#plasma membrane

Basics of Cell Biology

Which of the following is true about the Fluid Mosaic Model?

Fluid Mosaic Model postulated by Singer and Nicolson in 1972 integrated thermodynamic considerations and experimental data that membranes behave as two-dimensional fluid solutions. Phospholipids and sphingolipids undergo rapid lateral diffusion measured 1-3 µm2/s, rotation around axis, transbilayer flip-flop rare without flippase catalysis, demonstrated by Frye and Edidin fusion of human and mouse fibroblasts where surface antigens intermix within 40 minutes, and later single-particle tracking revealing Brownian, confined, and directed motions due to cytoskeletal fences. Integral membrane prot

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10: Fluid Mosaic Model, Lateral Diffusion.

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Basics of Cell Biology

Which component of the plasma membrane is responsible for fluidity?

Membrane fluidity describes lateral mobility, rotation, and flexing of components, parameter influencing permeability, fusion, and signaling complex formation. Phospholipids primarily determine fluidity through acyl chain composition: presence of cis double bonds as in oleoyl 18:1 and arachidonoyl 20:4 introduces kinks preventing close packing, lowering melting temperature Tm and increasing lateral diffusion coefficient about 10^-8 cm2/s measured by fluorescence recovery after photobleaching and fluorescence anisotropy. Shorter chains also increase fluidity. Cholesterol buffers fluidity by int

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Phospholipids and Fluidity Regulation.

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Basics of Cell Biology

Which scientist proposed the Unit Membrane Model?

Early electron microscopy after osmium fixation revealed trilaminar image of membranes about 7.5 nm thick: two dark lines 2 nm each representing protein stained by osmium, light core of lipid. J. David Robertson at Harvard analyzed myelin, chloroplast, and plasma membranes and in 1959 proposed Unit Membrane Model asserting all biological membranes share same basic construction of lipid bilayer sandwiched between non-lipid layers, with asymmetry that inner and outer surfaces differ chemically, explaining functional polarity. Robertson emphasized continuity of membranes through ER and nuclear en

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Unit Membrane Model, Robertson.

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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-p

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

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Basics of Cell Biology

Which of the following is not a function of the plasma membrane?

Plasma membrane confers cell individuality by separating intracellular milieu from external environment and actively regulating exchange to maintain homeostasis. Its phospholipid bilayer plus embedded transport machinery including Na+/K+ ATPase establishing 140 mM potassium inside and 145 mM sodium outside, glucose carriers GLUT family facilitating uptake, amino acid exchangers, and aquaporins ensuring selective permeability and volume control, together with tight junctions restricting paracellular diffusion. Signaling platforms include receptor tyrosine kinases like EGFR that dimerize upon EG

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Plasma Membrane Functions Overview.

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Plasma Membrane

Which of the following is NOT a feature of plasma membrane lipids?

Plasma membrane lipids assemble through non-covalent forces rather than covalent polymerization. Each phospholipid or cholesterol monomer inserts via hydrophobic effect with tails buried together while polar headgroups contact water, stabilized by van der Waals contacts, hydrogen bonding between phosphate oxygens and water, and electrostatic interactions. This allows lateral diffusion, rotation, and exchange between leaflets catalyzed by flippases, and reversible desorption to cytosolic carriers. Membranes exhibit selective permeability because polar headgroup region impedes hydrophobic solute

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Non-Covalent Assembly of Lipids.

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Plasma Membrane

Which type of lipid is NOT a part of the plasma membrane?

Biological membrane formation is governed by amphipathic geometry where polar headgroup cross-section balances hydrophobic tail volume, satisfying packing parameter close to 1 for bilayer assembly and stability. Phospholipids such as phosphatidylserine, phosphatidylcholine, phosphatidylethanolamine, sphingolipids like sphingomyelin and cerebroside, and cholesterol as small amphiphile intercalated among chains all meet this criterion and readily incorporate into 5 nanometer fluid mosaic film with integral proteins diffusing laterally. Triglycerides, triacylglycerols composed of glycerol esterif

Ref: Lehninger Principles of Biochemistry, 8th ed., Chapter 10: Triglycerides vs Membrane Lipids.

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Plasma Membrane

Which of the following phospholipids is most abundant in the outer leaflet of the plasma membrane?

Quantitative lipidomics using phospholipase treatment, chemical labeling, and mass spectrometry shows outer leaflet of mammalian plasma membrane is dominated by choline-containing lipids: phosphatidylcholine roughly 60% of outer phospholipid and sphingomyelin nearly exclusively outer, together creating relatively neutral, saturated, raft-competent surface that interacts with extracellular environment and immune system. Their cylindrical shape stabilizes flat outer leaflet and protects from opsonization by complement. In contrast phosphatidylserine and phosphatidylethanolamine concentrate 75 to

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Outer Leaflet Enrichment of PC.

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Plasma Membrane

Which of the following is NOT a function of cholesterol in the plasma membrane?

Cholesterol's structure with rigid tetracyclic ring system and short iso-octyl tail allows it to insert into phospholipid bilayers with hydroxyl near phospholipid carbonyls and ring system interacting with upper acyl chains. At low temperatures it disrupts tight packing of saturated chains, preventing transition to gel phase and preserving lateral diffusion essential for protein function, while at high temperatures its rigid ring restricts chain motion, decreasing permeability to small polar molecules, protons, and sodium ions, maintaining electrochemical gradients. Within membranes cholestero

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Cholesterol Functions and Steroidogenesis.

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Plasma Membrane

What is the primary function of glycolipids in the plasma membrane?

Glycolipids are restricted to the exoplasmic leaflet where their oligosaccharide chains, assembled in Golgi by glycosyltransferases and glycosidases, face extracellular milieu forming glycocalyx extending beyond protein domains. This topology makes them poorly suited to regulate fluidity, which is governed by fatty acid saturation and cholesterol content, or to transport ions, mediated by channels and pumps. Their principal contribution lies in recognition and signaling. Carbohydrate moieties constitute ABO and Lewis blood group antigens, selectin ligands like sialyl Lewis X that direct leukoc

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Glycocalyx and Cell Recognition.

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Plasma Membrane

Which of the following is a major glycolipid found in the plasma membrane?

Gangliosides are prominent glycosphingolipids of vertebrate plasma membranes, characterized by a hydrophobic ceramide anchor embedded in the outer leaflet linked to a complex oligosaccharide carrying one or more sialic acid residues. This bulky carbohydrate headgroup projects into the extracellular space and participates in the glycocalyx, mediating cell-cell recognition, modulation of receptor tyrosine kinases such as EGFR, and adhesion during neural development and immune synapse formation. Unlike sphingomyelin which incorporates phosphocholine as headgroup via phosphodiester bond, or phosph

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Membrane Structure, Glycosphingolipids and Gangliosides.

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Plasma Membrane

Which of the following stabilizes the plasma membrane by modulating its fluidity?

Modulation membrane order cholesterol textbook example fluidity homeostasis buffering mechanism dual action concentration dependent. Cholesterol inserts three beta hydroxyl near phospholipid ester carbonyl forming hydrogen bond rigid tetracycle parallel upper ten carbons acyl chains restricting trans gauche isomerization high temperatures decreasing lateral diffusion coefficient from one to zero point five micrometer squared per second and passive permeability small solutes two to three fold measured calcein leakage assays. Low temperatures cholesterol disrupts all trans crystalline lattice in

Ref: Krause & Regen, JACS 2005, Cholesterol regulates fluidity stability buffering phase transition mechanism.

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