SCOPE,
SEQUENCE,
COORDINATION
and
A National Curriculum Project for High School Science Education
This project was funded in part by the National Science
Foundation. Opinions expressed are those of the authors
and not necessarily those of the Foundation. The SS&C
Project encourages reproduction of these materials for
distribution in the classroom. For permission for any other
use, please contact SS&C, National Science Teachers
Association, 1840 Wilson Blvd., Arlington, VA 22201-3000.
Copyright 1996 National ScienceTeachers Association.
SCOPE, SEQUENCE,
and
COORDINATION
SS&C Research and Development Center
Gerry Wheeler, Principal Investigator
Erma M. Anderson, Project Director
Nancy Erwin, Project Editor
Rick McGolerick, Project Coordinator
Arlington, Va., 703.312.9256
lowa School Sites and Lead Teachers
Pleasant Valley H.S., William Roberts
North Scott H.S., Mike Brown
North Carolina Coordination Center
Evaluation Center
Charles Coble, Center Co-Director
Jessie Jones, School Coordinator
East Carolina University, 919.328.6172
Frances Lawrenz, Center Director
Doug Huffman, Associate Director
Wayne Welch, Consultant
University of Minnesota, 612.625.2046
North Carolina School Sites and Lead Teachers
Tarboro H.S., Ernestine Smith
Northside H.S., Glenda Burrus
Houston SS&C Materials Development
and Coordination Center
Puerto Rico Coordination Center*
Linda W. Crow, Center Director
Godrej H. Sethna, School Coordinator
University of Houston-Downtown, 713.221.8583
Manuel Gomez, Center Co-Director
Acenet Bernacet, Center Co-Director
University of Puerto Rico, 809.765.5170
Houston School Sites and Lead Teachers
Jefferson Davis H.S., Lois Range
Lee H.S., Thomas Ivy
Jack Yates H.S., Diane Schranck
Puerto Rico School Site
UPR Lab H.S.
* * * * * * * * * * * *
California Coordination Center
Tom Hinojosa, Center Coordinator
Santa Clara, Calif., 408.244.3080
California School Sites and Lead Teachers
Sherman Indian H.S., Mary Yarger
Sacramento H.S., Brian Jacobs
Pilot Sites
Site Coordinator and Lead Teacher
Fox Lane H.S., New York, Arthur Eisenkraft
Georgetown Day School, Washington, D.C.,
William George
Flathead H.S., Montana, Gary Freebury
Clinton H.S., New York, John Laffan*
Iowa Coordination Center
Robert Yager, Center Director
University of Iowa, 319.335.1189
*not part of the NSF-funded SS&C Project.
Advisory Board
Project Associates
Dr. Rodney L. Doran (Chairperson),
University of Buffalo
Bill G. Aldridge
SciEdSol, Henderson, Nev.
Dr. Albert V. Baez, Vivamos Mejor/USA
Dorothy L. Gabel
Indiana University
Dr. Shirley M. Malcom, American Association
for the Advancement of Science
Dr. Shirley M. McBay, Quality Education for Minorities
Dr. Paul Saltman, University of California-San Diego
Dr. Kendall N. Starkweather, International
Technology Education Association
Dr. Kathryn Sullivan, Ohio Center of
Science and Industry
Stephen D. Druger
Northwestern University
George Miller
University of California-Irvine
National Science Education Standard—Life Science
The Cell
Cells have particular structures that underlie their functions. Every cell is
surrounded by a membrane that separates it from the outside world. Inside the
cell is a concentrated mixture of thousands of different molecules that form a
variety of specialized structures that carry out such cell functions as energy
production, transport of molecules, waste disposal, synthesis of new molecules,
and the storage of genetic material.
Teacher Materials
Learning Sequence Item:
1025
Cell Size and Shape; Diffusion and Osmosis Processes
March 1997
Adapted by: Bill George, Duane Dawson, and Tom Hinojosa
Cell Structures That Underlie Cell Functions. Students should investigate diffusion and osmosis as important
processes in cell maintenance and distinguish between hypotonic, hypertonic, and isotonic solutions. They should
understand that a membrane is a boundary and relate the structure of the cell membrane to the observed processes of
diffusion and osmosis. Students should understand how cell size and shape are related to surface-to-volume ratio and
how that ratio limits cell size and function (Biology, A Framework for High School Science Education, p. 86).
Contents
Matrix
Suggested Sequence of Events
Lab Activities
1. Solutions and Cells
2. Onion Soup
3. Osmosis Demonstration
4. Movin’ On In, Movin’ On Out
5. There’s Always Room for Gel-Oh!
6. A Cube Is a Cube Is a Cube
Assessments
1. Outside Lookin’ In
2. Salty Chops
3. Speaking of Boundaries . . .
This micro-unit was adapted by Bill George (Georgetown Day School, Washington, D.C.), Duane Dawson (Pleasant Valley High School,
Pleasant Valley, Iowa), and Tom Hinojosa (California SS&C Project, Santa Clara)
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Cell Structures That Underlie Cell Functions. Students should investigate diffusion and osmosis as important
processes in cell maintenance and distinguish between hypotonic, hypertonic, and isotonic solutions. They
should understand that a membrane is a boundary and relate the structure of the cell membrane to the observed
processes of diffusion and osmosis. Students should understand how cell size and shape are related to surfaceto-volume ratio and how that ratio limits cell size and function (Biology, A Framework for High School Science
Education, p. 86).
