The Cell Membrane

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Objective

Students will be able to identify and model the proteins, carbohydrates and lipids functions in the plasma membrane.

Big Idea

Regardless of cell type, cellular components function together to maintain homeostasis.

Warm-Up

5 minutes

Warm-up: How is a window screen like a cell membrane?

This warm-up question is a preview of the content that will be taught in the lesson.  It allows students to associate what they know about window screens with what they know about the cell membrane.  Remind students that they learned about cell organelles in a previous lesson, Cell Parts.  Look for students to be able to describe the characteristics that make them similar, even though they won’t likely be able to correctly name the property of selective permeability since it hasn't yet been taught. 

Once it is established how a window screen is like a cell membrane, ask students to consider why some things can cross a window screen and others cannot.  Allow 2-3 students to share their thoughts.  Look for students to identify that some things are small enough to pass through the window screen openings but others are not.  If students are not able to make this connection on their own, prompt them with questions to help them make the connection.  For example, “Why do you think air can enter the room with an open window but bugs cannot?

Inform students that they will learn more about the characteristics of the cell membrane in today’s lesson.

Introduce New Material

20 minutes

Begin the lesson by introducing the vocabulary associated with the lesson: regulate, homeostasis, barrier, phospholipid bilayer, polar, nonpolar, hydrophilic, hydrophobic, selective permeability

Say each word aloud and ask students to repeat the term after you.  Clap out the syllables for the terms with 3 or more syllables.  This helps students hear the word parts of more complex words so that they can pronounce them correctly. 

Instruct students to add the bolded terms to their Vocabulary Map. Remind students that the bolded terms contain prefixes, suffixes, Greek or Latin root words.  Provide explicit instruction of each term when it arises during the course of instruction.

Inform students of the learning target for this lesson:

  • I can explain how the structure of the plasma membrane acts as a selectively permeable barrier that maintains homeostasis by regulating the passage of molecules in and out of the cell.
  • I can explain the function of proteins, lipids and carbohydrates in the functioning of the cell membrane.

Display visual information as you instruct and ensure students take notes using guided notes that you have provided or use a note-taking strategy that you have taught.  Guided notes provide greater support for the different learning styles of students. 

As you teach the characteristics and parts of the cell membrane, use of visual aids will help anchor abstract ideas to increase students’ abilities to conceptualize the learning.  For example, a paper towel role can be used to model the protein channel function. Here are a few examples of visual aids to help students conceptualize how the barrier works:

  • Start by explaining that the cell membrane is like a boundary or fence between the cell and its environment.  Talks about how a fence acts like a barrier between a yard and the street.  This type of real-world comparisons helps students “solidify” the concepts.
  • Next, use old produce bag that is made like a window screen (typically an orange or lemon bag) and tell students to consider the produce bag a cell membrane of a cell. 
  • Wave the bag through the air and ask students, what molecules are passing the barrier and entering the bag.  Also, ask why.  Students should be able to identify air passes through the bag because air molecules are small.   
  • Ask students to predict if water can cross the barrier of the cell membrane (the produce bag).  Students should be able to correctly identify that water can easily cross the membrane.  Pour water through the bag over a trash can to reinforce how easily the water crosses the membrane. 
  • Show students a tennis ball.  Tell students to consider the tennis ball a large molecule. Ask students to predict if the tennis ball can cross the barrier. Show students how the tennis ball cannot cross through the small openings of the produce bag because it is too big.
  • Show students a marble or similar sized object.  Ask students to predict if the marble can fit through the cell membrane.  Show students that the marble is too big to fit through the small openings of the produce bag.  However, inform students that the marble can cross the cell membrane.  Give students  1-2 minutes to talk with a nearby classmate about how this might occur.   Allow a few students to share their idea for how the marble might cross into the cell (bag) even though it is too big to go through the small openings of the produce bag.  After listening to a few ideas, show students one way that this can occur by taking a paper towel roll and place it into a pre-cut opening in the bag.  Explain and model how the paper towel roll acts a channel or hallway that “helps” the marble enter the cell.
  • Compare a protein receptor to a cell phone tower that receives signals and sends messages to the inside of the cell.

Explicitly teach the terms homeostasis, hydrophilic, hydrophobic and phospholipid bilayer.  Allow students to identify what each term means without using the knowledge they have about the prefixes or suffixes, and Latin or Greek root words in each term.  Provide guidance by helping students first identify the word parts then attach meaning to each of the word parts.  For example, for Phospholipid bilayer ask students to identify at least one prefix in the term.    Look for students to correctly identify “bi” as a prefix that means two.  Point out that this prefix is in many words we use each day. Use the terms bicycle and binary as examples of terms that use this prefix.  Ask students to think about what they know about the phospholipid bilayer just by knowing what the prefix, bi- means.  Look for students to know that the bilayer is a two-layer structure.

Lastly, show students pictures of different mosaics (mosaic tree, mosaic lily and mosiac fish).  Point out how the mosaic is a complete artwork made of different parts. Explain that the cell membrane is called a “fluid mosaic”.   Provide students an opportunity to practice verbal fluency.  Instruct students to “turn and talk” for 2 minutes with a seatmate about why they think the cell membrane is called a fluid mosaic.  At the end of the time, allow a few students to share their ideas with the class.  Look for students to identify that the cell membrane is called a fluid mosaic because 1) it moves, and 2) a mosaic is made of a lot of different pieces like the cell membrane is made of a lot of different structures.

Guided Practice

5 minutes

After providing instruction on the structures that help cells maintain homeostasis, share an animation, The Cell Membrane: Just passing through. Establish the purpose for viewing the animation by explaining to students that they are to watch closely for a visual of how different types of materials pass through the membrane of a cell at any one time. 

Refer back the number of objects that were shown attempting to pass the “membrane” of the produce bag.  Explain that the job of the membrane is to regulate this movement in order to maintain the proper balance of ions, water, oxygen, carbon dioxide, nutrients, and other molecules. Point out what is occurring as the animation illustrates the movement of some of these materials and describes the structures that make it possible.

Instruct students to note any questions they have watching the animation segments. Between the different visual segments, stop and allow students to ask any questions that they have.   

Independent Practice

20 minutes

Display and distribute instructions, Modeling the Cell Membrane.  Briefly summarize the activity and preview the materials that will be used. Instruct students to work in groups of two to complete a model of the cell membrane. 

Instruct each student to complete the questions that follow the modeling activity. Before students begin, ask probing questions to hep students gain a clear understanding of why they are engaging in this activity.  For example, "What do you think you will use the cotton swabs to represent?  What do you think you will use the pipe cleaners to represent?"

Walk around to observe students as they work. Look for students to be able to work cooperatively to create the cell membrane and engage in discussion to develop responses to the questions.  Because of the different reading levels in a classroom, expect that some students will need assistance with understanding the instruction well enough to build the cell membrane.

Both student work 1 and student work 2 show that students are able to gain understanding from the modeling activity that helps them to correctly answer the analysis questions.  It is clear that student work 1 shows a higher level of thought and depth of understanding, based on the correct use of academic vocabulary and the depth of the responses, particularly for question 1.  

Close

5 minutes

Have students to reflect on their own learning using a 3-2-1 formative assessment:

3—Key ideas or important words

2—examples or words from the content

1—summary statement or question that might be asked on a test

I like to recycle old copy paper that I cut into strips for this type of ticket out the door.  Give students an opportunity to briefly share from their written responses before leaving the classroom so that any misconceptions can be addressed before class ends.