#membrane lipids

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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Plasma Membrane

What is the role of phosphatidylinositol in the membrane?

Phosphatidylinositol's inositol ring protruding into cytosol can be phosphorylated to produce phosphoinositides that act as transient second messenger precursors. Upon activation of GPCR coupling to Gq or RTK activating PLC-gamma, phospholipase C cleaves PIP2 into diacylglycerol which stays embedded to recruit C1 domain of protein kinase C and Munc13 for exocytosis, and soluble IP3 that binds tetrameric IP3 receptor calcium channels on ER releasing calcium that activates calmodulin and NFAT. Concurrently class I PI3K phosphorylates PIP2 to PIP3 which accumulates transiently and recruits proteins containing PH domain such as Akt, PDK1, and BTK to plasma membrane, triggering Akt-mTOR pathway controlling survival, protein synthesis, and glucose metabolism. Dephosphorylation by PTEN and SHIP phosphatases terminates signal. Additional roles include PI4P at Golgi recruiting adaptors for vesicle budding and PIP2 binding to actin regulators like profilin. Thus PI cycle provides rapid, reversible production of lipid and soluble messengers, distinguishing it from structural roles or ion conduction.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 15: Phosphoinositide Signaling Cascade.

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Plasma Membrane

Which of the following forms mixed bilayers with phosphoglycerides?

Phosphoglycerides and sphingomyelin can co-assemble into stable mixed bilayers because both possess two hydrophobic tails and cylindrical geometry compatible with lamellar phase, unlike lysophospholipids that form micelles or cardiolipin that favors hexagonal inverted phases due to small headgroup. Sphingomyelin's fatty acid amidated to sphingosine resembles phosphatidylcholine's diacylglycerol moiety in hydrophobic thickness, allowing van der Waals matching and hydrophobic mismatch avoidance, enabling fluid mosaic continuity. Moreover sphingomyelin synthesis itself links the two classes: sphingomyelin synthase SMS1 and SMS2 in Golgi and plasma membrane transfer phosphocholine from phosphatidylcholine onto ceramide producing diacylglycerol signaling lipid activating protein kinase D. In model membranes, sphingomyelin-phosphatidylcholine mixtures exhibit miscibility with cholesterol-induced phase separation into liquid-ordered rafts, explaining outer leaflet enrichment and organization of GPI-anchored proteins. Cholesterol alone is too small with single hydroxyl to form bilayer alone, requiring phospholipid matrix, while phosphatidylinositol prefers inner leaflet due to polyphosphate negative charge. Thus sphingomyelin uniquely partners with phosphoglycerides to build heterogeneous mosaic bilayer with domains.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Mixed Bilayers of Sphingomyelin.

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Plasma Membrane

Which of the following molecules is the main precursor for sphingolipids?

Building sphingolipids requires creation of sphingoid long-chain base de novo in endoplasmic reticulum membrane through highly regulated steps essential for barrier formation. Serine palmitoyltransferase, heterodimer of SPTLC1 and SPTLC2 regulated by ORMDL feedback inhibition and small subunits ssSPTa/b that determine acyl chain preference, condenses L-serine and palmitoyl-CoA producing 3-ketodihydrosphingosine, reduced by 3-ketodihydrosphingosine reductase KDSR to dihydrosphingosine, also called sphinganine. Dihydrosphingosine is N-acylated by six ceramide synthases CerS isoforms with fatty-acyl specificity ranging C14 to C26 to dihydroceramide and desaturated by dihydroceramide desaturase DES1 to ceramide whose backbone is sphingosine, 2-amino-4-octadecene-1,3-diol. This amino alcohol thus represents defining precursor whose fatty acid chain can be remodeled via salvage pathway where sphingosine generated from sphingomyelin hydrolysis by acid sphingomyelinase is rapidly reacylated. Subsequent phosphorylation by sphingosine kinases 1 and 2 yields sphingosine-1-phosphate high-affinity ligand for S1PR1-5 GPCRs controlling lymphocyte egress from lymph nodes, angiogenesis, and vascular barrier integrity and immune regulation in mammals critically.

Ref: Lehninger Principles of Biochemistry, 8th ed., Chapter 21: Sphingosine Precursor and S1P Signaling.

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Plasma Membrane

Which type of linkage stabilizes the structure of ceramide?

Ceramide comprises sphingosine, an 18-carbon amino alcohol with trans double bond between C4-C5, linked to fatty acid of 14 to 26 carbons via amide bond at C2 amino position providing chemical stability. Reaction catalyzed by ceramide synthases CerS1-6 in endoplasmic reticulum shows chain-length specificity determining biological outcome such as apoptosis versus differentiation. Amide linkage is more resistant to hydrolysis than ester bond and participates actively in intermolecular hydrogen bonding through NH as donor and carbonyl oxygen as acceptor, interacting with adjacent hydroxyl groups at C1 and C3, creating tight lateral network that elevates melting point, reduces permeability, and promotes formation of ceramide-rich platforms inducing negative membrane curvature important for vesicle budding and mitochondrial outer membrane permeabilization via Bax pore formation. Beyond structural role, ceramide is generated rapidly by neutral and acid sphingomyelinases hydrolyzing sphingomyelin in response to TNF-alpha, radiation, and stress, activating protein phosphatase 2A and cathepsin D cascades that arrest cell cycle and promote death, linking chemistry to signaling pathways.