Learning Sequence
Science as Inquiry
Science in Personal
and Social Perspectives
Science and Technology
Solutions and Cells
Activity 1
Onion Soup
Activity 2
Osmosis Demonstration
Activity 3
Movin’ On In, Movin’ On
Out
Activity 4
There’s Always Room for
Gel-Oh!
Activity 5
A Cube Is a Cube Is a Cube
Activity 6
Outside Lookin’ In
Assessment 1
Salty Chops
Assessment 2
Speaking of Boundaries ...
Assessment 3
4
History and Nature
of Science
Suggested Sequence of Events
Event #1
Lab Activity
1. Solutions and Cells (45 minutes)
Alternate or Additional Activities
2. Onion Soup (40 minutes)
3. Osmosis Demonstration (20 minutes)
Event #2
Lab Activity
4. Movin’ On In, Movin’ On Out
Part 1: Diffusion (45 minutes)
Part 2: Osmosis (45 minutes)
Part 3: Optional Exercise (45 minutes)
Event #3
Lab Activity
5. There’s Always Room for Gel-Oh! (20 minutes)
Event #4
Lab Activity
6. A Cube Is a Cube Is a Cube (30 minutes)
Event #5
Readings from Science as Inquiry, Science and Technology, Science in Personal and
Social Perspectives, and History and Nature of Science
Suggested readings:
Thomas, Lewis, “The World’s Biggest Membrane.” The Lives of a Cell: Notes of a
Biology Watcher. New York: Bantam Books, Inc., 1974.
Assessment items are at the back of this volume.
5
Assessment Recommendations
This teacher materials packet contains a few items suggested for classroom assessment. Often, three
types of items are included. Some have been tested and reviewed, but not all.
1. Multiple-choice questions accompanied by short essays, called justification, that allow teachers to
find out if students really understand their selections on the multiple choice.
2. Open-ended questions asking for essay responses.
3. Suggestions for performance tasks, usually including laboratory work, questions to be answered,
data to be graphed and processed, and inferences to be made. Some tasks include proposals for
student design of such tasks. These may sometimes closely resemble a good laboratory task, since
the best types of laboratories are assessing student skills and performance at all times. Special
assessment tasks will not be needed if measures such as questions, tabulations, graphs, calculations,
etc., are incorporated into regular lab activities.
Teachers are encouraged to make changes in these items to suit their own classroom situations and to
develop further items of their own, hopefully finding inspiration in the models we have provided. We
hope you may consider adding your best items to our pool. We also will be very pleased to hear of
proposed revisions to our items when you think they are needed.
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Activity 1
Teacher Sheet
Science As Inquiry
Solutions and Cells
How do substances move in and out of cells?
Overview:
This activity begins the micro-unit by providing students with a measurable example of movement of
a substance into and out of a model cell—a salmon egg. Students create a hypertonic solution of salt and
water and compare its effect on salmon eggs with that of distilled water. The activity does not attempt to
explain how this happens but provides students with the empirical evidence that it does happen. The
concept of osmosis as a form of diffusion is introduced, as well as the terms hypotonic solution, isotonic
solution, and hypertonic solution. Subsequent activities in the micro-unit will incorporate the role of the
cell membrane in controlling transport of materials into and out of the cell and explore how this process
is related to cell size.
Remind students to use caution when using the scalpel or scissors to cut open eggs.
Materials:
Per lab group (2 or 3):
balance
beakers (100 mL), 2
distilled water
petri dishes, 2
ruler (mm)
salmon eggs, 12
salt (to make saltwater solution),
approx. 15 g of salt per 250 mL water)
scalpel or dissection scissors
Procedure:
Students should first examine and measure their eggs prior to the experimental treatment. Have them
carefully line up at least 10 eggs and measure the length of the row to the nearest millimeter. They
should then record the length of the row and their observations on the general appearance and state of the
eggs, for example, their hardness and texture. It is recommended that they use at least 10 eggs for their
measured rows because any change in egg size during the experiment will be more easily measured as a
total of 10 eggs rather than as a single egg. Millimeter rulers with a center groove will allow easy measurement. To get the expected experimental results, measurements must be made very carefully, and lab
partners should verify each other’s measurements.
While one lab partner measures and records observations on the eggs, another lab partner should
prepare the distilled water and the “concentrated” saltwater solution, recording the amounts of salt and
water actually used. Be sure students are given enough salt to create a solution roughly equal to 15 g of
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Activity 1
salt per 250 mL water, and that they label each beaker. Each beaker will require at least 25–35 mL of
water.
