Various techniques have been used to measure the motion of individual lipid molecules and their components. One can construct a lipid molecule, for example, with a fluorescent dye or a small gold particle attached to its polar head group and follow the diffusion of even individual molecules in a membrane. Alternatively, one can modify a lipid head group to carry a “spin label,” such as a nitroxide shape of packing of molecule molecules in water
Figure 10–7 Packing arrangements of amphiphilic molecules in an aqueous environment.
(A) these molecules spontaneously form micelles or bilayers in water, depending on their shape. cone-shaped amphiphilic molecules (above) form micelles, whereas cylinder-shaped amphiphilic molecules such as phospholipids (below) form bilayers. (B) A micelle and a lipid bilayer seen in cross section. note that micelles of amphiphilic molecules are thought to be much more irregular than drawn here (see Figure 10–26c).
Figure 10–6 How hydrophilic and hydrophobic molecules interact differently with water. (A) Because acetone is polar,  it can form hydrogen bonds (red) and favorable electrostatic interactions (yellow) with water molecules, which are also polar. thus, acetone readily dissolves in water.
(B) By contrast, 2-methyl propane is entirely hydrophobic. Because it cannot form favorable interactions with water, it forces adjacent water molecules to reorganize into icelike cage structures, which increases the free energy. this compound is therefore virtually insoluble in water. the symbol δ– indicates a partial negative charge, and δ+ indicates a partial positive charge. polar atoms are shown in color and nonpolar groups are shown in gray.
planar phospholipid bilayer with edges exposed to water
Figure 10–8 The spontaneous closure of a phospholipid bilayer to form a sealed compartment. the closed structure is stable because it avoids the exposure of the hydrophobic hydrocarbon tails to water, which would be energetically unfavorable.
Figure 10–9 Liposomes. (A) An electron micrograph of unfixed, unstained, synthetic phospholipid vesicles—liposomes—in water, which have been rapidly frozen at liquid-nitrogen temperature. (B) A drawing of a small spherical liposome seen in cross section. liposomes are commonly used as model membranes in experimental studies, especially to study incorporated membrane proteins. (A, from p. Frederik and D. hubert, Methods Enzymol. 391:431–448, 2005. With permission from elsevier.) group (=N–O); this contains an unpaired electron whose spin creates a paramagnetic signal that can be detected by electron spin resonance (ESR) spectroscopy, the principles of which are similar to those of nuclear magnetic resonance (NMR), discussed in Chapter 8. The motion and orientation of a spin-labeled lipid in a bilayer can be deduced from the ESR spectrum. Such studies show that phospholipid molecules in synthetic bilayers very rarely migrate from the monolayer (also called a leaflet) on one side to that on the other. This process, known as “flip-flop,” occurs on a time scale of hours for any individual molecule, although cholesterol is an exception to this rule and can flip-flop rapidly. In contrast, lipid molecules rapidly exchange places with their neighbors within a monolayer (~107 times per second). This gives rise to a rapid lateral diffusion, with a diffusion coefficient (D) of about 10–8 cm2/sec, which means that an average lipid molecule diffuses the length of a large bacterial cell (~2 μm) in about 1 second. These studies have also shown that individual lipid molecules rotate very rapidly about their long axis and have flexible hydrocarbon chains. Computer simulations show that lipid molecules in synthetic bilayers are very disordered, presenting an irregular surface of variously spaced and oriented head groups to the water phase on either side of the bilayer (Figure 10–10).