#lipid composition

Plasma Membrane

Which type of lipid is NOT a part of the plasma membrane?

Biological membrane formation is governed by amphipathic geometry where polar headgroup cross-section balances hydrophobic tail volume, satisfying packing parameter close to 1 for bilayer assembly and stability. Phospholipids such as phosphatidylserine, phosphatidylcholine, phosphatidylethanolamine, sphingolipids like sphingomyelin and cerebroside, and cholesterol as small amphiphile intercalated among chains all meet this criterion and readily incorporate into 5 nanometer fluid mosaic film with integral proteins diffusing laterally. Triglycerides, triacylglycerols composed of glycerol esterified to three fatty acids lacking phosphate, sugar, or charged moiety, are completely apolar with no amphipathic character, unable to orient with one end in water. In aqueous environment they coalesce into bulk oil phase, phase separating rather than forming lamellae. Cells store triglycerides in lipid droplet organelles comprising neutral hydrophobic core surrounded by phospholipid monolayer decorated with perilipins and seipin, located in cytosol or within ER lumen for VLDL assembly in liver. Adipocytes contain large droplets for energy reserve mobilized by hormone-sensitive lipase and lipolysis. Therefore plasma membrane excludes triglycerides, which would compromise barrier integrity and fluidity.

Ref: Lehninger Principles of Biochemistry, 8th ed., Chapter 10: Triglycerides vs Membrane Lipids.

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Glycolipid and Phospholipid

Which of the following is NOT abundant in lipid rafts?

Isolation detergent resistant membranes quantitative shotgun lipidomics show selective lipid sorting excluding certain anionic glycerophospholipids and enriching sphingolipid cholesterol species. Sphingomyelin cholesterol ceramide cerebrosides galactosylceramide gangliosides GM1 associate preferentially due saturated acyl chains fourteen to twenty four carbons hydrogen bonding amide hydroxyl enabling tight packing into liquid ordered phase high chain order low fluidity lateral diffusion moderate. Glycolipids project bulky headgroups outward stabilizing raft extracellular leaflet sterically hindering phospholipase access. Phosphatidylserine with often unsaturated oleoyl arachidonoyl tail serine carboxyl amine conferring net negative charge pH seven favors liquid disordered phase interior leaflet interacting annexins PKC KRAS polybasic clusters via electrostatic attraction membrane potential negative inside. Hence PS consistently underrepresented raft fractions measured TLC mass spec relative enrichment factor zero point three versus SM factor three fold enrichment. Differential partitioning underpins transbilayer coupling model inner PS clustering reciprocally stabilizes outer raft via long chain interdigitation cholesterol flipping. Recognizing PS depletion explains why calcium induced PS externalization disrupts raft organization triggers coagulation tenase assembly apoptosis signaling distinct raft functions interleaflet communication examined biophysics and cell biology questions advanced level.

Ref: Levental I et al., PNAS 2010, Lipid raft composition PS depleted not enriched and coupling.

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Properties of Plasma Membrane

What is the primary determinant of lipid bilayer curvature?

Spontaneous curvature stems from imbalance between optimal headgroup repulsion and hydrocarbon chain pressure described by Israelachvili packing parameter P equals v divided by a times l. When headgroup area a large relative to volume v molecule resembles inverted cone P less than one yielding positive curvature forming micelles as lysophosphatidylcholine with one chain generating outward bulges. Small headgroup like PE PA DAG yields cone P greater than one favoring negative curvature invaginations and HII tubes needed for fusion fission. PC sphingomyelin balanced a and v give cylinder P about one stabilizing flat bilayers low curvature elastic stress. Cells manipulate curvature enzymatic remodeling PC to PA via phospholipase D reducing head size or insertion ENTH BAR amphipathic helices wedging into leaflet creating asymmetry. Cholesterol modestly increases order but near cylindrical shape contributes little curvature per se. Glycolipid large oligosaccharide exerts steric pressure outside. Linking head size to curvature clarifies tubular ER reticulons caveolae caveolin scaffolding and budding mechanisms examined in cell biology and biophysics exams requiring shape understanding and bending energetics quantitative analysis for vesicle formation.

Ref: McMahon & Gallop, Nature 2005, Mechanisms membrane curvature determinants headgroup size.

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Properties of Plasma Membrane

Which of the following affects the transition temperature (Tm) of membranes?

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.

Ref: McElhaney, Lipid Phase Transition and Chain Length Effect, Biochim Biophys Acta 1984.

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