Have students place the measured eggs into a labeled beaker of distilled water for at least 10 minutes.
While waiting, they could examine extra eggs with a dissecting scope or hand lens, calculate the exact
concentration of their saltwater solution, or do related reading. After 10 minutes they should remove the
eggs from the water and line them up and measure them as before. The eggs are then returned to the
water bath for another 10-minute interval and removed and measured again.
Next, students place the eggs into the saltwater solution. (Note: If the eggs sink in this solution,
students may not have used enough salt.) Using the same procedure as with the distilled water, they
should immerse the eggs and then remove and measure them. They should also note any observations
about the texture, hardness, and general appearance of the eggs. All measurements and observations
should be recorded in a suitable data table created by students in their lab notebooks.
Background:
It is expected that the eggs will swell when placed in the distilled water and shrink slightly when
placed in the salt solution, both results due to osmosis. Water should diffuse into the eggs in the distilled
water (hypotonic) condition, thus causing the eggs to swell. When compared to their original condition,
this row of eggs should measure 3 to 10 mm longer. Water should diffuse out of the eggs in the saltwater
(hypertonic) condition, causing the eggs to shrink slightly. This row of eggs can be expected to measure
up to 1–3 mm less in length than it did prior to being placed in the salty water.
Salmon eggs are a common fish bait and can be easily obtained at local bait shops or fishing supply
outlets (a bottle of about 80 eggs is approximately $5). Do not use synthetic versions. You should test the
egg supply before purchasing large quantities to be sure they work as expected in this activity (due to
variations in preparation and packaging, some brands work better than others).
It is assumed that students have previously studied properties of aqueous solutions, concentration, and
the basic phenomenon of diffusion (see micro-unit 1024). If not, you may need to spend extra time in
this activity covering the concepts of solution, solute, and concentration. This will help students to better
understand the terms hypotonic, hypertonic, and isotonic introduced here. Some useful terms and definitions for you are:
Concentration—amount of solute dissolved in a given amount of solvent
Diffusion—movement of molecules of a substance from areas of high concentration of that substance
to areas of lower concentration
Osmosis—diffusion of water through a membrane
Hypertonic solution—solution in which the concentration of the solutes outside a cell is greater than
that inside the cell
Hypotonic solution—solution in which the concentration of solutes outside a cell is lower than that
inside
Isotonic solution—solution in which the concentration of solutes outside a cell is the same as that
inside the cell
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Activity 1
Water makes up 70 to 95 percent of a living cell. Since water is the most abundant substance in cells,
its movement into and out of cells is of vital importance. The cell has no control over osmosis. Water
will flow into or out of a cell depending on the concentration of water molecules on either side of the
membrane. Water moves across the membrane in order to reach a state of equilibrium. Water will continue to diffuse back and forth across the cell membrane even after equilibrium is established. However,
at equilibrium the number of molecules moving into the cell equals the number moving out.
Cell Membrane
Isotonic Solution
Hypotonic Solution
Hypertonic Solution
Variations:
Students could follow this activity with an attempt to create an isotonic solution using salt and water.
Some students may wonder if the effects obtained here would be greater if the eggs were allowed to soak
longer. You could have them leave their materials to soak overnight after this activity to be measured and
observed the next day.
Adapted from:
Goodman, H.D., L.E. Graham, T.C. Emmel, and Y. Shechter, Biology Today, Orlando, Florida: Holt,
Rinehart and Winston, Inc., 1991.
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Activity 2
Alternative/extension activity for Event 1
Teacher Sheet
Science as Inquiry
Onion Soup
How do substances move in and out of cells?
Overview:
This activity can be done as an alternative to Activity 1 or as a follow-up activity. Here students are
provided with a readily observable example of movement of a substance (water) into and out of a representative plant cell. Students create a hypertonic solution of salt and water and compare its effect on
onion cells to that of distilled water.
The activity does not attempt to explain how this happens but provides students with the empirical
evidence that it does happen. The concept of osmosis as a form of diffusion is introduced, as well as the
terms hypotonic solution, isotonic solution, and hypertonic solution. Subsequent activities in the microunit will incorporate the role of the cell membrane in controlling transport of materials into and out of
the cell and how this process is related to cell size.
Materials:
Per lab group (2 or 3):
balance
beakers (50 mL), 2
cover slips, 2
forceps or tweezers
medicine droppers, 2
microscope
microscope slides, 2
onion
paper towels or absorbent paper, 2
salt, approx. 2 g
Procedure:
Have students carefully peel off a small piece of onion tissue with forceps and prepare a wet mount.
They should then observe this slide under a microscope to determine the “normal” appearance of onion
cells, noting to the degree possible the existence and appearance of cell walls and membranes. These
particular structures were studied in micro-units 932 and 933. All lab partners should examine the cells
and record their appearance in their lab notebooks.
