Using Bubbles to Explore Cell Membranes

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The Fluid Mosaic Model

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Every day, single-celled organisms, as well as multicellular organisms, must interact with their respective watery environments. Whether it’s a paramecium surviving day to day in the ever-changing health of local waterways, or a human’s bone tissue bathed in blood-like fluid bringing oxygen and nutrients to the cells while removing carbon dioxide and other wastes from the cells, all cells must have a way to maintain a consistent internal environment.

 

One way to maintain this consistency is by the actions of the cell membrane. The cell membrane separates a cell from other cells and from its surrounding fluids. It holds the cell together and can give shape to the cell. Membranes are not solid barriers- certain molecules can pass through them. They are called “selectively permeable” because some molecules are allowed to pass through, but others are not. What types of molecules do you think need to pass through the cell membrane?

 

The cell membrane consists of two layers. It has a double layer of lipid molecules, called phospholipids, with protein molecules embedded within the lipid bilayer. The phospholipids and the proteins are not rigidly fixed in place – they can move about within the membrane. For this reason, cell membranes are considered “fluid”.

         

  • What is the structure of the cell membrane and what unique properties does it have?
  • How do these properties relate to how it works?
  • How can we visualize the fluidity of a cell membrane?

 

 

GOALS:  In this lab activity, you will…

  • Examine the structure of the cell membrane by creating a model.
  • Analyze the properties of the cell membrane.
  • Relate the structure of the cell membrane to its function.

 

 

MAIN IDEAS:  The important concepts and skills covered in this activity are…

  • Cell membranes consist of two layers of phospholipid molecules with proteins embedded within this double layer.
  • Cell membranes are not rigid structures. Rather, cell membranes are fluid and dynamic. The phospholipids and the proteins may shift among each other within the cell membrane.

 

 

 

 

CONNECTIONS

 

Scientific Content –

  • In order to establish and maintain their complex organization and structure, organisms must obtain, transform and transport matter and energy, eliminate waste products, and coordinate their internal activities.
  • The cell membrane is dynamic and interacts with internal membranous structures as materials are transported into and out of the cell.
  • The cell membrane, which is composed of proteins and two layers of phospholipids, is selectively permeable and regulates what enters and leaves the cell.

 

 

Scientific Process –

  • Students will make predictions and inferences based on their observations during this activity.

 

Math/Graphing –

  • There are no math skills or graphing skills associated with this activity.

 

 

 

 

Let’s Investigate:

 

Pre-Lab

 

  1. Draw the structure of the cell membrane in the space below. Indicate which areas of the membrane are hydrophobic and which areas are hydrophilic. Include proteins that are embedded within the membrane in your drawing.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In this investigation, you will use bubbles to explore how a cell membrane might work and how its structure is related to its function.

 

 

Procedure

 

  1. Spread newspapers or paper towels on top of your lab tables or work area.

 

  1. Each group will need the following materials:  2 straws, cotton string, cotton thread, scissors, glass stirring rod, tray, aluminum wire 6-8 inches in length, and bubble solution (to a depth of 1-2 cm once placed in the tray).

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  1. Cut the straws to lengths of 15-20 cm to make 4 equal lengths.

 

 

  1. Put the cotton string through the straws to make a rectangle about 3/4 of the tray size and knot the end. Cut the excess string.

 

  1. Cut a piece of cotton string 6-7 cm in length and knot. Place this aside.

 

 

  1. Form a circle at the end of the aluminum wire approximately 2-3 cm in diameter. Place this aside.

 

  1. Form a film of bubble solution on your straw device by dipping it into the bubble solution in your tray. Show the flexible nature of membranes by bending and folding the film.

 

          Predict:  Why might this characteristic be important to a cell’s survival?

 

 

 

 

 

 

 

  1. Form an opening in the membrane by floating a circle of thread on the film, popping the inside of it, and then gently removing it by using your glass stirring rod.

 

     What happens to the membrane? Relate this ability to a function that      a cell must perform in order to survive.

 

 

 

 

 

 

 

  1. To demonstrate movement of proteins within the lipid bilayer, insert a pencil or the glass stirring rod through the membrane and move it around. Your bubble solution should not break.

 

     Predict:  Why might it be important for proteins to be able to move from one part of the cell membrane to another?

 

 

 

 

 

 

 

Optional:

  1. Demonstrate how a eukaryotic cell could “evolve” from a prokaryotic cell using the aluminum “magic wand” and your straw device. (Hint: You must float a bubble within a bubble.)

 

     Infer: How does this represent how eukaryotic cells may have evolved from prokaryotic cells?

 

 

 

 

 

 

 

 

 

Investigating Further…

 

 

The cell membrane is not the only membrane consisting of a double layer of phospholipids. Research other phospholipid bilayers and their use or function and present your findings to the class.

 

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Summary of Activity…

 

 

Compare and contrast soap bubbles and cell membranes with regard to structure and function.

           

 

 

 

 

 

 

 

 

 

 

Applying what you have learned…

           

While on a hunting trip in a remote location in Delaware, a friend of yours was bitten by a copperhead. He nearly died from hemolysis, or breakage of many of his red blood cells. The copperhead’s venom was sent to a lab and the lab technician found three different enzymes present: phospholipase, which breaks down phospholipids; neuraminidase, which removes carbohydrates from cells; and protease, which degrades proteins. Which of these enzymes do you think was responsible for his near fatal red blood cell hemolysis? Explain your reasoning.