Lesson 3 of 7
Objective: SWBAT create a model of the cell membrane and develop a narration explaining movement across the cell membrane to maintain homeostasis.
This lesson as an introduction to the cell membrane with an emphasis on the semi-permeable nature of the phospholipid bilayer. This lesson contributes to student mastery of the NGSS Life Science Standard 1-1 by introducing the components and functions of the cell membrane.
It is recommended that this more specific lesson be implemented as a follow-up to the Art for Cell Lesson which is a generalized introduction to cell organelles.
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As an introduction to the semi-permeable characteristic of the cell membrane, students will be asked to respond to the following prompts comparing the cell membrane to a window screen.
- Create a list of all the things that can fit through a window screen.
- What is the determining factor to decide which things can pass through the window screen.
- What is the term that describes the characteristic of allowing some things\substances through while not allowing others?
- How do these characteristics relate to the function of the cell membrane?
Sample of Student Response - Student responses to the anticipatory activity are usually short and to the point. This sample student response demonstrates a strong understanding of the definition of semi-permeable and a real world analogy to illustrate the movement across the membrane.
The teacher will lead a whole-class discussion to review responses and identify similarities between a cell membrane and a window screen. Students seem to connect to the analogy with greater success if the teacher is able to bring in an actual window screen to encourage students to visually connect with the comparison.
Students are encouraged to get out the lecture notes they have transcribed in a previous lesson and add additional comments during the whole-class review session of the cell membrane. Students will also have the opportunity to ask clarifying questions related to the semi-permeable nature of the cell membrane.
After a brief review, students will organize into their assigned lab groups of four students each. This activity could be modified to be an independent or partner activity to meet the needs of your classroom. Each lab group will receive a block of clay for each color: red, blue, yellow, and green. Student groups will use their lecture notes and diagrams in the student textbook to construct a clay model to represent the phospholipid bilayer. Students will need to identify each component of the three-dimensional clay model and explain the role of that structure in the overall function of the cell membrane. Students will be asked to draft a statement explaining how the cell membrane supports the cell to maintain homeostasis.
Example of Student Work #1: Cell Membrane Model: This group paid great attention to overall detail of their clay model and placed emphasis in the scale between the individual structures in the fluid mosaic model. The term "fluid mosaic model" is another term used to describe the cell membrane due to the variety of individual structures that work in concert to compose the cell membrane. These students also paid careful attention identify each component of the cell membrane. The paper labels had fallen off at the time the picture was taken, but each group member was able to discuss the components of the cell membrane and describe its role in supporting movement into and out of the cell.
Example of Student Work #2: Cell Membrane Model: This group successfully recreated the cell membrane with their clay model, but were unable to accurately identify and describe the parts of the individual components that construct the cell membrane.
As students complete their clay models of the cell membrane, they will be given the Selective Permeability Lab Sheet that describes the procedure of the two-day lab that will demonstrate the movement of molecules across a semi-permeable membrane. Students will use pre-cut sections of dialysis tubing about 3-4 inches long to create a closed tube using a short piece of string to tie the end of the tubing closed. If you are unfamiliar with dialysis tubing, it is tough plastic material until it gets wet and then becomes pliable which opens into a narrow tube. Students are always amazed to observe this change and will struggle with with opening the tubing by rubbing their thumb and forefinger together like opening a produce bag. The greatest source of student error in this lab is that students do not secure the end of the tubing and the liquid solution will leak out leading to inaccurate results. Once one end of each section of tubing is closed, prepare the rest of the lab as follows:
- Beaker A: 100mL of water and 20 drops of iodine
- Bag A: 10mL of starch and tie the end of the tubing closed with a piece of string
- Beaker B: 100mL of starch
- Bag B: 10 mL of water and 20 drops of iodine and tie the end of the tubing with a piece of string
Once students prepare their beakers and bags according to the directions above, they need to mass each, note the starting color, and record both values in the data table on the Lab Sheet. Students will then be asked to label each beaker with the correct letter, the class period number, and the assigned group number and place their beakers on the counter to allow the molecular movement to occur over night.
Anticipated Molecular Activity: The small iodine molecules in Beaker A will move through the semi-permeable dialysis tubing and will experience a chemical reaction and turn deep violet when it comes into contact with the starch in Bag A. Even though students cannot see the molecules move, the color change inside of the bag signifies the molecular movement of the smaller iodine molecules into the bag.
In Beaker B the small iodine moved in the opposite direction. The small iodine molecules moved from the Bag B into the beaker across the semi-permeable membrane to cause the color change of the starch in Beaker B. Students have a difficult time visualizing the molecular movement and benefit greatly by having the teacher draw color- coded diagrams of each set-up, A and B, on the board in a color-coded diagram.
Student Expectations for the Second Day: Students will arrive to class and find their beakers from Day #1. The students will mass the beakers and bags, take note of the color of each, and record the measurements and observations in their data table. Students will then collaborate with their lab partners to answer the Lab Conclusion Questions. After about 10-15 minutes the teacher will answer any clarifying questions and then collect the lab sheets to assess.
Image of The Osmosis Lab Diagram - This diagram was created on the front board to represent what students should have observed in Beaker A and Beaker B for both Day #1 and #2. It is important to use different colors to represent the iodine and starch molecules AND draw the molecules representative of their relative size. The color and size differential will assist students in their understanding of how the iodine molecules can move through the membrane, but the starch molecules remain in the same place.
At the end of Day #1 of the lab, the teacher will draw a diagram on the board for students to hypothesize which direction the molecules will move to achieve equilibrium. Students will need to either create a simple statement that explains their rationale OR draw a depiction to describe the proposed molecular movement.
In and Out - Student Written Responses: Some students find that it is easier to demonstrate their understanding of the content through written paragraphs.
In and Out - Student Response Diagrams: Other students believe that they can more effectively prove their comprehension by creating sketches of each of the scenarios (hypertonic, isotonic, hypotonic) that are associated with the cell membrane and movement into and out of a cell.
It is important to note that both strategies to assess student understanding are effective and depends on the types of student learners you have in your classes or the time/resources you have available in your classroom.