Students then use a medicine dropper to introduce a 10% salt water solution onto the slide, thoroughly soaking the onion cells. The solution can be easily introduced by placing a few drops on the left
edge of the cover slip and absorbent paper on the opposite side of the cover slip. This will help draw the
fresh water out and create a mild pressure gradient that will encourage the saltwater to move under the
cover slip. Students should then observe what happens to the onion cells in the saltwater condition.
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Activity 2
Finally, have students use absorbent paper to draw out the saltwater while they irrigate the slide with
fresh water. As before, they should observe and record the effects of the irrigation on the onion cells.
Background:
It is expected that the cell membranes will shrink away from the cell walls when placed in the saltwater and swell against the cell walls in the distilled water condition, both results due to osmosis. Water
should diffuse out of the cells in the saltwater (hypertonic) condition, thus causing the cell membranes to
appear shriveled or crenated. When fresh water is reintroduced to the onion cell environment, the water
should diffuse back into the cells, causing the cell membranes to press against the cell walls It will take
quite a bit of effort to get enough distilled water back under the cover slip to counteract the effects of the
saltwater.
It is assumed that students have previously studied properties of aqueous solutions, concentration, and
the basic phenomenon of diffusion (see Micro-unit 1024). If not, you may need to spend extra time in
this activity covering the concepts of solution, solute, and concentration. This will help students to better
understand the terms hypotonic, hypertonic, and isotonic introduced here. Some useful terms and definitions for you are:
Concentration—amount of solute dissolved in a given amount of solvent
Diffusion—movement of molecules of a substance from areas of high concentration of that substance
to areas of lower concentration
Osmosis—diffusion of water through a membrane
Hypertonic solution—solution in which the concentration of the solutes outside a cell is greater than
that inside the cell
Hypotonic solution—solution in which the concentration of solutes outside a cell is lower than that
inside
Isotonic solution—solution in which the concentration of solutes outside a cell is the same as that
inside the cell
Water makes up 70 to 95 percent of a living cell. Since water is the most abundant substance in cells,
its movement into and out of cells is of vital importance. The cell has no control over osmosis. Water
will flow into or out of a cell depending on the concentration of water molecules on either side of the
membrane. Water moves across the membrane in order to reach a state of equilibrium. Water will continue to diffuse back and forth across the cell membrane even after equilibrium is established. However,
at equilibrium the number of molecules moving into the cell equals the number moving out.
Freshwater plants often exist in hypotonic solutions. As water diffuses into the cell, the cell swells and
internal pressure is increased. The pressure that builds in a plant cell as a result of osmosis is called
turgor or turgor pressure. The excess water entering a plant cell is often stored in a large central vacuole.
Increases in turgor pressure force the cytoplasm and the cell membrane against the plant cell wall,
causing the cell to become stiff. The cell wall prevents the cell membrane from bursting.
Animal cells do not have a cell wall and therefore cannot reach equilibrium in a hypotonic solution.
As water flows in, the cell may swell and burst unless the cell is able to efficiently remove excess water
from the cytoplasm. A number of mechanisms have developed to remove such excess water and generally require substantial energy to pump excess water from the cell before any damage results.
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Activity 2
Variations:
Other plant cell types could be used in place of onion. It might be a challenging activity for students to
try to create an isotonic solution for the onion cells using appropriate calculations to determine the exact
amount of solute concentration required.
Adapted from:
Goodman, H.D., L.E. Graham, T.C. Emmel, and Y. Shechter, Biology Today, Orlando, Florida: Holt,
Rinehart and Winston, Inc., 1991.
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Activity 3
Alternative/extension activity for Event 1
Teacher Sheet
Science as Inquiry
Osmosis Demonstration
How do substances move in and out of cells?
Overview:
This simple demonstration gives students a visual image and understanding of osmosis in terms of
movement of water into and out of the cell. You may wish to use this as a supplement to either Activity 1
or 2. There is no student sheet for this activity.
Materials:
Per class:
beaker, 250–500 mL
molasses (heavy), approx. 6 oz
ring stand and clamp
rubber band (small)
semipermeable membrane (e.g., goldbeater’s, cellophane tubing, dialysis tubing)
thistle tube
distilled water
Procedure:
This demonstration can be carried out when discussing
the concept of osmosis and what happens to cells when
they are placed in a hypotonic solution.
Fill the bulb of a thistle tube with heavy molasses
while holding a finger over the tube opening. (Instead
of holding a finger over the opening, some teachers
attach a short piece of rubber tubing with a clamp.)
Thistle tube filled with molasses
Next, cover the bulb with a wet semipermeable
membrane and immerse in a beaker of water (see
figure). Soon the water in the beaker diffuses
Semipermeable membrane
through the membrane, causing the level of
molasses in the thistle tube to rise.
Background:
This special diffusion of molecules of water
through the membrane is called osmosis. As the
water level rises in the tube, its pressure will
eventually stop further upward diffusion of water.
