Petri Dish: The plasma membrane
A cell is surrounded by a plasma membrane that forms a barrier between the inside of the cell and the outside environment.
Plasma membranes are not solid barriers—they are liquid films, similar to soap bubbles. These films consist of two layers of phospholipid molecules, with embedded proteins that selectively transport molecules into and out of the cell.
Polar and nonpolar molecules
Phospholipids belong to a class of molecules known as surfactants. These molecules have a polar head at one end and a nonpolar tail at the other. Polar molecules—like water—are attracted to one another and behave like tiny magnets. Nonpolar molecules—like fats and oils—are not attracted to one another.
In this simulation, the gray oblongs are nonpolar fat molecules. Notice how the fat molecules ignore each other except when they are colliding.
On the other hand, in the second simulation, the red circles representing water molecules are drawn together and clump up. This happens because the polar water molecules are attracted to one another.
What happens when our polar and nonpolar molecules are mixed together? In the third simulation, the gray fat molecules ignore both the red water molecules and the other gray fat molecules. The red water molecules ignore the gray fat molecules, but are drawn to the other red water molecules. As the red water molecules clump up, the gray fat molecules get pushed to the side—and the water and fat molecules separate.
Because polar and nonpolar molecules separate instead of mixing, polar molecules are often described as water-soluble, and nonpolar molecules as fat-soluble.
Surfactants
Using water to wash oil off of a spoon is not very effective because the oil and water do not mix. The water will dissolve and lift polar molecules off of the spoon, but not the nonpolar oil molecules. To clean the spoon in water, we need to add a bit of soap.
Soap is a surfactant—it has a polar head and a nonpolar tail. In a mixture of oil and water, surfactant molecules will form tiny bubbles called micelles. When washing dishes, these micelles will surround the oil molecules on the spoon, giving them a polar coating. Notice how the soap molecules (the gray oblongs with red circles) are oriented so their polar heads are facing outward toward the water and their nonpolar tails are facing inward toward the oil. This enables the water to lift away the oil molecules.
In a salad dressing, the micelles are reversed. Egg yolks and mustard both contain surfactant molecules that enable the polar vinegar molecules in a salad dressing to mix with the nonpolar oil molecules, forming an emulsion. But since salad dressings contain more oil than vinegar, the micelles end up surrounding the vinegar molecules instead of the oil—and the polar heads face inward, not outward.
Soap bubbles
Micelles form naturally when mixing oil and vinegar with a surfactant. All you have to do is shake up your salad dressing vigorously and you’ll end up with an emulsification—microscopic droplets of vinegar surrounded by the surfactant and suspended in oil.
Soap bubbles also form naturally when soapy water is mixed with air. The molecules in air are generally nonpolar, so you’ll end up with a thin film of water sandwiched between two layers of soap.
In water, phospholipid molecules naturally form a similar bubble, except the two phospholipid layers are polar on the outside and nonpolar on the inside. You can think of the plasma membrane surrounding a cell as a bubble of fat. Because it is a bubble of fat, polar molecules have a hard time passing through the plasma membrane. Because the bubble of fat is liquid, plasma membranes can stretch, shrink, flow, merge, split and deform in the same way as soap bubbles.
