SAM Teachers Guide
Diffusion, Osmosis, and Active Transport
Overview
Students investigate diffusion, osmosis, and active transport in order to study the
movement of molecules into and out of cells. They explore the way concentration and
surface area are related to the rate of ion and molecule flow across a cell membrane.
They apply what they have learned to a blood cell traveling through different
concentrations of oxygen. Then they explore a 3D aquapore embedded in a membrane.
Finally, students explore situations where concentrations on either side of the
membrane are not equal due to the use of ATP as an energy source to transport
molecules against the concentration gradient.
Learning Objectives
Students will be able to:
 Compare diffusion and osmosis.
 Explain how concentration differences affect the overall flow of molecules.
 Describe the dynamic nature of equilibrium.
 Apply the principles of diffusion to red blood cells in a real biological system.
 Explain how increased surface area increases the rate of diffusion.
 Explore active transport by experimenting with the relationship between ion
concentration and ATP concentration, and observing the formation of electrical
energy.
Possible Student Pre/Misconceptions
 Molecules move with a purpose. They “know” to move from areas of high
concentration to areas of low concentration.
 Molecular motion stops when equilibrium is reached.
 Diffusion happens at the same speed and is not affected by concentration
differences or surface area.
Models to Highlight and Possible Discussion Questions
After completion of Part 1 of the activity:
Models to Highlight:
 Page 1 – Simple Diffusion Example
o Highlight the random motion of particles and the fact that
collisions change the direction of molecular motion.


o Link to other SAM activities: Newton’s Laws. Review concept that
atoms are in constant motion.
Page 2 – Diffusion Model
o Observe the overall flow of molecules from high to low
concentration and point out the difference between the net flow of
the molecules and the movement of individual molecules (which is
random and not directed).
Page 3 – Diffusion Model
o Highlight what equilibrium “looks” like. It is a state of balance, but
molecules are still in motion.
Possible Discussion Questions:
 Why is it necessary for materials to be able to move across cell
membranes?
 Can you think of some conditions that might speed up or slow down the
process of diffusion?
 What are some of our biological processes that are possible because of
diffusion of molecules?
 Explain the journey of a red blood cell “picking up” and “dropping off”
oxygen. Start in the lungs and go from there, using your understanding of
diffusion.
 How do you think a lack of iron in the blood would affect oxygen levels?
After completion of Part 2 of the activity:
Models to Highlight:
 Page 6 – Surface Area Model
o Highlight the importance of increased surface area in the
effectiveness of diffusion in biological systems.
o Review the importance of surface area in both organic and
inorganic systems.
 Page 7 – Determining Which Molecules Need Pores
o Highlight the concept of selective permeability in determining
which items can pass through the membrane easily.
 Page 8 – Osmosis
o Highlight the similarities and differences between diffusion and
osmosis as well as the importance of water in biological systems.
 Page 9 – Active Transport and the Production of Voltage

o Highlight the concept of active transport and the presence of an
electric potential when ions cross cell membranes.
o Link to other SAM activities: Electric Current. Review electric
potential and the connection to voltage.
Page 10 – Chemical Energy of ATP is Converted into Electrical Energy
o Point out that the chemical energy decreases every time ATP reacts
with the calcium transport channel/protein, and that this pushes a
calcium ion to the other side of the membrane.
o By building up the concentration of positive ions to a greater
degree on one side of the membrane an electric potential is built up.
Possible Discussion Questions:
 What is the role of surface area in diffusion? What are some examples of
specific biological functions that rely on increased surface area?
 Extensions: Correlation of how a muscle cell is like a battery; example of
sodium / potassium pump.

Demonstration/Laboratory Ideas:
o Spray a bottle of perfume at the front of the classroom and see how
long it takes for students to smell it. Discuss in terms of diffusion.
o Drop food coloring into a beaker of water. Have students
hypothesize what will happen. Have students draw cartoons /
animations at the molecular level. Repeat with water of varying
temperatures and integrate the effect of temperature on molecular
motion and random collisions.
o Diffusion Baggie Lab – Use starch and iodine to show diffusion.
Refer to link below for possible procedure:
http://www.biologycorner.com/worksheets/diffusion.htm
Connections to Other SAM Activities
The goal of this activity is for students to explore the concepts of passive diffusion,
osmosis (and osmotic pressure), and pumping materials across a membrane against the
natural equilibrium, known as active transport. From Atoms and Energy students learn
about conservation of energy and in Electrostatics they learn about voltage, which is
important when studying chemical and electrical potential in active transport. A
background in Atomic Structure is necessary for students to understand that atoms are
made of protons, neutrons, and electrons. Ions exist as a result of atoms that have
gained or lost electrons. Newton’s Laws at the Atomic Scale focuses on the concept that
atoms are in constant motion, moving in a straight line until they collide. This
underscores the randomness of the motion involved in diffusion.