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Activity 3
Variations:
The living cells of a potato may be used to show the passage of water through the semipermeable
membranes that surround the cells. Use an apple corer to remove a cylinder from the center of a raw
potato. Do not plunge the corer through the entire length of the potato; leave about 1/2 inch of the potato
at one end. Slowly pour a concentrated sucrose solution into the core and close with a one-hole stopper
through which a piece of glass tubing has been inserted. Place in a beaker of water, using a clamp to hold
the tube upright. Then seal the stopper with melted paraffin to prevent leakage.
Adapted from:
Morholt, E., P.F. Brandwein, A. Joseph, A Sourcebook for the Biological Sciences, 2nd ed. California
State Series, California State Department of Education, Sacramento, Calif., 1967.
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Activity 4
Teacher Sheet
Science as Inquiry
Movin’ On In, Movin On Out
How do molecules move in and out of cells?
Overview:
Based on Activities 1–3, students will have an understanding that substances such as water are able to
move into and out of cells through the membrane. Together with their experience with micro-unit 1024
they should understand the concepts of concentration, concentration gradients, and diffusion. This
activity will give students a closer look at the factors involved in diffusion and osmosis through a semipermeable membrane and give them a better understanding of how cells maintain homeostasis.
Materials:
Per lab group (3-4 students):
balance
beaker (200 mL)
beaker (250 mL)
beakers (500 mL), 5
corn syrup
dialysis tubing, 25 mm flat width,
four 15-cm pieces and one
30-cm piece, all presoaked in water
eggs (fresh), 12
experimental solutions 1 and 2 (see footnote, p. 16)
funnel (small)
glucose solution (50%), 400 mL
glucose/starch solution (15% glucose/
1% starch), 20 mL
glucose test tape
graduated cylinder (25 mL)
labels
labeling tape
Lugol’s solution (IKI)
marking pen
paper towels
test tubes, 16
thread or 8 rubber bands
vinegar
Procedure:
This investigation is designed for four students per lab group. It is assumed that students are familiar
with the concepts of solids, liquids, gases, density, volume (Micro-unit 908), and the important cellular
organelles (Micro-unit 933). The egg activity (Part III) in this investigation can be done as a demonstration with students involved in the setup.
Part I: Illustrating Diffusion
Students first obtain a 30-cm piece of 25-mm dialysis tubing (presoaked in water). They should tie off
one end of the tubing with thread to form a bag. Note: It is important that the knots are tight to prevent
leaks. Have them pour 15 mL of 15% glucose/1% starch solution in the bag and tie off the open end,
leaving sufficient space for expansion of the contents. They should then record the solution color and the
color of the glucose test tape when placed in the glucose/starch solution.
Next, have students fill a 250-mL beaker with distilled water and add about 4 mL of Lugol’s solution
to the water. At this point they should record the color of the solution. They then test the solution with
glucose test tape. Now have students immerse their dialysis tubing bag in the Lugol’s bath and allow it to
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Activity 4
stand for 30 minutes. While waiting for results, students can design a data table to record their observations. After 30 minutes they should test the liquid in the beaker and in the bag with glucose test tape and
record their results in their data table.
Part II: Illustrating Osmosis
Here students investigate the relationship between solute concentration and the movement of water
through a selectively permeable membrane by the process of osmosis. They first place 10 mL of each of
the following solutions in appropriately numbered and labeled test tubes: 1) distilled water; 2) 50%
glucose solution; 3) distilled water; and 4) students’ experimental solution 1*.
Students then obtain four pieces of presoaked dialysis tubing and tie off one end of each with thread.
Note: knots must be tight to prevent leaks. They then place each of the above solutions into a dialysis
bag and tie off the open end. They should place the dialysis bags on labeled paper towels to ensure
proper identification. Each bag should be weighed to the nearest 0.1 gram.
Have students number and label four empty 500-mL beakers to correspond to the solutions in the four
dialysis bags and then fill each beaker with 350 mL of the following solutions in the order given: 1)
distilled water; 2) distilled water; 3) 50% glucose solution; and 4) students’ experimental solution 2*.
They then place each dialysis bag in its corresponding 500-mL beaker (by matching numbers—bag 1 in
beaker 1, etc.), noting the time of placement. Students should make sure in each case that the entire
dialysis bag is submerged in the solution.
Students should weigh the dialysis bags three times, at 10-minute intervals. After each 10-minute
interval, they remove the bags at the same time from their beakers, and carefully rinse them and blot off
the excess water. Each bag should be carefully weighed and returned to the appropriate beaker. After the
third weighing students can discard the dialysis bags. Note: For accurate results, dialysis bags should
be removed and returned from the beakers at the same time.
Have students design a data table to record the initial mass of the dialysis bag and the mass after each
10-minute interval.
Part III: Chicken Egg Model for a Living Cell Membrane (Optional Exercise/Demonstration)
This activity takes two days to complete. Final measurements are made on the third day. Have students determine the mass of two raw eggs to the nearest 0.1 gram and record this as the mass of the raw
egg and shell. They should then label one 500-mL beaker “ distilled water” and another 500-mL beaker
“syrup”. Have them pour 200 mL of vinegar into each labeled beaker. Caution: Vinegar is a mild
irritant. Safety goggles should be used.
