#lipid bilayer

Basics of Cell Biology

Which model of the plasma membrane first introduced the idea of a lipid bilayer?

Early membrane theories debated whether lipid or protein formed continuous barrier. Charles Overton's permeability studies correlated anesthetic potency with oil solubility hinting at lipid membrane, but direct bilayer demonstration came from Evert Gorter and François Grendel at Leiden in 1925. They extracted erythrocyte lipids with acetone, spread them as monolayer at Langmuir trough air-water interface, and measured area at collapse pressure. Total lipid area approximated double surface area of red cells used, leading to inference that membrane consists of two opposed lipid layers with non-polar tails inward, polar heads outward. Despite experimental limitations underestimating lipid amount and ignoring protein contribution, conceptual breakthrough established lipid bilayer as core structure. Subsequent refinements included Davson-Danielli sandwich model proposing protein layers coating bilayer based on surface tension, Robertson's unit membrane EM trilaminar interpretation, and finally Singer-Nicolson fluid mosaic incorporating proteins within fluid lipid sea. Thus Gorter-Grendel experiment stands as first bilayer proposal foundational for modern membrane biology.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 7: Gorter and Grendel Bilayer Experiment.

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Plasma Membrane

Which of the following influences membrane curvature?

Intrinsic lipid shape contributes substantially to membrane bending energy as described by Helfrich elastic theory and packing parameter concepts. Phospholipids with small headgroup relative to bulky unsaturated tail volume possess negative intrinsic curvature favoring concave bending of monolayer. Phosphatidylethanolamine possesses ethanolamine headgroup substantially smaller than choline of phosphatidylcholine conferring cone shape with packing parameter greater than one, preferring inverted hexagonal HII phases under stress and reducing bilayer stability. Enrichment of PE in cytoplasmic leaflet promotes formation of fusion intermediates like stalks and hemifusion diaphragms required for SNARE-mediated exocytosis, supports high curvature of endocytic buds and cristae junctions stabilized by ATP synthase dimers and OPA1. Phosphatidylethanolamine is generated from phosphatidylserine decarboxylation by phosphatidylserine decarboxylase PSD in mitochondria inner membrane and via CDP-ethanolamine Kennedy pathway in ER. Cholesterol modulates bending rigidity but induces less curvature than PE, sphingomyelin cylindrical shape stabilizes flat domains. Therefore PE distribution is major lipid factor shaping vesicle budding and organelle morphology during trafficking.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Membrane Curvature and PE Shape.

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Plasma Membrane

Which property of lipid bilayers allows self-repair of membranes?

Spontaneous assembly and healing of lipid bilayers arises from hydrophobic effect, fundamentally entropic driving force where water molecules form highly ordered clathrate-like cages around exposed hydrocarbon, reducing entropy of system. Aggregating acyl chains releases ordered water, increasing bulk water entropy and lowering overall Gibbs free energy by approximately 5 kJ per methylene group, favoring self-association. Van der Waals interactions between parallel acyl chains contribute additional enthalpic stabilization. If membrane suffers pore or tear exposing hydrocarbon edge to water, line tension at edge with exposed aliphatic chains contacting water is thermodynamically highly unfavorable, prompting rapid lateral flow of neighboring phospholipids to seal defect within milliseconds, minimizing hydrocarbon-water contact, process occurring without enzymatic assistance or ATP consumption. This self-repair underlies liposome formation when phospholipids are hydrated and vortexed, recovery after mechanical shearing and electroporation, and fusion of synaptic vesicles where stalk intermediates resolve to maintain continuity. High melting temperature would rigidify chains hindering closure, indicating hydrophobic interactions provide intrinsic resilience essential for maintaining barrier during stress and movement.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 7: Hydrophobic Effect and Self-Sealing.

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Plasma Membrane

Which of the following best describes lipid bilayer asymmetry?

