= Solution
A <lipid bilayer> has two opposed leaflets: <hydrophilic> heads contact the water on either side, while <hydrophobic> tails meet in its interior. This arrangement reduces exposure of the tails to <water> through the <hydrophobic effect>. Approximately cylindrical molecular packing favors a <lipid bilayer>; a strongly wedge-shaped <amphiphile> can instead form a <micelle>. Closing a sheet into a <lipid vesicle> removes its costly exposed edge.
Within a fluid membrane, lipids diffuse laterally, while transfer of a polar head through the hydrophobic core is much slower without assistance. The core limits passive transport of ions and many polar solutes; membrane <proteins> provide selective routes. Bending costs <free energy> but does not require the large area compression associated with stretching the molecular packing, so a thin membrane can bend readily while strongly resisting area change. <Cholesterol> and tail saturation alter packing and fluidity. In a cell, the bilayer also supports embedded <proteins> and a difference of <electric potential> across the membrane.
\Image[/past-exam-of-the-quantum-condensed-matter-course-of-the-university-of-cambridge/2009-membrane-and-nematic.png]
{title=Lipid bilayer organization and the low-field and field-aligned states of a twisted nematic cell}
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