Students now put an egg into each beaker, placing a 200-mL beaker containing 100 mL of water over
each egg to keep it submerged. Have them add more vinegar if the egg is not completely covered.
Beakers should be stored for 24 hours.
After 24 hours students should carefully pour the vinegar into the sink, remove and rinse the eggs, and
place them onto paper towels labeled “water” and “syrup.” Have them determine and record the mass of
*Student Experimental Solutions. Discuss with students their ideas for beaker #4, the experimental
setup. You can suggest variables such as different glucose concentrations, temperature, or disturbance of
the solutions. Provide additional solutions for them to choose from, such as 1% starch solution or 1%
egg albumen solution.
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Activity 4
each egg. They should now place the “syrup” egg in the syrup beaker and add syrup until the egg is
covered. Similarly, they should place the “water” egg in the “water” beaker and add distilled water to
cover. The beakers and eggs should be stored for 24 hours. After the allotted time students remove the
eggs from the beakers and measure and record their final masses.
Background:
Diffusion is the process by which molecules spread throughout a space until they are equally distributed. Some molecules, like water, will diffuse freely through a membrane. Diffusion and osmosis are two
passive processes that move molecules into and out of the cells.
In addition to free diffusion, material may get into and out of a cell by at least two carrier-mediated
transport mechanisms. If transport is driven by a diffusion gradient and involves an interaction between
specific membrane-embedded molecules (carriers) and the molecules of a substance tending to enter the
cell, the process is called facilitated diffusion. The name indicates that the carrier improves the cell’s
permeability to a substance without altering the direction in which the substance would tend to move on
its own. In other instances, transport is thermodynamically unfavorable and can be achieved only with an
expenditure of energy supplied by the cell. Such translocations are referred to as active transport. For
example, glucose uptake by microorganisms and by cells lining the intestines and kidney tubules of
animals is typically active; in most other animal cells, it is achieved by facilitated diffusion.
Part I: Dialysis tubing can be opened by rubbing the end between your fingers until the edges separate. Students may need help and/or instruction on how to carefully fill and tie the tubing. Molecules of
Lugol’s iodine solution will pass through the membrane, while undigested starch does not. Thus, the
characteristic blue-black color appears in the starch solution inside the dialysis tubing bag. Similarly, the
glucose molecules are soluble and therefore small enough to diffuse through the membrane and into the
solution in the beaker. Students should therefore obtain a positive result when testing the beaker solution
with glucose test tape after 30 minutes. Of course, water will move across the membrane due to osmosis
and will cause the contents inside the dialysis tubing bag to expand as the water moves from the hypotonic environment of the beaker into the hypertonic environment inside the bag.
Part II: Discuss with students their ideas for beaker 4, their experimental setup. You can suggest
variables such as different glucose concentrations, temperature, or disturbance of the solution. Provide
additional solutions for them to choose from, such as 1% starch solution or 1% egg albumen solution.
Students should find that the bag in beaker 1 is at equilibrium and will show no change in mass. The bag
in beaker 2 will experience water diffusing across the membrane into the glucose solution so the mass of
the bag will increase. In beaker 3 the opposite will occur; the water diffuses out of the bag and into the
glucose solution in the beaker, so the bag loses mass. In beaker 4, the diffusion of substances will depend
entirely on the contents of the bag and the solution in the beaker.
Part III: Since this activity actually requires three class days to complete, you may want to run it as a
demonstration or an additional activity concurrent with the other activities from Parts I and II. The
vinegar will chemically remove the shell from the egg. The goal here is to determine if either the water
or the syrup will diffuse through the membrane and into the egg. If the syrup is dense enough (hypertonic to the egg), the egg placed in the syrup will loose mass while the egg placed in the water will gain
mass (due to osmosis).
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Activity 4
Variations:
To save time and/or materials you could have half of each lab group do Part I of the procedure while
the other two partners complete Part II. Meanwhile, you could do Part III as a concurrent demonstration.
Adapted from:
College Board, AP Biology Laboratory. ETS, 1995.
Dyson, R.D., Cell Biology, A Molecular Approach, Boston: Allyn and Bacon, Inc., 1978.
Johnson and Raven, Biology: Principles and Explorations, 1996.
Towle, Modern Biology Laboratories. HRW, 1995.
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Activity 5
Teacher Sheet
Science as Inquiry
There’s Always Room for Gel-Oh!
What is the relationship between cell size and diffusion?
Overview:
After students have had experience with diffusion and osmosis in actual cells, it is appropriate for
them to study these concepts in terms of cell models or physical analogies. In this activity students
become aware of the importance of the surface-to-volume ratio of cells and how cell size is related to the
processes of diffusion and osmosis. They use agar cubes of increasing size to examine the extent and rate
of diffusion as a function of cell volume.