Ref: Lehninger Principles of Biochemistry, 8th ed., Chapter 10: Ceramide and Sphingolipid Metabolism.

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Plasma Membrane

Which of the following is NOT a function of cholesterol in the plasma membrane?

Cholesterol's structure with rigid tetracyclic ring system and short iso-octyl tail allows it to insert into phospholipid bilayers with hydroxyl near phospholipid carbonyls and ring system interacting with upper acyl chains. At low temperatures it disrupts tight packing of saturated chains, preventing transition to gel phase and preserving lateral diffusion essential for protein function, while at high temperatures its rigid ring restricts chain motion, decreasing permeability to small polar molecules, protons, and sodium ions, maintaining electrochemical gradients. Within membranes cholesterol drives formation of liquid-ordered lipid rafts that concentrate GPI-anchored proteins and Src kinases for signaling. Beyond barrier modulation, cholesterol is substrate for mitochondrial P450scc that cleaves side chain to pregnenolone, precursor for glucocorticoids, mineralocorticoids, sex steroids, bile acids, and vitamin D. Passive transport of glucose through GLUTs or water through aquaporins does not require cholesterol as facilitator; proteinaceous pores and carriers mediate downhill movement independent of sterol content, so cholesterol does not act as transport facilitator.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Cholesterol Functions and Steroidogenesis.

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Plasma Membrane

Which of the following linkages is found in sphingomyelin but not in phosphoglycerides?

Phosphoglycerides use glycerol backbone where fatty acids are linked to sn-1 and sn-2 hydroxyls via O-ester bonds, susceptible to phospholipase A1 and A2 hydrolysis generating lysophospholipids and free fatty acids like arachidonic acid. Sphingomyelin architecture departs: its backbone is sphingosine, containing long-chain amino alcohol with double bond, where fatty acid attaches at amino group at C2 through N-amide bond formed by ceramide synthase in ER, producing ceramide intermediate. This amide bond affords additional hydrogen bond donor and acceptor, raising melting temperature and strengthening interaction with cholesterol's hydroxyl through hydrogen bonding network, favoring liquid-ordered domains resistant to detergent extraction. After ceramide formation, phosphocholine is transferred from phosphatidylcholine to ceramide's C1 hydroxyl by sphingomyelin synthase, creating phosphodiester linkage shared with phosphoglycerides but distinct acyl linkage. Therefore presence of amide bond fundamentally distinguishes sphingomyelin, influencing its metabolic stability, raft partitioning, and role as source for bioactive ceramide upon sphingomyelinase action during stress signaling.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10: Sphingomyelin Structure, Amide Linkage.

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Plasma Membrane

What is the primary function of glycolipids in the plasma membrane?

Glycolipids are restricted to the exoplasmic leaflet where their oligosaccharide chains, assembled in Golgi by glycosyltransferases and glycosidases, face extracellular milieu forming glycocalyx extending beyond protein domains. This topology makes them poorly suited to regulate fluidity, which is governed by fatty acid saturation and cholesterol content, or to transport ions, mediated by channels and pumps. Their principal contribution lies in recognition and signaling. Carbohydrate moieties constitute ABO and Lewis blood group antigens, selectin ligands like sialyl Lewis X that direct leukocyte rolling and extravasation, and specific receptors for microbes: GM1 ganglioside binds cholera toxin B subunit, Gb3 binds Shiga toxin, and sulfatide binds influenza virus. Clustering in cholesterol-rich lipid rafts amplifies avidity through multivalent carbohydrate-lectin interactions, organizing signal transduction and sorting in polarized epithelia. In nervous system, gangliosides modulate Trk receptor activity and neurite outgrowth. Thus glycolipid function exemplifies how membrane lipid chemical diversity translates into identity cues guiding development, immunity, and pathogen interactions rather than bulk membrane mechanics.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Glycocalyx and Cell Recognition.

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Plasma Membrane

Which of the following phospholipids is primarily involved in apoptosis?

Phosphatidylserine is normally confined to the cytoplasmic leaflet of the plasma membrane at greater than 85% asymmetry, maintained by P4-ATPase flippases ATP11A and ATP11C that use ATP to translocate aminophospholipids inward while floppases move lipids outward. Early in apoptosis, effector caspase-3 cleaves and inactivates flippases and simultaneously activates calcium-dependent scramblases such as Xkr8 and ANO6, causing rapid collapse of asymmetry and exposure of phosphatidylserine on the external surface. This externalized lipid functions as dominant eat-me signal recognized by dedicated phagocyte receptors TIM4, BAI1, stabilin-2, and MerTK via bridging molecule Gas6 stimulating Rac1-dependent cytoskeletal engulfment without release of proinflammatory cytokines. In vitro, Annexin V which binds phosphatidylserine in calcium-dependent manner marks apoptotic cells for flow cytometry detection. Sustained surface phosphatidylserine also provides catalytic surface for tenase and prothrombinase complexes in blood coagulation cascade. Failure to clear phosphatidylserine-positive apoptotic bodies contributes to systemic lupus-like autoimmunity, illustrating how lipid topology directly couples biochemistry to immune tolerance and homeostasis.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Lipid Asymmetry and Apoptosis.

