Practice question

Phase transition of lipid bilayers from lamellar gel L beta where acyl chains fully extended ordered tight orthorhombic packing low mobility to liquid crystalline L alpha where chains disordered gauche isomers high mobility monitored by differential scanning calorimetry DSC exhibiting endothermic peak at Tm and by fluorescence anisotropy probes DPH TMA-DPH showing sharp decrease. Factors affecting Tm: chain length longer sixteen to eighteen carbons increases van der Waals interactions raising Tm dipalmitoyl phosphatidylcholine sixteen zero Tm forty one degrees centigrade distearoyl eighteen zero Tm fifty five degrees, unsaturation one cis double bond drastically lowers Tm dioleoyl eighteen one Tm minus twenty degrees due kink disruption, cholesterol broadens transition forming liquid-ordered phase no cooperative melting, headgroup type minor. Transmembrane proteins broadens but not set intrinsic lipid Tm, ATP hydrolysis fuels pumps not lipid melting, lipopolysaccharides outer membrane Gram-negative influences permeability distinct. Organisms regulate fatty acid synthase and desaturases homeoviscous adaptation keeping membrane above Tm fluid at growth temperature. Hence fatty acid chain length primary determinant affecting Tm together with unsaturation, illustrating hydrocarbon packing energetics governing membrane phase behavior and adaptation.