Materials:
Per lab group (2-3 students)
agar-agar, 3%
beaker, 250 mL
cake pan, 6 × 9 in
NaOH solution (0.04%), 2 L
phenolphthalein powder
razor blade
plastic spoon (small)
rinse bottle with distilled water
Procedure:
You will have to prepare the agar mixture the day before the activity is done. Follow the directions for
the preparation of agar gel (3% agar-agar). As the agar is cooling, add 1/4 teaspoon of phenolphthalein
powder and stir well. Pour the agar mixture into the cake pan. The depth of the agar in the cake pan
should be 3 cm. Let the agar cool and harden overnight.
After the agar has hardened, give each lab group enough of the gel to make three cubes measuring 1
cm, 2 cm, and 3 cm on a side respectively. Students then place all three cubes in one 250-mL beaker and
cover them with NaOH solution. They should allow the setup to stand for 15 minutes. During this time
students should occasionally swirl the NaOH solution around the blocks. Warn students to swirl gently—do not break up agar cubes. While they are waiting they can calculate the surface-to-volume ratio
of the three blocks.
After the appropriate time, have students carefully pour off the NaOH solution and rinse the agar
cubes with distilled water. Using plastic spoons, they should then carefully remove the agar cubes from
the beaker and place them on a paper towel. Have them cut the cubes in half and examine and compare
their inside appearance.
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Activity 5
Background:
Phenolphthalein is a biological indicator that reacts with a salt solution such as sodium hydroxide,
NaOH. The phenolphthalein will turn pink when mixed with NaOH. The NaOH diffuses into the agar
cubes at an equal rate for each cube, but because of the differing volumes of the cubes, the results will
not appear to be the same. When students cut into the larger block of agar the center will not have picked
up the stain (i.e., the NaOH solution will not have diffused all the way through the block). The larger the
block of agar, the greater the unstained area. This should indicate to students the importance of cell size
in the diffusion of substances into cells.
Diffusion is a major transport mechanism for moving substances into and out of the cell. It occurs
passively; that is, it does not require expenditure of energy by the cell like some other transport systems
(e.g., active transport using carrier proteins within the cell membrane.) If cells were too large, they
could not efficiently absorb materials and excrete wastes via diffusion.
Variations:
None suggested.
Adapted from: none
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Activity 6
Teacher Sheet
Science as Inquiry
A Cube Is a Cube
Is a Cube Is a Cube!
What happens to the surface area of a cube as the volume increases?
Overview:
In this activity students measure how much greater the increase in volume is than the increase in
surface area when an object increases in size. It follows nicely after the Gel-Oh! activity and helps
students fully understand what happened in that activity.
Materials:
Per lab group (2 students):
graph paper
metric ruler
transparent tape
pencil
scissors
Procedure:
Part I
Have students follow the pattern below to build two cubes. The first cube should be 1cm on each side
and the second cube 2 cm on each side. Using graph paper, students should measure out the two patterns,
using dashed lines to represent areas where folds will be made to form the cube. Measurements should
be accurate so that the cubes fit easily together. Students then cut out the cubes, taking care to cut only
on the solid lines. They then assemble the cubes, using tape to secure them.
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Activity 6
Have students calculate the surface area of each cube by multiplying the area of one face by six,
expressing their answer in square centimeters. Working together, student groups should then assemble
the completed 1-cm cubes into a single cube that has a volume equal to the 2-cm cube. Students are
asked to compare the total surface area of the assembled cube to that of the single 2-cm cube.
Additional:
4 mm each side
Have students observe the cube-shaped cells in the
diagram pictured here. The smallest cell has 1-mm
sides, a surface area of 6 mm squared, and a
volume of l mm cubed. If the side of the cell
is doubled to 2 mm, the surface area will
increase fourfold to 6 × 2 × 2, or 24
mm squared. Have students calculate
1 mm each side
the change in surface area and
volume as the cell doubles in size
again to 4 mm on a side.
2 mm each side
Surface area = 24 mm2
Volume = 8 mm3
Surface area = 6 mm2
Volume = 1 mm3
Background:
Before this exercise, students should review Micro-units 932 and 933. You may need to review the
formula for finding the area of a square. Focus the students’ attention on why the majority of cells
making up any organisms are small. Students should be asking these questions: Is big better? What does
this mean for cells? How does it affect cell function? If cell size doubled, the cell would require eight
times more nutrients and would have eight times more waste to excrete. The surface area, however,
would increase by a factor of only four. Thus, the plasma membrane would not have enough surface area
through which oxygen, nutrients, and wastes could diffuse. The cell would either starve to death or be
poisoned from the buildup of wastes products. In fact, cells divide before they reach this point.
Variations:
None suggested.
Adapted from:
Biology Dynamics of Life. Glenco/McGraw Hill, 1996.
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Assessment 1
Science as Inquiry
Salty Chops
Item:
Use your understanding of osmosis to describe why putting salt on a pork chop before cooking it
on a grill is likely to result in a dry and tough piece of meat.