Phase Change serves as a reference to the states of matter (particularly solids and
liquids) that are addressed in this activity. Gas Laws highlights the random motion of
gas particles, which behave much like particles dissolved in water or other solvents
seen in this unit. Understanding osmotic pressure is also quite similar to the underlying
principles behind gas pressure. The Solubility activity helps students understand why
some things can or cannot cross membranes.
Cellular Respiration highlights how ATP is used to move materials across the cell
membrane. Four Levels of Protein Structure gives students background in how
proteins are organized; student can thus appreciate the details about aquapores. The
Molecular Recognition activity allows students to recognize that one function of
proteins is to act as transmembrane molecules. Finally, in the Lipids and
Carbohydrates activity, students gain background into the structure and function of
lipids, which form the membranes across which the molecules are being transported.
Activity Answer Guide
Page 1:
1. After looking at the path taken by several
red dye molecules, their motion can best be
described as:
(b)
2. It is often stated that substances diffuse
from highly concentrated areas to areas of
low concentration. Describe how this works
if a molecule can't tell that it is in a region of
high concentration and just diffuses
randomly through collisions.
The molecules are in constant motion, colliding
with other molecules in the area. As they move
they spread out because they are bumping into
these other molecules and changing directions.
Each individual molecule moves randomly, and
can't tell if it’s in an area of high or low
concentration. However, there are so many
more molecules of that type in the highly
concentrated area that there is a greater chance
that one of them will move to the area of low
concentration.
Sample snapshot: There are many more carbon
dioxide molecules outside the cell than inside
the cell.
4. Set up the model so that it is IN
equilibrium. Then use the "snapshot" button
below the model to take a picture of your
setup. Use the "open" button below to place
that image here.
Page 2:
1. Set up the model so that there is a high
concentration of something outside and a
low concentration of that same thing inside.
What are the chances that a molecule will
move into or out of the cell? (b)
2. Describe the flow of molecules when
there is an area of high and low
concentration.
(c)
Page 3:
1. What is true of the concentrations when
equilibrium has been reached?
(d)
2. What is true of the rate at which
molecules move into and out of the cell at
equilibrium?
(c)
3. Set up the model so that it is NOT in
equilibrium. Then use the "snapshot" button
below the model to take a picture of your
setup. Use the "open" button below to place
that image here.
Sample snapshot: The system is now in
equilibrium. The concentration of oxygen and
carbon dioxide molecules is the same inside and
outside of the cell.
Page 4:
1. What happens to the oxygen
concentration of the cell when you move it to
a new environment?
(d)
2. Explain how a red blood cell delivers
oxygen from your lungs to the rest of your
body.
The oxygen concentration is highest in the
lungs, so oxygen will diffuse into the cell. In
other parts of the body, the oxygen
concentration is lower, so the red blood cell can
"deliver" oxygen as it diffuses out of the cell to
move toward equilibrium.
Page 5:
1. Describe what happens when the red
blood cell contains hemoglobin:
(c)
2. Explain how hemoglobin helps transport
more oxygen than could normally be done
with simple diffusion:
Iron in the hemoglobin molecule sticks to the
oxygen. There are several hemes groups in a
hemoglobin and each bind to oxygen.These
oxygen molecules are no longer free to diffuse
back out of the red blood cell.
Page 6:
1. How does surface area affect diffusion
rates?
(d)
Sample snapshot: Side view of an aquapore.
2. Take an image showing water going
through the channel formed by the
aquapore:
2. Single-celled organisms absorb
everything they need directly through their
"skin," their cell membrane. However, you
could never get enough oxygen if oxygen
could only diffuse through your skin. Explain
why it is necessary to have lungs with large
surface areas:
The increased surface area of lungs allows more
opportunities for oxygen to diffuse into the
surrounding cells.
Page 7:
1. Take an image showing a side view of an
aquapore poking out of both sides of a piece
of cell membrane. (Note: You will need to
find the right scene on the molecule tour and
rotate the molecule yourself to get the
correct image.)
Sample snapshot: Water traveling through the
channel.
3. Which are the only types of molecules
that pass easily through the cell membrane?
(c)
4. What is true of most naturally occurring
pores?
(d)
Page 8:
1. Osmotic pressure is related to salt
concentrations (or other dissolved
substances) in what way?
