isotonic to the surrounding environment. Marine invertebrates like lobsters, crabs, and shrimp taste "salty" because their bodies must contain lots of salt to keep their body fluids isotonic to their salt-water homes!

Fresh-water fishes: Fresh water fishes maintain body fluid concentrations live in a hypotonic environment and thus obtain water directly from osmotic uptake from their environment. These fish generally have highly developed kidneys and active excretory systems to keep the osmotic uptake of water from getting out of hand.

Marine teleosts (most fish): The salty environment draws water from the fish via osmosis. The fish compensate by greatly increasing their water intake. Teleost gills have chloride-secreting cells that help to put ingested salts back into the environment. Teleosts also have highly developed kidneys and excretory systems.

Sharks, Skates, and Rays: The main form of osmoregulation in sharks, skates, and rays is a specialized salt excretion gland (such as that found in the spiny dogfish).

Lamprey: The lamprey has osmoregulatory mechanisms similar to those of marine teleosts.

Sea Turtle: The sea turtle, like other reptiles, has specialized salt glands that excrete salt. The shell also provides a barrier to the loss of water to the hypertonic environment.

Marine mammals: Excretory organs are designed to help conserve water. Urine is excreted as a semi fluid paste.

Salmon: Salmon have well designed excretory systems that can adjust to their dual fresh-water and salt-water habitats.


The assessment of the students’ learning should be based on answers to the questions in the two "Discussion and Conclusion" sections. If the students have fully understood the concepts of the station the questions should be easily answered. Additionally, the concluding discussion of osmoregulation in marine and freshwater organisms gives students the opportunity to apply what they have learned to conditions faced by fresh-water and salt-water organisms. Students should be able to describe the different osmotic problems faced by fresh-water and salt-water organisms.


Beyond the Activity
Further activities which relate to and extend the complexity of the experiment.

Students could be asked to use the concepts learned from the potato experiment to explain why their fingertips shrivel when they go swimming. Students should be able to explain the osmotic state (hyper/hypo/isotonic) of the water relative to their fingers.

Web Resources
A web address with information on the topic of the activity.

Web Address

Additional References


Karnaky Jr., Karl, J. (1998). "Osmotic and Ionic Regulation." In The Physiology of Fishes. 2nd ed. Boca Raton: CRC Press.

Krogh, August. (1965). Osmotic Regulation in Aquatic Animals. New York: Dover Publications.

Lutz, Peter L. (1997). "Salt, Water, and pH Balance in the Sea Turtle." In The Biology of Sea Turtles. Boca Raton: CRC Press.

Vernberg, Winona, B. Vernberg, F. John. (1972). Environmental Physiology of Marine Animals. New York: Springer-Verlag.


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