Answer:
Putting salt on the pork chop will result in a hypertonic solution of the pork chop’s surface, causing
water and other liquids in the cells of the meat to be drawn to the surface of the pork chop and turned
into vapors by the heat.
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Assessment 2
Science as Inquiry
Outside Looking In
Item:
Explain the importance of surface-to-volume ratio in a cell.
Answer:
If cells grow too large, they will outstrip their ability to supply needed materials to their interiors.
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Assessment 3
Science as Inquiry
Speaking of Boundaries . . .
Item:
The membrane around a cell of a living organism is considered a boundary. However, this membrane
is described as “selectively permeable.” Explain what is meant by the term selectively permeable. Use
observations from the activities you’ve done here to support your answer.
Answer:
Selectively permeable means only certain substances can pass through the membrane. For example,
water passes through cell membranes, but the substances dissolved in the water may or may not pass
through.
Students observed water passing through a membrane around salmon eggs in Activity 1. In Activity 2
water diffused into and out of plant (onion) cells In Activity 3 students observed water diffusing through
a semi- or differentially permeable membrane (modeling the selectively permeable nature of cell membranes) into a molasses solution, while the molasses was prevented from moving across the membrane
and into the water of the beaker. In Activity 4 students observed water, glucose, and iodine (Lugol’s
solution) diffusing across a membrane while (undigested) starch in solution was prevented from crossing
the membrane.
Note: In addition to free diffusion, material may get into and out of a cell by at least two carriermediated transport mechanisms. If transport is driven by a diffusion gradient and involves an interaction
between specific membrane-embedded molecules (carriers) and the molecules of a substance tending to
enter the cell, the process is called facilitated diffusion. The name indicates that the carrier improves the
cell’s permeability to a substance without altering the direction in which the substance would tend to
move on its own. In other instances transport is thermodynamically unfavorable and can be achieved
only with the expenditure of energy supplied by the cell. Such translocations are referred to as active
transport. For example, glucose uptake by microorganisms and by cells lining the intestines and kidney
tubules of animals is typically active; in most other animal cells, it is achieved by facilitated diffusion.
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Unit Materials/References
Consumables
Item
Quantity (per lab group)
agar-agar (3%)
—
corn syrup
—
dialysis tubing (25 mm flat width)
4 15-cm pieces; 1 30-cm piece
distilled water
—
eggs (fresh)
12
glucose solution (50%)
400 mL
glucose/starch solution (15% glucose/1% starch)
20 mL
glucose test tape
—
graph paper
—
labels
—
labeling tape
—
molasses (heavy)
—
NaOH solution (0.04%)
2L
onion
1
paper towels
—
phenolphthalein powder
—
plastic spoon (small)
1
razor blade
1
rubber band
1
salmon eggs
12
salt (to make solution)
15 g per 250 mL water
salt
approx. 2 grams
semipermeable membrane
—
(e.g., goldbeater’s, cellophane tubing,
dialysis tubing)
thread or 8 rubber bands
—
transparent tape
—
vinegar
—
water
—
Event
5
4
4
1, 3*
4
4
4
4
6
4
4
3*
5
2
2, 4
5
5
5
3*
1
1
2
3*
4
6
4
1
Nonconsumables
Item
balance
beaker (200 mL)
beaker (250 mL)
beaker (250–500 mL)
beakers (50 mL)
beakers (100 mL)
beakers (500 mL)
cake pan (6 × 9 in)
cover slips
forceps or tweezers
funnel (small)
graduated cylinder (25 mL)
Quantity (per lab group)
1
1
1
1
2
2
5
1
2
1
1
1
26
Event
1, 2, 4
4
4, 5
3*
2
1
4
5
2
2
4
4
1025
Unit Materials/References
marking pen
medicine droppers
microscope
microscope slides
pencil
petri dishes
plastic spoon (small)
ring stand and clamp
rinse bottle with distilled water
ruler (mm)
scalpel or dissection scissors
scissors
test tubes
thistle tube
1
2
1
2
1
2
1
1
1
1
1
1
16
1
4
2
2
2
6
1
5
3
5
1, 6
1
6
4
3*
Key to activities:
1. Solutions and Cells
2. Onion Soup
3. Osmosis Demonstration
4. Movin’ On In, Movin’ On Out
5. There’s Always Room for Gel-Oh!
6. A Cube Is a Cube Is a Cube
Activity Sources
Biology Dynamics of Life. Glenco/McGraw Hill, 1996.
College Board, 1995. AP Biology Laboratory, ETS.
Dyson, R.D., Cell Biology, A Molecular Approach. Boston: Allyn and Bacon, Inc., 1978.
Goodman, H.D., L.E. Graham, T.C. Emmel, Y. Shechter, Biology Today. Orlando, Fla.: Holt, Rinehart, and Winston, Inc., 1991.
Morholt, E., P.F. Brandwein, A. Joseph, A Sourcebook for the Biological Sciences, 2nd ed. California State Series,
California Dept. of Education, Sacramento, 1967.
27