#membrane biology

Plasma Membrane

Which membrane component is responsible for forming a hydrophilic barrier?

Aqueous compartments surround cells, so membrane lipids must present hydrophilic surface to water while hiding hydrophobic interior away from solvent. Phospholipid headgroups such as phosphocholine, phosphoethanolamine, phosphoserine, and phosphoinositol are zwitterionic or anionic, heavily hydrated, forming hydrogen bonds with water and creating interfacial region of high polarity and dielectric constant. This polar curtain, two molecules thick around bilayer perimeter, repels hydrophobic compounds and prevents uncontrolled leakage of cellular metabolites, establishing hydrophilic barrier essential for compartmentalization and selective permeability. Hydrophobic effect drives spontaneous bilayer assembly: burying acyl chains minimizes ordered water cages surrounding hydrocarbons, increasing entropy and lowering free energy. Van der Waals interactions provide additional stabilization within core. Cholesterol's amphipathic nature contributes modestly, but protein channels actually provide aqueous pathways that bypass hydrophobic core for ions. Fatty acid tails form continuous hydrocarbon interior that is barrier to polar solutes. Energetic cost of exposing acyl chains explains spontaneous vesicle closure and rapid self-healing after injury.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Membrane Barrier and Headgroup Hydration.

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Glycolipid and Phospholipid

Which statement about lipid droplets is correct?

Lipid droplets defy classic bilayer vesicle definition possessing neutral core surrounded by monolayer explaining unique buoyancy density microscopy appearance. Emergence begins when DGAT1 DGAT2 synthesize triacylglycerol DAG acyl CoA ACAT1 ACAT2 forms cholesteryl ester accumulating between ER leaflets forming oil lens twenty to sixty nanometers growing via Ostwald ripening coalescence regulated Seipin LDAF1 perilipin recruitment. Lens buds toward cytosol coated phospholipid monolayer enriched PC PE lyso PC preventing coalescence decorated perilipin family PLIN1 five CIDE proteins Seipin Rab18 regulating lipolysis contacts mitochondria peroxisomes for beta oxidation fatty acid exchange energy provision. Function storing energy five kilocalories per gram sequestering toxic free fatty acids cholesterol preventing lipotoxic ER stress unfolded protein response providing precursors membrane synthesis eicosanoids prostaglandins. Mobilization ATGL PNPLA2 HSL LIPA lipases and lipophagy via Rab7 LC3 autophagy. They contain hundreds associated proteins contrary protein free notion and do not produce ATP via oxidative phosphorylation which occurs mitochondria matrix. Understanding monolayer architecture explains why droplets float density gradients expand obesity metabolic syndrome nonalcoholic fatty liver disease pathology and diabetes insulin resistance cellular metabolism context for biomedical exams.

Ref: Walther TC & Farese RV, Annu Rev Biochem 2012, Lipid droplets neutral lipid storage monolayer organization and functions.

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Glycolipid and Phospholipid

Which of the following is NOT a glycolipid?

Lipid classification relies headgroup chemistry: glycolipid defined direct glycosidic linkage sugar ceramide without phosphate intermediate distinct from phospholipid. Glucosylceramide galactosylceramide lactosylceramide GM1 ganglioside exemplify category synthesized UGCG glucosyltransferase B4GALT5 galactosyltransferases producing beta linkage to ceramide hydroxyl. Sphingomyelin instead ceramide one phosphate linked choline ethanolamine via phosphodiester synthesized sphingomyelin synthases SGMS1 Golgi SGMS2 plasma membrane transferring phosphocholine phosphatidylcholine ceramide releasing diacylglycerol DAG signaling second messenger activating PKD. Therefore SM belongs sphingophospholipid subset not glycolipid though shares sphingosine backbone amide fatty acid chain eighteen to twenty four carbons. It is zwitterionic neutral pH seven abundant outer leaflet twenty percent phospholipid myelin sheath thirty percent organizing cholesterol rafts via saturated chains tight packing. Its catabolism sphingomyelinase SMPD1 produces ceramide signaling apoptosis stress. Misclassifying SM as glycolipid overlooks phosphate presence distinct metabolic regulation crucial understanding Niemann Pick disease types A B due SM accumulation liver spleen neurodegeneration and raft biology experimental tools lysenin toxin binding SM specifically requires careful classification for lipidomics interpretation.