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Plasma Membrane

Which of the following is a major glycolipid found in the plasma membrane?

Gangliosides are prominent glycosphingolipids of vertebrate plasma membranes, characterized by a hydrophobic ceramide anchor embedded in the outer leaflet linked to a complex oligosaccharide carrying one or more sialic acid residues. This bulky carbohydrate headgroup projects into the extracellular space and participates in the glycocalyx, mediating cell-cell recognition, modulation of receptor tyrosine kinases such as EGFR, and adhesion during neural development and immune synapse formation. Unlike sphingomyelin which incorporates phosphocholine as headgroup via phosphodiester bond, or phosphatidylinositol which is a glycerophospholipid with inositol phosphate, gangliosides are synthesized stepwise in the Golgi apparatus by glycosyltransferases that sequentially add galactose, N-acetylgalactosamine, and sialic acid to ceramide arriving from ER on CERT transporter. Their saturated chains favor association with cholesterol in lipid rafts, increasing local concentration. Functionally they serve as antigenic determinants, regulators in neurogenesis, and docking sites for bacterial toxins like cholera and tetanus, illustrating how carbohydrate diversity confers recognition specificity beyond simple barrier function in physiology.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Membrane Structure, Glycosphingolipids and Gangliosides.

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Plasma Membrane

Which of the following membrane lipids carries a negative charge at physiological pH?

Electric field across membrane influenced anionic phospholipids clustered inner leaflet physiological pH seven point four generating negative surface potential crucial signaling recruitment. Phosphatidylserine headgroup contains phosphate pKa less two carboxyl pKa three amine pKa nine net charge minus one neutral pH despite zwitterionic potential measured zeta minus fifteen millivolts inner leaflet ten to fifteen percent total phospholipid outer almost zero. Approximately PS binds Annexin V calcium dependent detection apoptosis C2 domains PKC synaptotagmin polybasic KRas via electrostatic attraction enriching signaling inner surface recruitment MARCKS Effector Domain. Phosphatidylcholine quaternary ammonium constitutive positive balancing phosphate net neutral zero, sphingomyelin similarly neutral zwitterion with phosphocholine head balanced, cholesterol neutral small hydroxyl uncharged only. Maintaining PS inside requires ATP11C flippase CDC50A complex ATP dependent; externalization during apoptosis creates electronegative patch recognized phagocyte TIM receptors stabilin-2 BAI1 initiating clearance. Understanding negative charge carrier explains why inner leaflet more negative recruiting polybasic proteins why calcium influx upon activation neutralizes PS facilitating membrane fusion and blood coagulation tenase prothrombinase complexes assembly PS rich platelet surface supporting clotting amplification mechanisms thoroughly examined physiology and cell biology and immunology.

Ref: Levental & Grzybek, Annu Rev Physiol 2010, Phosphatidylserine anionic negative charge electrostatic signaling and clearance.

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Glycolipid and Phospholipid

Which type of lipid interaction is disrupted by methyl-β-cyclodextrin?

Lipid raft integrity pharmacologically perturbed using methyl beta cyclodextrin torus shaped oligosaccharide seven glucose units hydrophobic interior cavity zero point eight nanometer diameter fitting sterol. Cavity fits cholesterol high affinity one to two stoichiometry extracting sterol membrane into soluble inclusion complex without hydrolyzing phospholipids at low concentrations below ten millimolar preserving phospholipid mass. Removal dismantles cholesterol sphingolipid condensed complexes driven van der Waals packing hydrogen bonding amide hydroxyl interactions transforming liquid ordered liquid disordered dispersing GPI anchored proteins caveolin flotillin abolishing cholera toxin B GM1 clustering visualized fluorescence microscopy FRET, FRAP diffusion increase. Peptide bonds protein synthesis ribosomes remain intact unrelated processes. Low concentrations selectively deplete raft cholesterol leaving bulk phospholipids, high doses extract phospholipids causing toxicity membrane blebbing. Repletion cholesterol loaded cyclodextrin restores rafts proving reversibility causality gold standard control. Reagent standard tool testing raft dependent signaling Src kinase activation HIV entry APP processing amyloid beta production demonstrating dependence microdomains cholesterol sphingolipid association rather than protein scaffold alone concept widely taught cell biology immunology virology courses and assay design for microdomains investigation and antiviral strategies.

Ref: Zidovetzki & Levitan, Biochim Biophys Acta 2007, Cyclodextrin cholesterol sphingolipid raft disruption mechanism and reversibility.

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