Which characteristic of sterols makes them amphipathic?
Amphipathicity requires coexistence within single molecule hydrophilic region capable electrostatic hydrogen bonding interactions water and hydrophobic region driving entropic exclusion aggregation to minimize hydrocarbon water contact. Sterols satisfy this by small three beta hydroxyl group donating hydrogen bond acting polar anchor positioned near phospholipid glycerol carbonyl interface extracellular leaflet oriented aqueous phase while bulk consists four fused cyclopentanoperhydrophenanthrene rings three six membered one five membered plus branched isooctyl chain eight carbons providing large nonpolar surface interacting via London dispersion forces with acyl chains inserting deep hydrophobic core. This Janus topology yields minimal aqueous solubility about one nanomolar preferring bilayer insertion over micellization vesicle formation. Multiple fatty acid chains define triacylglycerol hydrophobic bulk storage not amphipathic, micelle formation typical lysophospholipids large head small tail high positive curvature, high water solubility opposite sterol property hydrophobic. Understanding both hydroxyl hydrocarbon regions clarifies why sterols cannot form bilayers alone but intercalate regulating order rigidity permeability curvature elasticity membrane physiology pharmacology statins targeting HMGCoA reductase synthesis pathway and raft formation modulation for signaling regulation therapeutic considerations.
Ref: Alberts et al., MBOC 7th ed., Chapter 10: Cholesterol amphipathic structure hydroxyl hydrocarbon explanation.