Spontaneously Formed Lipid Structures

Monolayers and Micelles

Amphipathic lipids spontaneously form a variety of structures when added to aqueous solution. All these structures form in ways that minimize contact between the hydrophobic lipid chains and the aqueous milieu. For example, when small amounts of a fatty acid are added to an aqueous solution, a mono-layer is formed at the air-water interface, with the polar head groups in contact with the water surface and the hydrophobic tails in contact with the air (Figure 9.2). Few lipid molecules are found as monomers in solution.

Further addition of fatty acid eventually results in the formation of micelles. Micelles formed from an amphipathic lipid in water position the hydrophobic tails in the center of the lipid aggregation with the polar head groups facing outward. Amphipathic molecules that form micelles are characterized by a unique critical micelle concentration, or CMC. Below the CMC, individual lipid molecules predominate. Nearly all the lipid added above the CMC, however, spontaneously forms micelles. Micelles are the preferred form of aggregation in water for detergents and soaps. Some typical CMC values are listed in Figure 9.3.

FIGURE 9.2 • Several spontaneously formed lipid structures.

Bilayer m w

Unilamellar vesicle

Unilamellar vesicle

Multilamellar vesicle

Multilamellar vesicle

FIGURE 9.4 • Drawings of (a) a bilayer, (b) a unilamellar vesicle, (c) a multilamellar vesicle, and (d) an electron micrograph of a multilamellar Golgi structure ( X 94,000).

(d, David Phillips/Visuals Unlimited!)

FIGURE 9.4 • Drawings of (a) a bilayer, (b) a unilamellar vesicle, (c) a multilamellar vesicle, and (d) an electron micrograph of a multilamellar Golgi structure ( X 94,000).

(d, David Phillips/Visuals Unlimited!)

Structure

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