Dianne Robinson
CEMSS Lesson Design
Focus: Cell Processes: The cell membrane, diffusion and osmosis
Critical Input experiences:
1. Cell membranes are selectively permeable, allowing substances required by the cell to
freely move across the membrane into and out of the cell.
2. The movement of molecules/particles in nature is always in the direction from a higher
concentration of particles to a lower concentration of particles.
3. Diffusion is the name of the process in nature whereby particles move from a higher to a
lower concentration.
4. Osmosis is a special form of diffusion that refers only to the movement of water from a
higher to a lower concentration.
Lesson 1: Cell Membranes and Permeability-Selectively permeable membranes
Critical Input Experience- Cell membranes are selectively permeable, allowing substances required by the cell to freely move across the membrane into and out of the cell.
Materials : Several different colored balloons each filled with a different scent (food flavorings),
blown up and tied off. (5-6)
http://www.flinnsci.com/documents/demopdfs/biology/bf10225.pdf
Vocabulary: Selectively permeable membrane, Concentration, Diffusion
Procedure: * Wait time techniques are used in all question-response modalities.
1. Teacher shows students each of the students the balloons so they can list the colors in their
science notebooks.
2. The balloons are then passed around so that each student can smell them and write down the
scent they think they smell next to the color of the balloon along with any other
observations.
3. When through ask students what they know about the process of smell? What has to happen
to be able to “smell “an odor ( The odor-containing molecules/particles must reach and enter
the nostril to record a “smell” in the brain).
4. Now ask how the odor or smell particles get out of the balloon and into the nose if no air
can escape the balloon? Allow students to work within their assigned groups for a few
minutes to discuss their ideas and write or illustrate their hypotheses in their science
notebooks.
5. The students can then present their ideas to the class and the teacher can lead a relevant
discussion on the topic.
6. Teacher then presents notes on the idea of a selectively permeable membrane and the
composition of the cell membrane. Using the balloon, teacher describes it as a membrane.
7. During the lecture and discussion teacher asks again” How can the odor particles or
molecules pass through the balloon membrane but not the air molecules?
The students will eventually realize that the balloon membrane is selectively permeable by
the size of the pores in the balloon membrane and the odor molecules are small enough to
pass through or DIFFUSE through the membrane pores but the air molecules are not.
8. Teacher reinforces the idea of the balloon as a selectively permeable membrane, one that
allows only certain substances to pass across and that this is determined by the size of the
pores in the membrane and the size of the molecule.
9. Teacher then asks the students if they can think of any other example of molecules passing
through a membrane that is selectively permeable. (ie. Sieve, cheesecloth, nylon etc)
10. Teacher asks “Why would a cell membrane allow substances to pass into and out of the cell?
And “ What substances do you think are necessary for the cell so as to continually move freely across the cell membrane into the cell? What substances do you think the cell would want to get rid of so they move freely out of the cell?” Students can discuss this in their groups and write their ideas in their science notebooks.
11. Students will then read a short section in their textbooks on cell membranes and take teacher
directed notes as well as draw and label a representation of a cell membrane in their science notebooks. Students will also define the vocabulary pertinent to this topic.
12. HW- worksheet : Into and out of the cell ( A worksheet on cell membranes)
Lesson 2- Diffusion
Critical Input experience: The movement of molecules/particles in nature is always from an area of
higher concentrated molecules to an area of lower concentrated molecules.
Materials needed:
Part 1 Demo- One half of a Petri dish filled with water, a drop of food coloring, overhead projector
Part 2- Diffusion lab-Cornstarch solution, iodine solution, baggies (1 per group), 1 test tube per
Group, 1 cup/beaker per group
Vocabulary: Diffusion, solution, Concentration, permeable
Procedure:
1. On the overhead projector, the teacher places a drop of food coloring ink into the middle of a
water-filled Petri dish and asks the students to write a prediction in their science notebooks as to
what they think will happen to the drop if left untouched.
2. While waiting, the teacher can review the previous lesson and ask “Why didn’t the odor molecules
stay in the balloon? Why did they pass out or diffuse out of the balloon through the balloon
membrane?
3. Students can discuss this question in their groups and will eventually see that the food coloring
is slowly migrating away from the center of the dish towards the sides .
4. The students can add this observation to their discussion and each group can share their ideas
with the class.
5. The students are then given the vocabulary words to define in their journals and a passage to read
from the textbook on the concept of diffusion.
6. The teacher will then give the students a lecture with notes and visual aids on the idea that
diffusion is the movement of particles always from a higher to a lower concentration of those
particles and that this process occurs naturally in nature.
7. Students will be given everyday examples of diffusion and then will be asked to brainstorm a few
of their own. Examples include:
The small of perfume/ air fresheners, Making Kool-Aid, coffee or tea, the spread of smoke
Part 2- Diffusion lab- Diffusion in selectively permeable membranes
1. Teacher demonstrates what happens when an iodine solution comes into contact with a starch
solution and defines the concept of an indicator.
2. Students in their lab groups are asked to set up a lab on diffusion after reading the introduction
and following the instructions. (See attached lab)
3. Each member of the group will be assigned a job responsibility within the group to assure that
each student is engaged and the lab runs smoothly. These include a principle investigator,
materials manager, recorder/reporter and timekeeper.
4. After completing the lab setup, students are asked to create a lab report in their science
notebooks that includes a title, a labeled illustration of the set-up and a prediction of what they
think will happen if the baggie sits in the iodine solution for 15 minutes (or overnight as needed).
