3. Amphipathic Lipids and the Creation of Micelles.

To understand the remarkable properties of amphipathic lipids, consider the following thought experiment.

A quantity of phospholipid is placed in a separatory funnel containing equal volumes of olive oil and a saline solution. Then the funnel is shaken vigorously, thoroughly mixing the initially separate (or immiscible) oil and water layers and dissolving the phospholipid. If the layers are now allowed to separate completely once again, both the upper, oil layer and the bottom, aqueous layer will be slightly cloudy and the interface will be slightly more evident than it was before introducing the phospholipid. If more phospholipid is added, and the experiment repeated, the upper and lower phases appear more cloudy; less phospholipid produces more transparent layers. What's happening?

When shaken the phospholipid molecules distribute themselves between the oil and the saline phases in an interesting manner, as illustrated to the right. Being amphipathic, phospholipid can enter both phases but cannot completely dissolve in either! In the saline solution, phospholipid breaks up into very small but organized structures called spherical micelles, each of which is formed from many individual molecules aggregating with their nonpolar tales pointing inwards and polar heads outward (interacting with the water molecules and saline ions). As if each molecule were "trying" to go into solution but can not! Similarily, spherical micelles are formed in the oil phase, but the phospholipids exhibit the opposite orientation (including trapped saline droplets at the micelle core). The micelles are large enough to scatter light and if enough are present, each phase become cloudy, even translucent. At the oil/water interface phospholipids readily form a different, extended structure called a planar micelle or monolayer.

Thus, phospholipids associate spontaneously into stable micelles because of their amphipathic properties. This particular mixing experiment simply illustrates what would happen naturally, over a longer period of time. What stabilizes these structures?  Moreover, what do micelles have to do with plasma membranes? To answer this second question, we next need to consider real data from an actual experiment.