MEMBRANE MODEL: The Bubble Lab
Soap bubbles are bilayers very similar to phospholipid membranes, so they can be
used to investigate some of the properties of the cell membrane. The soap bubble
bilayer is made of molecules with a hydrophilic (water –loving) head and a hydrophobic tail (water-fearing), exceptthat the surrounding medium (air) is non-polar (no positive or negative ends), so the tails of the bilayer face outwardand the polar heads form the inside. Other than this orientation, however, a soap membrane and a cell membrane behave pretty much the same.
Procedure
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.
- What do you observe?
- Based on your observations, do you think that the cell membrane is a solid, liquid, or gas? Explain your choice.
(The soap bilayer is actually less flexible than a cell membrane because a cell membrane is supported on both sides, one side by the cytoplasm and the other by the water that the cell is bathed in. So whatever you are doing to the soap film, plus more, can be done to cell membranes without breaking them.)
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. 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 for the water molecules and 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.
a)Do you think that the dye molecules are polar, or are they nonpolar? Why?
b)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, a type of alcohol).
b) Why did you draw your arrangement the way you did?
c)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.)
Questions to answer in your SUMMARY
Start the summary with a general statement of what the purpose of the activity was. Then go through the activity and answer the questions within it. Use your observations as the evidence. Show me in writing what your brain is thinking. Answers are not necessarily wrong if you show good thinking.
3) Is there any evidence to support the idea that molecules are always in motion?
4) Based on your observations, do you think that the cell membrane is a solid, liquid, or gas? Explain your choice
5-7) Tell 3 ways you could get the pencil through. How does this relate to how a cell membrane can let molecules through?
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 (give an educated guess) 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) Were you able to combine the small bubbles? Could you split the big bubble? If so, how did you do it?
13) Draw a model of what is happening? Try I do not grade on art ability. Label the picture.
14)
a)Do you think that the dye molecules are polar, or are they nonpolar? Why?
b)What observation did you make when you did this, and why does that support your claim for part a?
15) 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, a type of alcohol).
b) Why did you draw your arrangement the way you did?
c)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.)