Properties of Plasma Membrane

Properties of Plasma Membrane

Which technique is best for quantifying membrane lipid mobility?

Measuring lipid mobility in living membranes requires optical methods sensitive to diffusion rather than biochemical detection DNA proteins. Fluorescence recovery after photobleaching FRAP uses laser bleaching fluorescent analogs like NBD phosphatidylcholine Bodipy sphingomyelin in defined micron spot monitoring influx unbleached fluorophores over seconds to minutes. Analysis half recovery time yields diffusion coefficient and mobile fraction distinguishing freely diffusing from cytoskeleton anchored immobile pools. Alternative approaches single particle tracking trajectories mean squared displacement fluorescence correlation spectroscopy FCS autocorrelation provide micro diffusion coefficients but FRAP remains ensemble standard quantifying long range fluidity. PCR amplifies nucleic acids Western blot detects proteins gel electrophoresis separates charge size none track lipid movement. FRAP demonstrated mosaic corrals actin picket fence model Kusumi and distinct diffusion liquid ordered versus disordered phases with coefficients zero point one versus one micrometer squared per second. Technique textbook fluid mosaic validation routinely appears in NET GATE questions about membrane biophysics methodology and experimental design emphasizing live cell imaging approaches.

Ref: Axelrod et al., Biophys J 1976, FRAP principle measuring lipid diffusion. Lippincott-Schwartz.

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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 of the following statements about membrane proteins is true?

Fluid mosaic model does not imply unrestricted random diffusion all components; regulated anchoring is prevalent and functionally critical. Many membrane proteins undergo co translational or post translational lipidation targeting membranes microdomains. N myristoylation by NMT attaches fourteen carbon myristate via amide to N terminal glycine exposing after methionine removal in Src Yes Lck non receptor kinases localizing to plasma membrane rafts. S palmitoylation by DHHC PAT family forms reversible thioester on cysteine in Ras PSD95 SNAP25 enhancing raft partitioning and stabilizing membrane orientation regulated by APT thioesterases. Prenylation farnesyl fifteen or geranylgeranyl twenty via FTase GGTase on CaaX cysteine of Rab Rho ensures transient association regulated by GDI. GPI anchoring in ER involves transamidase replacing C terminal signal with ethanolamine phosphate mannose three glucosamine PI core tethering alkaline phosphatase Thy1 CD59. Cytoskeletal fencing ankyrin spectrin actin further corrals diffusion. Hence some proteins tethered by lipids distribution patchy thickness regulated hydrophobic mismatch rather than protein abundance alone concept essential for exams understanding membrane organization beyond fluid mosaic simplicity.

Ref: Lingwood & Simons, Science 2010, Lipidation membrane domains anchoring. Alberts Chapter 10.

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Properties of Plasma Membrane

Which molecule has the lowest transition temperature (Tm)?

Transition temperature dependence reflects chain packing efficiency influenced by length unsaturation. DOPC dioleoyl phosphatidylcholine contains two eighteen carbon chains each with cis delta nine double bond bending chain thirty degrees creating double kinks and large cross sectional volume reducing van der Waals contacts. Its Tm about minus seventeen to minus twenty Celsius lowest among common PCs. POPC one palmitoyl saturated sixteen plus one oleoyl unsaturated Tm minus two, DPPC dipalmitoyl two saturated sixteen packs tightly all trans Tm forty one due to high cooperative enthalpy, DMPC dimyristoyl two saturated fourteen shorter fewer contacts Tm twenty four despite saturation. Therefore degree of unsaturation dominates over length lowering Tm within this series. Psychrophilic bacteria enrich DOPC like lipids maintaining fluidity at zero degrees. Differential scanning calorimetry endotherm width also broadens with unsaturation. This ranking underpins liposome formulation for temperature sensitive drug delivery and calibration fluorescence anisotropy probes DPH TMA DPH used measuring fluidity in laboratory assessments and examination questions about membrane biophysics quantitative aspects and thermodynamic principles.

Ref: Lewis et al., Biochemistry 1987, Phosphatidylcholine Tm table DOPC lowest.

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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 of the following phospholipids is least abundant in biological membranes?

Bulk membrane construction requires millimolar quantities of phosphatidylcholine and phosphatidylethanolamine structural lipids while phosphatidic acid remains trace because it functions primarily as biosynthetic intermediate and transient second messenger rather than barrier component. Phosphatidic acid generated de novo via glycerol three phosphate acyltransferase GPAT and one acylglycerol three phosphate acyltransferase AGPAT and by phosphorylation diacylglycerol via diacylglycerol kinases DGK alpha phosphorylating DAG kinases. Immediate conversion proceeds via CDP DAG synthases CDS1 CDS2 to CDP DAG fueling PI PG and cardiolipin synthesis and via lipin phosphatidate phosphatases LPIN1 to DAG for PC PE TAG synthesis. Signaling PA produced by phospholipase D PLD1 PLD2 hydrolyzing PC rapidly degraded within minutes by lipid phosphate phosphatases LPP. Consequently steady state mole fraction below one percent contrasts PC forty percent. Low abundance grants high signaling fidelity for recruitment of mTOR FRB Sos PH NADPH oxidase via specific PA binding domains. This explains why PA quantification requires sensitive mass spectrometry versus abundant choline lipids easily detected in lipidomics workflows.

