Practice question

Membrane thickness determined by fatty acyl chain length unsaturation and sterol content influencing hydrophobic matching with integral protein transmembrane domains. Cholesterol small amphipathic sterol twenty seven carbons rigid tetracyclic ring structure with hydroxyl head and iso-octyl tail approximately one point five nanometer length intercalates among phospholipid acyl chains hydroxyl near glycerol carbonyl ester oxygen forming hydrogen bond, rigid rings restricting chain trans-gauche isomerization increasing order parameter measured by deuterium NMR and electron spin resonance, condensing average area per lipid from about zero point sixty to zero point fifty square nanometers and increasing hydrophobic thickness about zero point three to zero point five nanometers demonstrated by X-ray diffraction lamellar repeat. Sphingomyelin saturated long chains also increases thickness but cholesterol present up to fifty mole percent exerts pronounced thickening particularly in liquid-ordered raft domains enriched sphingomyelin cholesterol thicker than surrounding liquid-disordered phase containing unsaturated phosphatidylcholine. Proteins with longer transmembrane helices partition preferentially into thicker domains. Among listed lipids cholesterol therefore most directly modulates thickness and rigidity affecting Na K ATPase function and mechanosensitive channels.