The Huntington Library, Art Collections, and Botanical Gardens
The You in Diffusion
Overview
A peeled grape is used to demonstrate osmosis and water potential while the cells of
Elodea or a red onion skin, when put in a hypertonic solution, demonstrate plasmolysis.
Introduction
A molecule or ion dissolved in a liquid is called a solute, whereas the liquid that does the
dissolving is referred to as the solvent; the mixture is called a solution. Water is often called the
“universal solvent” due to its ability to dissolve so many molecules.
Due to the random movement of molecules, they have a net movement from areas of
higher concentration to lower concentration, until they become homogenous in a solution. This
is called diffusion. This can be demonstrated by opening a jar of perfume in the classroom, or
adding a drop of dye to a beaker of water.
Water also has the potential energy to flow from areas of higher to lower concentrations.
This energy is called water potential. Pure water has a water potential of zero, and any solution
of water has a negative potential (think of this as less-concentrated water). Therefore, water will
move from areas of high to low water potentials. This can also be described as the movement
from hypotonic (more water concentration, less solute concentration) to hypertonic (more solute,
less water). Water can do such across a biological membrane, such as a cell’s plasma membrane;
this process is called osmosis.
Plant cells are hypertonic to their environment. Therefore, water creates a hydrostatic
pressure, or turgor pressure, in the cell. Turgor pressure is involved in support, cell expansion,
and plant movements. When plants loose their turgor pressure, the cytoplasm will pull away
from the cell wall (plasmolysis). The loss of turgor and plasmolysis causes wilting in the plant.
The loss of turgor pressure due to a forced transport of solutes quickly out of leaf cells is thought
to be the mechanism that causes Venus’ fly traps to close.
Motivation
Why do your fingers look like raisins after spending a long time in the water? How do
Venus’ fly traps close upon their prey? Why do we put carrots in water to make them crisp after
they have started to look wilted? We can conduct a simple experiment with grapes to
demonstrate the science that is involved in all three of these scenarios.
Objectives
At the completion of this laboratory, students should be able to
1. Define water potential, hyper/hypotonic, osmosis, plasmolysis, turgor pressure, osmosis,
diffusion, solute, and solvent.
2. Predict the effects on a cell or tissue placed in hyper- and hypotonic solutions.
3. Set up and perform a visual illustration of osmosis and plasmolysis.
Materials
Salt
Distilled water
Graduated cylinder
3 grapes per group
3 beakers per group
Towels
Scale
Spoons
Elodea leaves and/or red onion
Microscope slides
Cover slips
Microscope
Kimwipes
Associated California Biology Standards:
1a. Students know cells are enclosed within semipermeable membranes that regulate their
interaction with their surroundings.
Procedure
Part I
1. You may wish to have the students work in groups with each group completing the entire lab
and compile the results as replications. Alternatively, you can divide up the tasks of the lab,
having each group work with a single concentration, compiling class results at the end.
2. Have students carefully peel the skins off three grapes, one for each concentration. The
grapes should then be dabbed dry with a towel and each weighed. Students must record the
weight of each grape, keeping them separate so they know which is which.
3. Have the students write “distilled,” “10%,” and “25%,” on separate labels. If you’d like, you
can make one unknown for your class as well. Place the labels on beakers.
4. Because 1 gram of water is equivalent to 1 milliliter, to make a 10% salt solution, you would
replace 10% of the volume with salt, i.e., dissolve 10g of salt in 90ml of water; for 25%, add
25g of salt to 75ml of water. Your students should work through these calculations. You may
wish to make an “unknown” for your students.
5. Have students gently place a grape in each labeled beaker.
6. At each 15-minute interval for an hour, students should use the spoon to the remove the
grapes from the beakers, carefully pat dry the grape with a towel, and weigh it.
7. Have students record the weight of the grapes each time and note any changes observed.
Part II
8. Instruct students in the proper way to prepare a wet mount of an Elodea leaf with distilled
water and have them prepare their own. You can also use the inside peeled skins of a red
onion.
9. Have students observe the leaf or onion skin at 40x and 100x and record observations.
10. Next, instruct students to remove the slide from the stage of the microscope and to place 2
drops of a 10% salt solution on the slide at the edge of the cover slip. Have them place a
Kimwipe on the slide at the edge of the cover slip that is opposite the side where the salt
solution was placed. It should begin to soak up water, drawing the salt solution under the
cover slip as it does so.
