#cell membrane

Basics of Cell Biology

Which of the following proteins is an integral membrane protein?

Integral membrane proteins are defined by hydrophobic transmembrane segments, typically 20-25 residues forming alpha helix crossing hydrocarbon core stabilized by van der Waals and hydrogen bonding, or beta-barrel of multiple antiparallel strands as in VDAC. Extraction requires detergent disrupting bilayer rather than high salt. Aquaporin family represents classic multipass integral protein: tetramer in plasma membrane where each monomer with six transmembrane helices forms hourglass pore lined by NPA motifs Asn-Pro-Ala that orient water molecules in single file and aromatic-arginine constriction selecting against protons, facilitating rapid osmotic water equilibration essential in kidney proximal tubule, collecting duct regulated by vasopressin, and astrocyte edema control. Spectrin is peripheral cytoskeletal tetramer binding actin via adducin and band 3 through ankyrin on inner leaflet, easily eluted with low ionic strength. Histone octamer wraps DNA in nucleus, tubulin forms cytosolic microtubules of 25 nm. Thus aquaporin exemplifies true transmembrane embedding with functional pore within bilayer, while others are soluble or peripheral.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Integral Membrane Proteins, Aquaporin.

View Details

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 EGF binding activating Ras-MAPK and PI3K-Akt cascades, heterotrimeric GPCRs coupling to adenylyl cyclase producing cAMP activating PKA, and integrins linking extracellular matrix fibronectin and laminin to actin cytoskeleton for mechanotransduction via focal adhesion kinase. Glycoproteins and glycolipids presenting sialylated and fucosylated epitopes mediate cell-cell recognition essential for immune surveillance, tissue patterning, and blood grouping. Protein synthesis occurs elsewhere: cytosolic free 80S ribosomes translate cytosolic proteins, ER-bound ribosomes synthesize secretory proteins translocated via Sec61 channel, with mRNA processing in nucleus. Therefore synthesis of proteins is not intrinsic plasma membrane function.

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

View Details

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.

View Details

Plasma Membrane

Which of the following best describes the Fluid Mosaic Model?

Proposed by S.J. Singer and G.L. Nicolson in 1972 based on freeze-fracture electron microscopy and cell fusion experiments, the Fluid Mosaic Model describes membrane as two-dimensional solution where amphipathic lipids form viscous fluid matrix, about 100-fold more viscous than water, within which integral membrane proteins float as mosaic of diverse functions including pumps, receptors, adhesion molecules, and enzymes. Phospholipids exhibit rapid lateral diffusion around 10^-8 cm2 per second, rotation around axis, flexing of chains, but slow transverse flip-flop unless catalyzed by flippases, evidence obtained by Frye-Edidin heterokaryon fluorescence showing intermixing of human and mouse antigens within minutes and later fluorescence recovery after photobleaching revealing mobile fraction. Peripheral proteins attach via electrostatic interactions with anionic headgroups or protein-protein interactions via adapters. Lipid composition variation creates phase coexistence: saturated sphingolipids and cholesterol form ordered microdomains rafts. Mosaic arrangement allows remodeling during endocytosis, formation of immunological synapse, and diffusion-reaction signaling. Membranes are not static crystalline arrays of Davson-Danielli sandwich nor composed solely of phospholipids, but dynamic mosaics enabling compartmentalization and adaptability essential for life.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Fluid Mosaic Model, Singer and Nicolson.

View Details

Plasma Membrane

Which enzyme is responsible for flipping lipids between membrane leaflets?

Movement of intact phospholipid with polar headgroup across hydrophobic core has activation energy exceeding 60 kJ per mol, making spontaneous flip-flop negligible within hours to days, so cells require dedicated translocase catalysts. Flippases are P-type P4-ATPases, ten members in mammals, that adopt E1-E2 conformational cycle phosphorylating conserved aspartate to translocate fluorescent NBD-labeled aminophospholipids from exoplasmic to cytoplasmic leaflet against concentration gradient at expense of ATP, maintaining phosphatidylserine inside. Floppases belong to ABC transporter family also using ATP to export choline-containing lipids and cholesterol outward, important for bile secretion and surfactant release in lungs. Scramblases such as TMEM16F ANO6 and Xkr8 are calcium-activated, ATP-independent, bidirectional, collapsing asymmetry during platelet activation allowing phosphatidylserine to promote thrombin generation via tenase assembly, and during apoptosis to flag dead cells for phagocytosis. Phospholipase A2 cleaves acyl chain generating lysophospholipid and arachidonate for prostaglandin synthesis, not translocating whole lipid. Genetic loss of ATP11C causes hemolytic anemia and B-cell lymphopenia illustrating physiological importance.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 10: Flippases, Floppases, Scramblases.

View Details

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.

View Details

Plasma Membrane

Which of the following is true for phosphatidylcholine?