Ref: Shayman JA, J Lipid Res, Sphingomyelin is sphingophospholipid not glycolipid distinction and metabolism.

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Glycolipid and Phospholipid

Which of the following best describes cholesterol’s role in lipid rafts?

Liquid ordered raft stability arises favorable packing between sphingolipid and sterol driven enthalpy entropy compensation. Cholesterol small polar hydroxyl rigid planar tetracycle inserts into voids between long saturated amide linked acyl chains sphingosine backbone maximizing van der Waals contacts permitting hydrogen bond between sphingosine C3 hydroxyl cholesterol hydroxyl and amide carbonyl to amide donor. This promotes condensed complex formation increasing bilayer thickness four angstroms decreasing permeability solutes and creating diffusion distinct from liquid disordered with FRAP coefficients point one micrometer squared per second versus one. Methyl beta cyclodextrin extraction collapses order increasing disorder and lateral mixing. Cholesterol does not thin membranes nor generically increase permeability; low concentrations increase permeability slightly gel phase but fluid phase reduces it significantly. Proteins with saturated GPI double palmitoyl anchors partition into ordered domains enriching signaling kinases Src Lyn. Describing stabilization sphingolipid cholesterol interaction distinguishes physical chemistry rafts from simple protein clustering and explains why raft size regulation affects immune receptor triggering BCR TCR activation and signaling thresholds examined in cell biology immunology questions about domain formation mechanism and biophysics of phase separation.

Ref: Simons & Sampaio, Cold Spring Harb Perspect Biol 2011, Cholesterol sphingolipid stabilization rafts formation.

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Properties of Plasma Membrane

Which of the following is NOT involved in maintaining plasma membrane asymmetry?

Maintenance of nonrandom phospholipid distribution across leaflets relies on specialized lipid translocators rather than hydrolytic enzymes. Flippases are P4 ATPases such as ATP11C and ATP11A complexed with CDC50A chaperone that consume ATP to move phosphatidylserine and phosphatidylethanolamine inward against gradient maintaining inner negativity. Floppases are ABC transporters ABCA1 ABCB1 ABCB4 that export phosphatidylcholine and cholesterol outward crucial for HDL biogenesis and bile secretion. Scramblases like TMEM16F ANO6 and Xkr8 family are calcium activated or caspase activated providing bidirectional nonselective pathways randomizing composition during platelet coagulation and apoptosis for phagocytosis. These activities are probed with NBD labeled phosphatidylserine internalization assays N ethylmaleimide inhibition and ionomycin calcium triggering. Lipases including phospholipase A2 cleaving sn two acyl, PLC hydrolyzing PIP2 to DAG IP3, PLD generating PA are catabolic signaling enzymes altering lipid identity but not transporting intact lipid across bilayer. Hence lipase unrelated to asymmetry maintenance distinguishing translocation from hydrolysis preventing confusion interpreting knockout phenotypes and drug targets in cell biology.

Ref: Pomorski & Menon, Annual Review Cell Biology, Lipid translocators flippase floppase scramblase functions.

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Properties of Plasma Membrane

Which of the following is NOT a function of lipid bilayers?

Biological membranes excel at compartmentalization, selective permeability, scaffolding receptors, and regulating fluidity, but they do not directly catalyze polymerization of deoxyribonucleotides into DNA strands. Lipid bilayers form continuous hydrophobic barriers five nanometers thick preventing free diffusion of ions and polar metabolites, enabling maintenance of sodium potassium gradients by Na K ATPase and creation of organelle specific lumens for oxidative folding. They anchor integral proteins via hydrophobic matching, recruit peripheral proteins through PIP2 and phosphatidylserine electrostatic interactions, and host signaling complexes within liquid ordered rafts to amplify receptor activation. DNA replication, by contrast, occurs in nucleoplasm, mitochondrial matrix, chloroplast nucleoids, or bacterial nucleoid and relies on helicase unwinding, SSB stabilization, primase RNA primers, DNA polymerases alpha delta epsilon, clamp PCNA, RNase H, and ligase sealing Okazaki fragments. No phospholipid chemically participates in phosphodiester bond formation between dNTPs. Membranes influence replication indirectly by nuclear envelope breakdown cues and dNTP transporter localization, distinguishing architectural support from enzymatic catalytic function important for conceptual clarity in competitive examinations.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Membrane functions and Chapter 5: DNA replication machinery.