Students are also asked to create a data table that includes the initial color of each solution and
the color after 15 minutes.
5. While waiting, the students can work on the vocabulary and the questions given in the lab (see
handout) that also include specific predictions.
6. After the time allotted the students can observe and record their results in the data
table and re-illustrate the setup along with the changes in their notebooks.
7. The students can discuss in their lab groups what they think happened between the two solutions
and the baggie and review their predictions.
8. The teacher can then randomly call on several groups to ask what they think happened backed up
by observable evidence.
The teacher asks” Which substance moved across the baggie membrane; the iodine or the starch?”
“Can you provide evidence to support this?”
“What substance was the baggie selectively permeable to: the iodine or the starch?”
9 The teacher can then sketch the setup and results and show the direction of diffusion with arrows.
10. Give the students time to understand the relevance of the two concepts:
1. The idea of particles always wanting to move from a higher to a lower concentration (diffusion)
2. The idea of particle s moving across a selectively permeable membrane by size. Some particles
may be allowed to naturally diffuse while others may not.
11. Ask the students what would happen if you switched the two solutions (iodine solution in
baggie, starch solution in cup) and be specific and detailed. Illustrations may be used.
12.Students are to write a conclusion/reflection to this lab with the following guidelines on
conclusion writing.
Directions: Fill in the blanks for the following statements with the vocabulary given and then rearrange the sentences into a logically-constructed conclusion. Students can rewrite the correct sequences or cut out each completed statement, rearrange them logically and glue them into their science notebooks in the correct order.
You may use the following words more than once if needed.
Control diffusion evidence higher indicator iodine lower observation(s) purple-black
selectively permeable solution starch yellow-brown
1. The color change outside the baggie provides ______that ______
diffused into the baggie and caused the color change.
2. Iodine acts as an ______because it changes color from ______to ______
in the presence of a starch.
3. The baggie membrane was ______to the ______but not to the ______.
4. We added a ______of starch to the baggie and a ______of iodine to the cup.
5. Our ______show that the starch solution in the baggie was initially white in color and
the iodine in the cup was yellow-brown.
6. Therefore the process of ______has occurred because the ______particles have
moved across the selectively permeable membrane of the baggie and turned the starch solution
purple-black.
7. The baggie was ______to ______but not to ______. The ______shows that the
color change only occurred in the baggie and not in the cup showing a one-way movement of
______molecules into the baggie and no movement of ______molecules out of the
baggie.
8. Therefore, the ______molecules moved across the selectively permeable baggie membrane
in the process of diffusion from a ______concentration in the cup to a ______
concentration in the baggie.
9. The ______molecules were too large to fit through the baggie membrane and no ______
of starch molecules occurred.
10. The starch solution in the test tube is used as a ______to show the color of the starch
solution when nothing is added to it.
11. My prediction was ______because I thought that…..
Correct/incorrect
http://www.biologycorner.com/worksheets/diffusionlab.html
Diffusion Lab
Introduction: In this lab you will observe the diffusion of a substance across a selectively permeable membrane. Iodine is a known indicator for starch. An indicator is a substance that chances color in the presence of the substance it indicates. Watch as your teacher demonstrates how iodine changes in the presence of starch.
Prelab Observations: Describe what happened when iodine came into contact with starch.
Procedure:
1. Fill a plastic baggie with a teaspoon of corn starch and a half a cup of water tie bag. (This may already have been done for you)
2. Fill a beaker halfway with water and add ten drops of iodine.
3. Place the baggie in the cup so that the cornstarch mixture is submerged in the iodine water mixture.
4. Wait fifteen minutes and record your observations in the data table
5. While you are waiting, answer the questions.
Questions:
1. Define diffusion.
2. Why is iodine called an indicator?
3. Molecules tend to move from areas of ______concentration to areas of ______concentration.
What’s in the Bag?
Were going to think about concentrations now, which substances are more or less concentrated depends on which one has the most stuff in it.
1. Is the baggie or beaker more concentrated in starch?
2. Is the baggie or beaker more concentrated in iodine?
Make Some Predictions
1. If the baggie was permeable to starch, which way would the starch move, into the bag or out of the bag? ______
2. If the baggie was permeable to iodine, which way would the iodine move, into or out of the bag? ______
3. If the baggie was permeable to iodine, what color would you expect the solution in the baggie to turn? ______What about the solution in the beaker? ______
4. If the baggie was permeable to starch, what color would you expect the solution in the baggie to turn? ______What about the solution in the beaker? ______
5. Make a prediction about what you think will happen:
Data Table
Starting Color / Color after 15 minutesSolution in Beaker
Solution in Bag
Post Lab Analysis
1. Based on your observations, which substance moved, the iodine or the starch?
2. How did you determine this?
3. The plastic baggie was permeable to which substance?
4. Is the plastic baggie selectively permeable?
5. Sketch the cup and baggie in the space below. Use arrows to illustrate how diffusion occurred in this lab.
6. What would happen if you did an experiment in which the iodine solution was placed in the baggie, and the starch solution was in the beaker? Be detailed in your description.
7. Why is it not a good idea to store iodine in a plastic bag?
Lesson 3- Osmosis
Critical Input experience; Osmosis is a special form of diffusion that refers only to the movement of
water from an area of high water concentration to an area of low water to
concentration.
Teacher introduces the concept of osmosis by asking students several questions that they answer in their science notebooks:
- What happens when you place a dry sponge in a bucket of water?
- What happens to a wilted plant when you water it?