Phosphatidylcholine dominates eukaryotic membranes, constituting 40-50% of total phospholipid, existing as zwitterion at neutral pH with positively charged trimethylammonium and negatively charged phosphate, yielding net neutral molecule that stabilizes lamellar bilayer phase and prevents excessive electrostatic repulsion between headgroups. Its cylindrical molecular geometry with roughly equal cross-section of headgroup and two acyl chains favors planar bilayer formation, contrasting cone-shaped PE that induces curvature. Physiologically outstanding example is lung surfactant synthesized by type II alveolar cells in lamellar bodies containing dipalmitoylphosphatidylcholine DPPC with two saturated palmitate chains plus surfactant proteins SP-B and SP-C; DPPC reduces alveolar surface tension to near zero at end-expiration, preventing alveolar collapse and decreasing work of breathing. Premature infants deficient in surfactant develop respiratory distress syndrome treated with exogenous surfactant replacement therapy. In plasma membrane outer leaflet, PC serves as reservoir for signaling lipid phosphatidic acid via phospholipase D and as substrate for sphingomyelin synthesis, illustrating structural and metabolic versatility beyond membrane structure.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10: Phosphatidylcholine and Pulmonary Surfactant.

View Details

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.

View Details

Plasma Membrane

Which statement about sphingolipids is correct?

Sphingolipids are defined by long-chain amino alcohol sphingosine, 18 carbons with amino group at C2 and hydroxyls at C1 and C3, rather than glycerol used in phosphoglycerides. Synthesis starts with serine palmitoyltransferase condensing serine and palmitoyl-CoA to 3-ketosphinganine, reduced to sphinganine, then N-acylated by ceramide synthase family CerS1-6 to dihydroceramide and desaturated by DES1 to ceramide. Ceramide is central metabolic hub; addition of phosphocholine headgroup by sphingomyelin synthase SMS1 in Golgi yields sphingomyelin, while addition of glucose by glucosylceramide synthase yields glucosylceramide precursor for lactosylceramide, globosides, and gangliosides via sequential glycosyltransferases. This explains why sphingolipid derivatives all retain amide-linked fatty acid, saturated acyl chain promoting ordered packing, extensive hydrogen bonding through amide and hydroxyl groups, and tendency to associate with cholesterol in membrane rafts. They are mostly neutral or zwitterionic, not constitutively anionic, and their backbone divergence from glycerolipids underlies distinct biophysical behavior crucial for myelin sheath, epidermal barrier, and signaling via ceramide and sphingosine-1-phosphate controlling fate decisions.

Ref: Lehninger Principles of Biochemistry, 8th ed., Chapter 10: Sphingolipids and Ceramide Structure.

View Details

Plasma Membrane

Which of the following lipids is involved in intracellular signaling pathways?

Phosphatidylinositol is quantitatively minor, about 5-8% of total phospholipid, but disproportionately important for intracellular signaling because its inositol ring can be phosphorylated at positions 3, 4, and 5 by dedicated kinases to generate combinatorial phosphoinositide codes. PI 4,5-bisphosphate resides in plasma membrane inner leaflet where phospholipase C, activated by Gq-coupled receptors or receptor tyrosine kinases, hydrolyzes it into two second messengers: diacylglycerol that remains membrane bound to recruit and activate conventional protein kinase C via C1 domain, and inositol 1,4,5-trisphosphate that diffuses rapidly to bind IP3 receptors on endoplasmic reticulum triggering calcium release from stores activating calmodulin pathways. PI 3-kinase phosphorylates PIP2 to PIP3, creating docking site for Akt/PKB via PH domain to activate mTOR survival, growth, and glucose transporter trafficking. Other species like PI3P marks early endosomes recruiting FYVE domain proteins for sorting. Thus phosphoinositide metabolism links extracellular cues to cytoskeletal remodeling, proliferation, and metabolism, a role cholesterol and neutral glycosphingolipids do not perform in membranes.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 16: Signaling, Phosphoinositide Second Messengers.

View Details

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.

View Details

Plasma Membrane

Which of the following lipids is most abundant in the plasma membrane?

Quantitative lipidomics mammalian plasma membrane places phospholipids dominant class representing fifty to sixty mole percent total lipid exceeding sterols thirty to forty sphingolipids fifteen to twenty glycolipids two to five minor outer leaflet exclusively. Within phospholipids phosphatidylcholine thirty to forty percent phosphatidylethanolamine fifteen to twenty five phosphatidylserine ten fifteen phosphatidylinositol five phosphatidic acid lysophospholipids minor species synthesized Kennedy CDP choline ethanolamine pathways PS synthase base exchange PSS1 PSS2 in ER mitochondria associated membranes MAM regulated by substrate availability. Abundance reflects role bilayer matrix providing lateral solvent integral proteins barrier function permeability regulation and signaling platform via PIP2. Sphingolipids crucial rafts less abundant overall but functional regulatory rafts signaling, cholesterol abundant single species class distinct sterol not phospholipid class. Hence when question asks most abundant class answer phospholipids aggregate consistent extraction Folch method yields phospholipid fraction largest. Understanding hierarchy informs reconstitution experiments PC PE mixture mimics physiological fluidity permeability and explains phospholipid biosynthesis defects severe growth phenotypes yeast cho mutants hampering proliferation membrane expansion during cell cycle progression for exams and synthetic biology liposome engineering.

Ref: van Meer G et al., Nature Rev Mol Cell Biol 2008, Membrane composition phospholipid predominant class and quantification.

View Details