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Plasma Membrane Proteins

Which of the following proteins spans the membrane 14 times?

Band 3 anion exchanger family SLC4A1 represents prototypical high abundance multi-pass transporter in human erythrocytes comprising about twenty five percent integral membrane protein. Full length nine hundred eleven amino acids organized into two domains: N-terminal cytoplasmic domain approximately four hundred three residues forming elongated dimeric structure binding ankyrin, protein 4.2, deoxyhemoglobin, and glycolytic enzymes including aldolase regulating metabolism; C-terminal membrane domain approximately five hundred residues comprising fourteen transmembrane helices arranged into two inverted repeats each seven helices forming core and gate domains exhibiting elevator-like transport movement confirmed by crystal structures of inward and outward facing states. Arg730 and Glu681 form selectivity filter catalyzing electroneutral exchange of chloride for bicarbonate at rate up to fifty thousand turnovers per second essential for carbon dioxide carriage. Glycophorin single-pass nineteen residues, GPCR seven-pass signaling, ankyrin peripheral adaptor not spanning. Mutations causing South Asian ovalocytosis, spherocytosis illustrate fourteen-span complexity enabling high-capacity anion flux without ATP hydrolysis using gradient energy.

Ref: Arakawa et al., Crystal Structure of AE1 Band 3 Anion Exchanger, Science 2015.

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Plasma Membrane Proteins

Which of the following is an example of a single-pass transmembrane protein?

Single-pass type I membrane protein topology illustrated by glycophorin A major sialoglycoprotein CD235a abundant about million copies per red cell. Gene GYPA encodes one hundred fifty amino acids including signal peptide nineteen residues removed cotranslationally. Extracellular domain seventy residues heavily O-glycosylated with fifteen O-linked tetrasaccharides NeuAc-alpha2-3Gal-beta1-3 GalNAc plus one N-linked complex glycan contributing sixty percent mass, terminal sialic acid provides negative zeta potential preventing aggregation and defines M and N blood group antigens recognized by anti-M anti-N antibodies. Transmembrane segment residues seventy three to ninety five forms nineteen residue alpha helix enriched leucine isoleucine valine with dimerization motif L75 IxxG79 V80 xxG83 V84 xxT87 allowing tight van der Waals packing and strong association measured minus twelve kilocalories per dimer widely studied for helix-helix energetics. Cytosolic tail thirty six residues acidic interacts with protein 4.1R linking to spectrin actin junction regulating lateral mobility. Unlike seven-pass GPCR and fourteen-pass Band 3, single-pass architecture simplifies biophysical and invasion studies as receptor for Plasmodium falciparum EBA-175.

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 10, Single-Pass Proteins – Glycophorin.

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Plasma Membrane Proteins

Which of the following interactions primarily stabilizes transmembrane proteins within the lipid bilayer?

Stabilization of transmembrane segments inside bilayer originates from hydrophobic effect plus van der Waals packing. Sec61 translocon provides aqueous pore laterally gated allowing nascent hydrophobic sequence to partition directly into lipid phase assessed by apparent free energy scale of Hessa and von Heijne measuring contribution per residue leucine minus zero point five six kilocalories, arginine positive two point five kilocalories. Burying non-polar side chains of leucine isoleucine valine phenylalanine methionine avoids forcing water to form ordered clathrate cages around them, releasing water to bulk increasing system entropy and providing favorable free energy. Once inserted, side chain interactions with surrounding acyl chains and annular shell lipids contribute additional stabilization minimizing hydrophobic mismatch where thickness adjusts locally via stretching or tilting. Flanking tryptophan tyrosine favor interfacial region aromatic belt via snorkeling, positively charged lysine arginine anchor cytosolic side following positive-inside rule. Ionic hydrogen bonding stabilizes extramembranous loops but not core insertion, covalent lipid anchors define separate class. Hydrophobic interactions therefore dominate stabilization.

Ref: White and von Heijne, How Translocons Select Transmembrane Helices, Annu Rev Biophys 2008.

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Lipid Link Protein

Which of the following lipids forms a thioester bond with cysteine?

