#membrane properties

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 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 statements about membrane fluidity is true?

Regulation of fluidity reflects balance between order and disorder of fatty acyl chains governing membrane physical properties and protein function. Saturated acyl chains straight all trans conformation maximize van der Waals contacts small area per lipid about forty eight square angstroms low fluidity high transition temperature. Cis double bond introduces kink thirty degree bend reducing contact area to about sixty square angstroms increasing free volume lowering transition temperature fifty degrees increasing lateral diffusion coefficient and rotational freedom measured by fluorescence anisotropy diphenylhexatriene EPR order parameter. Polyunsaturated arachidonic docosahexaenoic more fluid. Longer chains raise Tm via more contacts decreasing fluidity. Temperature increase disorders chains raising fluidity; decreasing induces gel phase. Cholesterol biphasic modulates ordering. Cells homeoviscously adapt bacteria desaturase induction at low temperature, poikilotherms increased unsaturation winter. Therefore statement that membrane fluidity increases with unsaturated fatty acids accurately describes molecular basis of chain packing disruption enabling maintenance of functional liquid crystalline state required for permeability protein activity vesicular trafficking and signaling platform formation.

Ref: Singer and Nicolson, Fluid Mosaic Model and Fluidity Dependence on Unsaturation, Science 1972.

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Plasma Membrane Proteins

Which of the following properties affects membrane protein mobility?

Diffusion behavior of membrane proteins governed by hydrodynamic and cytoskeletal factors. Saffman-Delbruck model relates diffusion coefficient D to membrane thickness h viscosity mu m and protein radius r through D proportional to natural log of membrane to protein size. Number of transmembrane domains increases effective radius and frictional drag because each helix engages annular lipids diffusing as complex and more extensive hydrophobic surface contacts viscous acyl chains. Single-pass glycophorin diffusion coefficient about five times ten to minus nine centimeters squared per second faster than seven-pass GPCR about one times ten to minus nine and fourteen-pass Band 3 about zero point five. Additional slowing originates from hop diffusion model developed by Kusumi showing actin cytoskeleton fences create compartments eighty to two hundred nanometers where proteins diffuse rapidly then hop between compartments limited by transient fence and picket interactions. Binding to immobilized ankyrin spectrin network reduces mobile fraction to fifty percent. Size of extracellular loops influences extracellular matrix interactions but less than transmembrane count, hydropathy index predicts insertion, pH charge limited effect. Number of transmembrane domains therefore primarily affects mobility.

Ref: Kusumi et al., Paradigm Shift in Membrane Protein Diffusion and Corral Model, Annu Rev Biophys.

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