SAM Teachers Guide Diffusion, Osmosis and Active Transport Overview Students investigate diffusion, osmosis and study the role of surface area in facilitating diffusion. 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. Using the Molecular Rover, they fly through the aquapore, and consider how it facilitated their travel from one side of the membrane to another. Learning Objectives: Students will be able to: • Compare diffusion and osmosis. • Explain how concentration differences affect the overall flow of molecules. • Contrast molecular movement of materials in and out of equilibrium and 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. • 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 difference. • The process of diffusion is dependent on the type of solute. 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 Activity: Newton’s Laws. Review concept that atoms are in constant motion. • Page 2 – Diffusion Model o Observe the flow of molecules from high to low concentration with the class and point out the difference between the 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? • Why do some materials move more easily than others? 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 RBC “picking up” and “dropping off” oxygen. Start in the lungs and explain 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 Link to Other SAM Activity: Heat and Temperature. 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 Activity: Electric Current. Review electric potential and the connection to voltage. 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? [Alveoli in lungs, villi in small intestines, matrices in mitochondria] • Extensions: Correlation of how a muscle cell is like a battery, example of sodium / potassium pump. • Demonstration Ideas: o Spray a bottle of perfume at the front of the classroom and see how long it takes for students to be able 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. [Link to previous SAM activities.] 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 focus 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 Energystudent learn about conservation of energyAnd in Electrostatics They learn about ion transfer 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. Subsequently, 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 motion 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 unit. Gas Laws highlights the random motion of gas particles, which behave quite similarly to particles dissolved in water or other solvents as seen in this unit. Understanding osmotic pressure is also quite similar to the underlying principles behind gas pressure. The Solubility unit is helpful for students to understand why some things can or cannot cross membranes.. The Chemical Reactions and Energy activity helps students learn about the chemical energy used to push ions across a membrane. ATP‐Biological Energy 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 so they can appreciate the details about aquapores given in this unit. Structure and Function of Proteins allows students to recognize that one function of proteins is to act as transmembrane molecules. Finally, in the Lipids and Carbohydrates unit, students gain background into the structure and function of lipids, which appear so prominently Photobiology shows many materials diffusing (and going against diffusion) or releasing ions for signals to occur. Two examples are the electron transport chain and light triggering a nerve. 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.
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. The 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. The use the "open" button below to
place that image here.
*Sample Pictures*
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 "drop off" 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. Take an image showing water going
through the channel formed by the
aquapore:
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. 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)
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 allows more
opportunities for diffusion.
Page 7:
1. Take an image showing a side view of an
aquapore poking out of boths 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.)
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 general 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.
Side view of an aquapore.
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)
4. Describe how surface area affects the
speed at which diffusing ions or molecules
reach equilibrium across both sides of the
surface:
The greater the surface area the greater the
diffusion rate.
Page 10:
5. Which of the following can pass through a
cell membrane without a specialized pore: (c)
1. Which best describes the motion of an
INDIVIDUAL ion or molecule: (c )
6. If you were to drink salt water from the
ocean which has a high concentration of
dissolved ions, what would happen? (a)
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.
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)
7. Describe how you can make an electric
potential occur across a membrane, and how
you can make 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 another.
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. What term would can be used to describe the state of the molecules in the model below.
Would the carbon dioxide and oxygen atoms in the model below still be in motion? Why
or why not?
3. Red blood cells (with the help of hemoglobin) pick up oxygen in the lungs and drop it off
to the rest of the body tissues. Explain how this is an example of diffusion in action.
4. 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 the cell membrane. Why are there
differences in how molecules can cross the cell membrane?
5. 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?