The way soap bubbles form suggests a possible mechanism for the formation of early cell membranes.

Where there is life, as far as we know, there is water. That's why scientists have devoted part of the Mars Rover's tasks to looking for signs of past or present water on the red planet.

What's soap got to do with it?

Soap, like cell membranes, is made of long molecules. One end of the molecule mixes easily with water (a property known as "hydrophilic"), the other end doesn't (a property known as "hydrophobic"). As students learn in this activity, those different ends account for the way both cell membranes and bubbles form, the shape they take, and how they allow things to pass through them.

How could this contribute to the formation of life? When a bubble forms, it traps other molecules inside it, making them more likely to interact with each other. Over millennia, these interactions may have lead to the formation of simple biological building blocks, which in turn may have lead to the formation of cells or protocells.

Molecules with opposing ends have been found in Martian meteorites collected on Earth. Do they represent the early building blocks of cell membranes? Maybe—if there was once water on the planet for them to combine with. Scientists hope the Mars Rover will give them some clues to the mystery of whether or not life has existed on Mars.

ACTIVITY
Cellular Soap Opera
(adapted for "Journey to Mars" from Human Body Explorations)

It's easiest to do this activity with at least two people. One person can make the soap film and hold the handle while the other person explores how the film behaves.

1) Cut the straw in half with the scissors. 2) Thread the longer string through the two half-straws and tie the ends together to make a loop. Cut the excess string from the ends of the loop's knot. Move the string through the half-straws so that the knot is hidden inside one of the straws. This is your bubble frame. 3) Create a handle for the frame by threading the shorter string through one of the straws and tying the ends together. An easy way to get the string through the straw is to tie it to the string loop you made in the previous step, and then pull the loop to bring the tied end through one of the straws. Then untie the knot and make the handle. The frame and handle should look something like the drawing in Figure 1.

Figure 1: The bubble film apparatus.

4) Fill the pie pan with the soap solution, at least 3/4 inch (2 cm) deep. 5) Shape the bubble frame into a rectangle (as shown in Figure 1). Holding the frame by the handle, immerse the entire frame in the bubble solution. 6) Lift the frame up by the handle until the bottom of the frame is slightly out of the bubble solution and the half-straws are parallel to the tabletop. You should have a rectangular soap film between the two half-straws. If there isn’t a soap film, try immersing and lifting the frame again. 7) Hold the soap film in front of a piece of black construction paper or other black material. Carefully observe the surface of the film. Blow gently on the film and watch what happens. 8) Wet your finger in the bubble solution. Gently poke it through the soap film. What happens? Can you move your finger around in the film? Now wet your finger in plain water and poke it into the film. What happens? 9) Try gently poking a dry finger through the soap film. What happens?

Students can try putting wet, then dry fingers through a bubble film.

10) Make a new film on the frame. Roll a film can (with the top and bottom removed) in the bubble solution to coat the surfaces of the can. Grasp the film can near one end and remove it from the solution. If films have formed across the openings of the can, pop them. Insert one end of the film can through the soap film on the frame. If the film pops, make another and try again. 11) When you successfully insert a bubble solution–coated film can through the soap film, hold the can in this position and have your partner or someone else pass an object (such as a pencil) through the openings in the can, from one side of the film to the other. Can you move the film can around in the soap film? 12) Try putting a dry film can through the soap film.

A young museum visitor puts his hand through the "Soap Film Painting" exhibit. The exhibit is a large-scale version of Cellular Soap Opera.

What’s going on?

Figure 2: The "water sandwich." There is water between the two layers of soap molecules, as there is between the two layers of a cell membrane.

The soap film is a "water sandwich"—a layer of water positioned between two layers of soap (or detergent) molecules. The hydrophilic heads of the soap molecules point inward toward the water layer, and the hydrophobic tails of the soap molecules point toward the outside of the film, in contact with the air (see Figure 2).

The surface tension that causes bubbles, or bubble films, is due to the tendency of water, through hydrogen bonding, to minimize its surface area. When a finger, film can, or other object is wetted with bubble solution and inserted through the soap film, the film remains in contact with a like solution, continues to minimize its surface area, and doesn’t burst. Likewise, with a water-coated object, the tendency of water to minimize its surface area causes the film to stretch at the point of entry but not burst. A dry object, however, mechanically shears the film.

Figure 3: An actual cell membrane resembles soap film in many ways.

Plasma membranes are similar to soap films in several respects. To begin with, they are both bilayered structures, formed by molecules with hydrophilic and hydrophobic ends (compare Figure 2 with Figure 3). Second, both allow objects to move laterally within them—moving the film can around in the soap film models the way that proteins are able to move around within the plasma membrane. Third, plasma membranes and soap films are both flexible, nonrigid structures.

In addition, plasma membranes and soap films are both selectively permeable. Plasma membranes allow hydrophobic substances to pass through them, much like the soap film allows the coated finger to pass through it, because hydrophobic substances are highly soluble in the lipid bilayer. Among polar (partially charged) molecules, only small ones such as H₂0 and CO₂, can easily pass through the spaces between the phospholipids. Very large polar molecules (such as glucose) and fully charged ions such as sodium (Na+) and potassium (K+) require transport processes to cross the membrane. These substances are similar to the dry finger inserted in the soap film—they cannot pass through without special modifications. (One major difference between soap films and plasma membranes is that plasma membranes do not "pop" when confronted with an unlike substance!)

Finally, both plasma membranes and soap films (with appropriate modification) can allow the passage of large or unlike substances. Substances that can’t cross the plasma membrane on their own are transported through special processes that involve the proteins embedded in the plasma membrane. In some cases, the proteins provide channels or tunnels for the passage of molecules or ions. The wet film cans inserted into soap film are much like these protein channels.

Going further

• Coat a finger with a different type of fluid (for example, vegetable oil or rubbing alcohol) and see if you can penetrate the film without popping it. Try it several times with different fluids. What do the fluids that work seem to have in common?

• Wet your finger again in the soap solution and put your finger in the bubble. What happens to the bubble when you pull your finger out? What does this imply about cell membranes?

• Cells of humans and many other organisms have a nucleus wrapped in a membrane floating inside the cell—a sort of bubble inside a bubble. How could this have evolved? Can you make a soap bubble using bubble solution and straw and pass that bubble through the soap film like you did with the film can?