Transverse asymmetry arises because lipid synthesis and active transport are polarized across bilayer and maintained continuously by energy-dependent enzymes. In ER, phosphatidylcholine is produced by CDP-choline pathway and partly enriches outer leaflet after vesicular trafficking, while phosphatidylserine synthesized by serine exchange enzymes PSS1 and PSS2 remains preferentially cytosolic. P4-ATPase flippases ATP11C and ATP8A1 hydrolyze ATP to move phosphatidylserine and phosphatidylethanolamine inward against gradient, while ABC floppases like ABCB4 and ABCC1 move phosphatidylcholine and cholesterol outward, and calcium-activated scramblases randomize quickly upon activation. Consequently phosphatidylcholine and sphingomyelin are about 60-75% outer, phosphatidylserine greater than 80% inner, phosphatidylethanolamine similarly inner, glycolipids exclusively outer. This selective asymmetry provides crucial functional cues: inner anionic phosphatidylserine binds polybasic motifs of K-Ras, protein kinase C, and synaptotagmin regulating membrane recruitment and exocytosis, while exposed phosphatidylserine signals engulfment or coagulation. Identical leaflet composition would erase electrochemical asymmetry, highlighting that asymmetry is actively maintained essential property for signaling, not random distribution.

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

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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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Plasma Membrane

Which property of phospholipids is essential for the formation of the lipid bilayer?

Formation continuous bilayer aqueous milieu requires molecules possessing opposing solubility tendencies same structure thermodynamic principle driving self assembly. Phospholipids classic amphipaths containing hydrophilic headgroup phosphate plus choline ethanolamine serine inositol capable ionic hydrogen bonding water and two hydrophobic long chain fatty acids fourteen to twenty four carbons driving entropic hydrophobic effect via release ordered water clathrates surrounding tails increasing system entropy. Above critical micelle concentration nanomolar they self assemble bilayer sheets closing vesicles eliminating high energy edges exposing hydrocarbon water cost about forty kilojoules per mole line tension. Packing parameter approximately zero point eight to one predicts lamellar preference unlike single chain lysolipids forming micelles conical shape parameter less than zero point five. High melting point hydrophobic heads cholesterol presence not prerequisite; pure egg PC spontaneously forms multilamellar vesicles room temperature without energy input. Self assembly cooperative entropically driven enthalpy compensated van der Waals. Recognizing amphipathic nature explains self healing sealing resealing liposome technology foundational fluid mosaic model Singer Nicolson where lipids provide two dimensional fluid scaffold protein function central biology exams conceptual basis membrane structure biophysics and origin of cells.

Ref: Tanford C., Science 1978, Amphipathic phospholipids bilayer assembly hydrophobic effect and thermodynamics.

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Properties of Plasma Membrane

Which of the following does NOT affect membrane thickness?

Thickness hydrocarbon core typically three to four nanometers varies with chemical features not cellular energy currency. Acyl chain length linearly increases thickness about one point five angstrom per CH2; cis double bond shortens effective length kink reducing thickness. Cholesterol ordering straightens chains increasing thickness about four angstroms in fluid membranes by extending acyl conformations and reducing gauche defects. Integral proteins impose hydrophobic mismatch leading local thickening thinning compensated tilting clustering recruitment matching lipids as shown for SERCA rhodopsin potassium channels. Experiments X ray scattering small angle neutron scattering AFM confirm adjustments. ATP concentrations regulate activity P type ATPases ABC transporters flippases actin polymerization remodeling domains but do not change hydrocarbon chain length chemically. Depleting ATP halts active asymmetry yet freeze fracture electron microscopy shows lamellar thickness preserved. Therefore ATP availability uncoupled from bilayer dimensions unlike compositional factors distinguishing metabolic state structural parameter relevant membrane protein reconstitution crystallization and understanding mismatch sensing by Mga2 and Ire1 stress sensors in ER quality control pathways and unfolded protein response signaling.

Ref: Sharpe et al., Annu Rev Biophysics 2010, Hydrophobic mismatch thickness and proteins.

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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 phospholipids contributes to membrane fusion?

Stalk hypothesis posits fusion proceeds through negatively curved hourglass intermediate where proximal monolayers merge before distal creating hemifusion diaphragm. Conical lipids like phosphatidylethanolamine with small ethanolamine headgroup relatively large chain volume spontaneously adopt inverse hexagonal HII phase and stabilize stalk reducing bending energy dehydration cost from twenty to ten kBT. Sites exocytosis enrich PE via locally activated scramblases and PSD decarboxylase converting PS to PE within mitochondria and plasma membrane contact sites. Reconstituted vesicles containing thirty percent PE fuse tenfold faster with calcium or polyethylene glycol than pure PC vesicles measured by lipid mixing dequenching and content release assays. DAG and PA further promote due even smaller heads. Viral fusogens influenza hemagglutinin HIV gp41 insert amphipathic loops creating PE clustering lowering barrier. Sphingomyelin PC oppose fusion favoring lamellar phase. Understanding PE curvature promotion explains why synaptic vesicle membranes contain forty percent PE and why phospholipid shape targeted antimicrobial peptides defensins and why SNARE mediated release tightly coupled to lipid composition for efficient neurotransmission.