11. Student should now return the slide to the microscope stage and repeat the observations of
the cells at 40x and 100x and prepare sketches of a group of the cells under each set of
conditions. Ensure that students label the sketches to note the cell structures that they can
identify. Be sure to highlight that they should look for and note any changes in the color,
size, and shape of the cell. If your students are not seeing a change, they may use a higher
concentration of salt solution.
12. Next, have students flush out the salt water with distilled by the same method as it was
introduced, repeat the previous step, and then record and draw observations.
13. Finally, instruct the students to properly clean the slide and the cover slip after they remove it
from the stage.
Evaluation
The following questions are listed under the Analysis section of the student handout and may be
used as part of a report, class discussion or assessment.
1. What happens to the grape when placed in distilled water? Why?
2. What happens to the grape when placed in 10% salt solution? Why? How does that compare
to an egg placed in 25% salt solution?
3. Why do you think, based upon what you learned from this experiment, that our skin tends to
wrinkle when we’ve been in water for an extended period of time?
4. What happens to the Elodea or onion cells as the salt water flows under the cover slip? Why?
5. What happens to the Elodea or onion cells when the salt water is flushed out with distilled
water? Why?
6. Elodea normally lives in fresh water. What changes would you observe in the cells of an
Elodea plant that was suddenly moved from fresh water to salt water? Why?
Extension Activities:
1. Try a “carrot pump” activity with your class, such as the one outlined at
http://resources.yesican-science.ca/lpdd/g08/demos/pump.html. How is osmosis involved and
what is happening?
Test Preparation:
1. An animal cell in a hypertonic solution would
(A)
Swell.
(B)
Swell and exhibit turgor.
(C)
Exhibit plasmolysis
(D)
Not be affected; only a plant cell would be affected.
2. Which one of the following would not normally diffuse through the lipid bilayer of a plasma
membrane?
(A)
CO2
(B)
Amino acid
(C)
starch
(D)
water
(E)
O2
Student Sheet: The “You” in Diffusion
Name:________________________
Procedure
Part I
1. Peel the skin from three grapes and pat them dry.
2. Weigh the grapes when dry and record the start weight.
3. Write “distilled,” “10%,” and “25%,” on separate labels. Place the labels on beakers.
4. Knowing that 1 gram of water is equivalent to 1 milliliter, devise a method to create a 10%
and 25% salt solution. Check this with your teacher and proceed to mix up the solutions
when approved.
5. Gently place a grape in each beaker and record the start time.
6. At 15 minute intervals, use the spoon to the remove the grape from the beaker, carefully pat
dry the grape with a towel, and weigh it.
7. Record the weight of the grape and note any changes you observe.
Table 1: Grape weights
Distilled water
0 min
15 min
30 min
45 min
1 hour
10%
25%
Part II
8. Prepare a wet mount of an Elodea leaf or red onion skin with distilled water. Observe the leaf
at 40x and 100x and record your observations on the back of this sheet. Include sketches of
the cells at each magnification. Label any cell structures that are visible
9. Remove the slide from the stage of the microscope. Place 2 drops of a 10% salt solution on
the slide at the edge of the cover slip. Place a Kimwipe on the slide at the edge of the cover
slip that is opposite the side where the salt solution was placed. It should begin to soak up
water, drawing the salt solution under the cover slip as it does so.
10. Return the slide to the microscope stage and repeat the observations of the cells at 40x and
100x. Again on the back of this sheet, prepare sketches of the cells you see under each set of
conditions and record your observations. Label the sketches to note the cell structures that
you can identify. Be sure to note any changes in the color, size, and shape of the cell.
11. Now flush out the salt water with distilled by the same method as it was introduced. Repeat
step 10 to record and draw observations.
12. Remove the slide from the stage and properly clean the slide and the cover slip.
Analysis
On a separate sheet of paper please complete the following:
1. What happens to the grape when placed in distilled water? Why?
2. What happens to the grape when placed in 10% salt solution? Why? How does that compare
to a grape placed in 25% salt solution?
3. Why do you think, based upon what you learned from this experiment, that our skin tends to
wrinkle when we’ve been in water for an extended period of time?
4. What happens to the Elodea or onion cells as the salt water flows under the cover slip? Why?
5. What happens to the Elodea or onion cells when the salt water is flushed out with distilled
water? Why?
6. Elodea normally lives in fresh water. What changes would you observe in the cells of an
Elodea plant that was suddenly moved from fresh water to salt water? Why?