(a)
2. If you want water to flow out of the cell
faster than into the cell you should: (c)
3. Cells generally stay in equilibrium with
their surroundings. What are two ways you
know the cell has reached equilibrium?
(b) (c)
4. Describe the similarities and differences
between diffusion and osmosis.
Diffusion and osmosis both involve random
movement of molecules from areas of high to
low concentration. Osmosis is specific to water
molecules. Both proceed to equilibrium.
Page 9:
1. What must be done to get an electric
potential (a voltage) across the membrane?
(c)
2. How can you get the maximum voltage
across the cell membrane?
(e)
Page 11:
1. Which best describes the motion of an
INDIVIDUAL ion or molecule? (c)
2. Describe in as many ways as possible how
you know when a system has reached
equilibrium:
The concentration remains constant even
though the molecules continue to move
randomly, yet equally in all directions. The rate
of molecules moving into and out of the cell will
be equal.
3. Describe what will happen if a cell has an
oxygen concentration that is higher outside
of the cell compared with inside of the cell
(see illustration to the right): (d)
4. Describe how surface area affects the
speed at which diffusing ions or molecules
reach equilibrium across both sides of the
surface:
Page 10:
The greater the surface area, the greater the
diffusion rate.
1. What factor most affects how much
chemical energy you start with? (b)
5. Which of the following can pass through a
cell membrane without a specialized pore: (c)
2. Describe how the chemical energy in ATP
is converted into electric potential energy.
6. If you were to drink salt water from the
ocean, which has a high concentration of
dissolved ions, what would happen? (a)
The ATP molecules react with the calcium
transport channel to push ions across the
membrane. This causes an imbalance in the
charge on both sides of the membrane, creating
a voltage across the membrane. The chemical
energy of the ATP has been turned into electric
potential energy of different ions’ concentrations.
7. Describe how you can make an electric
potential occur across a membrane, and how
you can maximize this potential energy:
To create an electric potential you need to make
the concentration of one ion different from
another across a membrane. To maximize this,
you should move all the positive ions to one side
and all the negative ions to the other.
SAM HOMEWORK QUESTIONS
Diffusion, Osmosis, and Active Transport
Directions: After completing the unit, answer the following questions to review.
1. During diffusion molecules move from areas of high concentration to areas of low
concentration. Is this process spontaneous or directed by the cell? Explain.
2. Look at the two images below. Which one represents a system that is in equilibrium?
Describe the expected motion of particles in each of the systems shown.
*Note: All particles shown are carbon dioxide.
3. Give an example of a molecule that can move easily through a cell membrane. Give an
example of a molecule that requires assistance to cross a cell membrane. Why are there
differences in how molecules can cross the cell membrane?
4. Diffusion and osmosis refer to the movement of particles until they reach equilibrium.
What is active transport? Why is this process sometimes necessary for cells?
SAM HOMEWORK QUESTIONS
Diffusion, Osmosis, and Active Transport – With Suggested Answers for Teachers
Directions: After completing the unit, answer the following questions to review.
1. During diffusion molecules move from areas of high concentration to areas of low
concentration. Is this process spontaneous or directed by the cell? Explain.
Spontaneous. As the molecules move, they bump into one another and spread out from regions of high
concentration to regions of low concentration.
2. Look at the two images below. Which one represents a system that is in equilibrium?
Describe the expected motion of particles in each of the systems shown.
*Note: All particles shown are carbon dioxide.
System B is in equilibrium. The particles would still be in motion, but there would be equal amounts of carbon
dioxide on either side of the membrane. In system A, there are more carbon dioxide molecules on the left of the
membrane. Therefore, you would expect more molecules to move to the right until that system also reached
equilibrium.
3. Give an example of a molecule that can move easily through a cell membrane. Give an
example of a molecule that requires assistance to cross a cell membrane. Why are there
differences in how molecules can cross the cell membrane?
Only small non-polar molecules, such as carbon dioxide, can easily pass through the membrane. The non-polar
molecules pass easily through the non-polar tails of the lipids following the principle of “like dissolves like.” Large
molecules, ions, and polar molecules all need assistance crossing the cell membrane because the lipid bilayer is not
conducive for them to dissolve. They cross with the help of a protein pore.
4. Diffusion and osmosis refer to the movement of particles until they reach equilibrium.
What is active transport? Why is this process sometimes necessary for cells?
Active transport refers to the process when energy, in the form of ATP, is used to move molecules across the cell
membrane. The chemical energy in ATP is converted into electric potential energy to create a voltage. This is
necessary to move molecules, such as ions, into and out of the cell as necessary.