Ref: Chernomordik & Kozlov, Nature Struct Mol Biol 2008, PE and stalk fusion mechanism.

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Properties of Plasma Membrane

Which feature of lipid bilayers makes them selectively permeable?

Selective permeability originates from continuous oily interior separating cytosol from extracellular milieu. Phospholipid tails containing fourteen to twenty four carbon hydrocarbon chains exclude water creating low dielectric slab with Born energy penalty greater than one hundred fifty kilojoules per mol for moving sodium potassium chloride through effectively blocking ions and large polar molecules like glucose nucleotides sucrose. Small nonpolar gases oxygen carbon dioxide nitrogen dissolve readily and cross by solubility diffusion while water crosses slowly via transient pores permeability about ten minus three centimeters per second. Ion movement depends entirely on integral proteins channels carriers pumps providing hydrophilic pathways regulated by gating. Pure liposomes lacking proteins demonstrate identical barrier confirming lipid core sufficient. Glycolipid carbohydrate and asymmetry provide recognition not permeability. This core hydrophobicity underlies Nernst potentials generation secondary active transport driving nutrient uptake by SLC transporters and predictive rules for drug logP absorption. Appreciating barrier energetics explains why transporter mutations cause channelopathies like cystic fibrosis and why lipophilic drugs cross blood brain barrier efficiently for central action.

Ref: Alberts et al., MBOC 7th ed., Chapter 11: Permeability of hydrophobic core and channels.

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Properties of Plasma Membrane

Which property of lipid bilayers allows self-healing of membranes?

Self sealing of membranes is a spontaneous thermodynamic process driven by amphipathic design and the hydrophobic effect. Each phospholipid carries polar phosphate head that hydrogen bonds with water and two nonpolar tails that release ordered water clathrates when aggregated gaining entropy. Edge exposure where tails contact water creates high line tension ten to twenty piconewtons so minimizing edge length is energetically favored. Small holes close via lateral diffusion of lipids at rates near micrometer squared per second without ATP while larger wounds recruit ESCRT III CHMP4B and annexin mediated patching for excision. This principle allows artificial liposomes to form after sonication, cells to reseal after microinjection, patch clamp gigaohm seals to stabilize, and protoplasts to recover. Glycolipids protein content modulate fluidity but are not requirement for resealing. Cholesterol depletion stiffens but still reseals via phospholipids alone. Understanding hydrophobic driven self assembly clarifies why bilayers are resilient barriers and underpins technologies from liposomal vaccines mRNA delivery to membrane repair assays routinely used in cell biology laboratories worldwide.

Ref: Tanford C., The Hydrophobic Effect, Wiley. Alberts et al., MBOC 7th ed., Chapter 10: Self-assembly of bilayers.

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Properties of Plasma Membrane

Which of the following lipid modifications increases curvature in membranes?

Membrane bending energy is governed by molecular geometry quantified as shape parameter defined as volume over area times length. Phosphatidylethanolamine possesses small ethanolamine headgroup about zero point two eight square nanometers versus larger hydrocarbon volume due to two acyl chains, creating conical shape with parameter greater than one favoring negative spontaneous curvature typical of inner leaflet concave tubules and fusion stalk intermediates. Enrichment of PE in cytoplasmic leaflet at SNARE assembly sites reduces hydration repulsion and elastic cost of merging outer leaflets first forming hemifusion diaphragm. Influenza hemagglutinin fusion peptide inserts and recruits PE; reconstituted PE containing liposomes fuse rapidly upon calcium addition while pure phosphatidylcholine liposomes are fusogen resistant. Enzymes phosphatidylserine decarboxylase PSD in mitochondria inner membrane generate curvature for cristae formation. Flippases maintain PE asymmetry. Lysophospholipids opposite shape induce positive curvature inhibiting stalk formation. Cholesterol buffers rigidity but modest curvature effect. Appreciating cone versus cylinder concept connects lipid metabolism to exocytosis cytokinesis and viral entry mechanisms thoroughly asked in examinations.

Ref: Chernomordik & Kozlov, Nature Structural Biology 2003, PE and membrane curvature in fusion.

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