Bubble Lab: (from Jen Sosnowski and Jeremy Conn http://www.haikudeck.com/p/CGr1dLyYXe/cell-membrane-bubble-lab# )


flat pans

straw-and-string membrane holders (see http://bit.ly/19tIlas for holder and soap solution instructions)

extra straws

soap solution

thread (~6 inches long, already knotted into a circle)


2-inches of plastic tubing big enough to fit a pencil through

paper clip

100 mL beaker

olive oil in dropper bottles

ethanol or rubbing alcohol in dropper bottles

food coloring


Propose and determine the qualities & properties of an artificial membrane and develop a membrane model that is supported by those qualities with evidence. Explain the limitations of the model.


1) Pour soap solution to about 1 inch depth in your pan. Be careful not to make

froth as you pour.

2) Holding the straws of the membrane holder, immerse it into the pan of soap

solution. Slowly raise it out of the pan and allow the excess soap to drip off. Hold up

the soap film-filled membrane holder. You now have a sample of cell membrane to experiment with.

3) Raise the membrane holder up to the light. Is there any evidence to support the idea that molecules are always in motion? Describe your observations below, and explain how those observations support your answer.

4) Twist the straw handles in opposite directions and bend the film into different configurations.

  1. What do you observe?

  2. Based on your observations, do you think that the cell membrane is a solid, liquid, or gas? Explain your choice.

  3. Is the soap membrane an accurate cell membrane model? Explain.

5) Create a fresh bubble. Try to push the pencil through the membrane without popping it. You may use any of the materials provided for you on the side lab table. If you have an idea that requires materials that are not provided, ask me and I'll see if I can get them for you.

Describe the method you used to push the pencil through.

6) Now, find another way to get the pencil through the membrane. How did you do it?

7) Now, find another way to get the pencil through the membrane. How did you do it?

8) Using what you figured out from the pencil techniques, see if you can form a hole in the membrane, and then seal it back up again. How did you do it?

9) Based on what you know about how lipid (and soap) molecules behave, speculate as to why your methods work.

10) If you were a cell, how might you use these type of techniques to get molecules back and forth across your membrane?

11) Use a straw to create a few bubbles in your soap solution. Coax the bubbles

toward each other and try to get them to fuse into a single big bubble. Then try and split a small bubble off the big bubble. Can you put another bubble inside the first bubble? Cells do this all the time with their membranes, although we'll see more of it when we start talking about eukaryotic cells.

MEMBRANE MODEL: Breaking It Apart

12) Put the soapy pan aside for the moment, and get a 100 mL beaker and fill it with water.

13) By now, you should be able to predict what you will see when you add three drops of oil (olive, if you're curious) to the surface of the water. The harder part will be drawing a molecular representation of what's happening in the space below.

Use a “V” for the water molecules and “ O= “ for the lipids.

14) Again, you can probably predict what you would see if you added food coloring to the water. Instead, add a drop or two of food coloring right in the middle of an oil blob.

  1. Do you think that the dye molecules are polar, or are they nonpolar?

  2. What observation did you make when you did this, and why does that support your claim for part a?

15) Wash out your beaker (with soap, and make sure you rinse the beaker well). Refill it with water, and add three drops of oil again. Take the alcohol dropper and add three drops of alcohol to the beaker.

a) Draw a molecular representation of what the molecules in the beaker look like now. Use for the alcohol molecules (EtOH is the scientific abbreviation for ethanol).

b) Why did you draw your arrangement the way you did?

  1. With this information in mind, explain why we use alcohol to disinfect cuts and scrapes. (Bonus: see if you can also explain why it hurts like heck when we put alcohol into a cut.)

Write a reflection on your experiences with the soap bubble membrane model. What did you learn? How is the soap membrane like a real cell membrane? How is the soap membrane unlike a real cell membrane?.