Ref: Tanguy et al., Cellular Signalling, Phosphatidic acid metabolism low abundance and signaling.

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Properties of Plasma Membrane

Which component of membranes helps maintain fluidity at lower temperatures?

Preservation of fluidity when temperature drops relies on lipid composition adjustments preventing gel formation that would freeze transporters. Cholesterol with rigid tetracyclic nucleus and three beta hydroxyl inserts among phospholipids orientation hydroxyl near carbonyl oxygen aligning ring parallel to upper acyl chains. At low temperatures it sterically hinders all trans chain crystallization by creating free volume and disrupting van der Waals lattice lowering order parameter measured by DPH anisotropy from zero point eight to zero point four. At high temperatures same rigid ring dampens excessive motion limiting trans gauche isomerization reducing permeability to ions. This dual buffering narrows phase transition breadth maintaining intermediate liquid disorder fluidity required for protein rotation diffusion and signaling. Organisms lacking cholesterol such as many bacteria increase unsaturated branched fatty acids instead via FabA Des pathway. Experimental cholesterol depletion by methyl beta cyclodextrin increases membrane rigidity at cold and calcein leakage assays. Understanding cholesterol buffering clarifies raft liquid ordered stability adaptation poikilotherms and why animal plasma membranes remain functional across physiological temperature ranges relevant to physiology and pharmacology.

Ref: Mourtsen & Zuckermann, FEBS Letters 2004, Cholesterol broadens transition maintains fluidity.

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Properties of Plasma Membrane

Which phospholipid is associated with apoptosis when it flips to the outer leaflet?

Phosphatidylserine externalization is tightly regulated apoptotic signature converting silent inner lipid into eat me signal for phagocytes and coagulation cofactor. In viable cells phosphatidylserine restricted inside by flippase ATP11C ATP11A requiring CDC50A chaperone maintaining aminophospholipid gradient preventing thrombin generation and immune recognition. Apoptotic executioner caspases three and seven cleave ATP11C at conserved DEVD site inactivating inward pumping and cleave Xkr8 at C terminus unmasking scramblase activity requiring binding to basigin neuroplastin heterodimer. External PS binds Annexin V calcium dependent manner enabling flow cytometric detection of apoptosis, bridges MFG E8 to integrin alphaVbeta3, engages Tim4 stabilin two and BAI1 receptors initiating engulfment without inflammation. Procoagulant platelets also expose PS via TMEM16F providing scaffold for factors Va Xa tenase prothrombinase amplifying clot formation. Other lipids PC SM remain outside constitutively without signaling role. Mutations causing Scott syndrome impair TMEM16F leading to bleeding diathesis linking lipid topology to immunity hemostasis and therapeutic annexin imaging applications.

Ref: Nagata et al., Cell Death & Differentiation 2016, PS exposure via flippase scramblase in apoptosis and coagulation.

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Properties of Plasma Membrane

Which lipid movement occurs the fastest in a membrane?

Biological membranes support diverse motions separated by orders of magnitude based on activation energy required to traverse hydrocarbon core. Lateral diffusion within same leaflet requires only breaking weak van der Waals contacts and creating small free volume achieving diffusion coefficient about ten to minus eight centimeters squared per second meaning lipid circumnavigates Escherichia coli in microseconds and mammalian cell in seconds. Axial rotation and wobbling of acyl chains are even faster nanoseconds observed by ESR. In contrast flip flop forcing zwitterionic or anionic headgroup through low dielectric interior faces energy barrier near sixty to ninety kilojoules per mole yielding spontaneous half times hours to days for phosphatidylcholine without catalysis. Cells overcome slow step using flippase scramblase lowering barrier via hydrophilic groove. FRAP bleaching NBD PC spot shows rapid recovery via lateral exchange while spin label EPR assays of flip flop show negligible recovery. Cytoskeletal picket fence model by Kusumi explains macroscopic confinement despite local speed. Understanding rapid lateral versus sluggish transverse motions explains how compositional asymmetry persists despite fluid mosaic mobility fundamental for exams.

Ref: Alberts et al., MBOC 7th ed., Chapter 10: Rates of lipid movement lateral vs transverse.

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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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