Distinction between cysteine lipidations clarifies regulation of membrane affinity. S-palmitoylation involves nucleophilic attack of cysteine thiolate on palmitoyl-CoA thioester producing thioester bond between sixteen-carbon saturated palmitate and side chain catalyzed by membrane-integrated DHHC zinc finger enzymes containing conserved Asp-His-His-Cys motif utilizing palmitoyl-CoA donor. This thioester is reversible rapidly cleaved by serine hydrolases APT1, APT2, PPT1 and ABHD17 allowing depalmitoylation cycles controlling localization of HRas trafficking between Golgi and plasma membrane, G-alpha subunits signal termination, synaptic scaffolds PSD-95 and glutamate receptors GluN2A. Hydroxylamine at neutral pH specifically hydrolyzes thioester providing biochemical diagnostic cleavage while thioether and amide resist. N-myristoylation forms stable amide to N-terminal glycine, prenylation forms thioether to C-terminal CAAX cysteine via farnesyltransferase or geranylgeranyltransferase resistant to hydroxylamine. GPI attachment uses amide to ethanolamine. Hence palmitic acid uniquely creates hydroxylamine-labile thioester with cysteine enabling dynamic raft association. Functional assays using hydroxylamine cleavage and metabolic labeling with alkyne palmitate analogues confirm dynamic palmitoylation turnover in living cells.

Ref: Resh, Palmitoylation of Ligands, Receptors and Intracellular Signaling Molecules, Sci STKE 2006.

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Lipid Link Protein

What happens when detergent concentration is below the CMC?

Understanding CMC highlights phase behavior of amphiphiles. At subsaturation detergent monomers exist dissolved in aqueous phase hydration shell disordered, partial insertion into outer leaflet may increase permeability but leaves bilayer structure largely intact. When monomer concentration reaches thermodynamic threshold where entropic cost of ordering water around hydrophobic tails outweighs micellization free energy, spontaneous aggregation into micelles occurs with hydrocarbon tails sequestered interior and polar heads facing water forming spherical or ellipsoidal particles thirty to fifty angstroms diameter. Below this point no micelles detected by dye solubilization, surface tension or NMR; membranes not disrupted into mixed micelles, so integral proteins remain embedded and organelle barriers persist demonstrated by retention of luminal markers. This intentional sub-CMC usage allows delivery of hydrophobic drugs or limited permeabilization. For complete lysis and purification supra-CMC concentrations mandatory providing sufficient micelles to sequester phospholipids and proteins into soluble particles preventing aggregation and inactivation during biochemical isolation. Experimental determination of CMC using light scattering fluorescent probes ANS or N-phenyl naphthylamine tracks abrupt intensity increase indicating micellization.

Ref: le Maire et al., Interaction of Membrane Proteins and Detergents, Biochim Biophys Acta 2000.

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Lipid Link Protein

Which of the following is a membrane-permeable detergent?

Detergent physicochemical properties determine permeabilization without denaturation. Triton X-100 tert-octylphenyl polyethoxyethanol consists of hydrophobic tert-octylphenol moiety and hydrophilic polyoxyethylene chain averaging nine to ten units, conferring CMC near zero point two four millimolar, aggregation number about one hundred to one fifty and micellar weight around ninety kilodalton. It partitions efficiently into phospholipid bilayers increasing permeability to macromolecules like antibodies and substrates, forming mixed micelles that solubilize lipids while protein-protein electrostatic contacts and hydrophobic core packing of soluble domains remain undisturbed because headgroup uncharged does not bind backbone cooperatively. SDS anionic twelve-carbon sulfate displays low CMC but strong cooperative binding unfolding helices and beta sheets into rodlike micelles. Sodium deoxycholate bile salt anionic steroid forms smaller micelles relatively harsh. For gentle permeabilization preserving enzyme activity, antigenicity and protein complexes, Triton X-100 remains laboratory standard, explaining identification as membrane-permeable non-denaturing detergent distinct from ionic denaturants used for SDS-PAGE. Quantitative assessment of protein activity after extraction confirms retention of ligand binding and catalytic turnover essential for biochemical characterization.

Ref: Seddon et al., Methods Mol Biol, Membrane Protein Solubilization and Detergent Properties.

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