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 disp

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 rhodo

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

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

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

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 d

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 permeabilit

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 t

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

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. Externa

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 th

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 providin

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

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