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#cholesterol

11 public questions tagged with this topic.

Paracrine Factor

What role does cholesterol play in Hedgehog protein processing?

Hedgehog proteins undergo unique autoprocessing requiring intein-like activity and lipid modifications for gradient formation. C-terminal domain cleaves N-terminal signaling peptide, covalently attaches cholesterol to C-terminus, and palmitate attaches to N-terminal cysteine via Skinny hedgehog HHAT acyltransferase in endoplasmic reticulum. Cholesterol moiety anchors Hedgehog to membranes of producing cells, restricts diffusion, enables multimeric sterol-rich puncta for long-range gradient and facilitates secretion via Dispatched transporter RND protein requiring sterol sensing domain. Without cholesterol, Hedgehog diffuses uncontrolled but fails to signal at distance, loses apical sorting. Thus lipid modifications crucial for morphogen gradient shaping.

Ref: Porter et al., Science 1996: Cholesterol modification of Hedgehog required for secretion, multimerization and gradient.

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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 cholesterol drives formation of liquid-ordered lipid rafts that concentrate GPI-anchored proteins and Src kinases for signaling. Beyond barrier modulation, cholesterol is substrate for mitochondrial P450scc that cleaves side chain to pregnenolone, precursor for glucocorticoids, mineralocorticoids, sex steroids, bile acids, and vitamin D. Passive transport of glucose through GLUTs or water through aquaporins does not require cholesterol as facilitator; proteinaceous pores and carriers mediate downhill movement independent of sterol content, so cholesterol does not act as transport facilitator.

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

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Plasma Membrane

Which component of the membrane forms lipid rafts?

Lipid raft genesis requires cooperative interaction two distinct lipid classes rather than single component protein scaffold alone driven thermodynamics enthalpy. Sphingolipids sphingomyelin glycosylceramide ganglioside with amide linked saturated acyl chains eighteen to twenty four carbons sphingosine backbone capable hydrogen bond donor acceptor network adopt extended conformation high melting temperature promoting condensation ordered domain formation. Cholesterol planar rigid ring three beta hydroxyl inserts voids between chains increasing order parameter thickness three point seven to four point four nanometers creating liquid ordered phase coexisting liquid disordered phosphatidylcholine phosphatidylethanolamine pools rich unsaturated tails kinked. Thermodynamic driving force favorable enthalpy van der Waals CH pi interactions outweighing entropy mixing observed ternary model membranes DOPC SM cholesterol forming domains ten to two hundred nanometers AFM fluorescence super resolution STED microscopy visualizing. Phosphatidylcholine unsaturated kink cardiolipin four chains mitochondrial inner membrane excluded rafts distinct organelle. Cholesterol sphingolipids combination therefore defines physical basis raft formation underlying sorting apical proteins GPI anchored signaling kinases pathogen entry HIV influenza Ebola mechanisms studied modern cell biology raft concepts fundamental exams and membrane microdomain investigations contemporary biophysics and virology and immunology research focus.

Ref: Sezgin et al., Biochim Biophys Acta 2017, Cholesterol sphingolipid cooperation forms lipid rafts domains and phase separation.

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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 introducing kinks increasing free volume preventing gel phase quantified decrease order parameter S zero point eight to zero point five ESR spin label DPH anisotropy experiments broadening DSC transition. Net effect broadened phase transition abolishing sharp DSC peak buffering fluidity optimal Na K ATPase receptor function signaling. Glycoproteins mediate adhesion migration immunity, phosphatidylinositol transiently phosphorylated signaling but bulk fluidity regulated sterol, integral proteins immobilize annular lipids via picket fence but not buffer globally. Understanding cholesterol stabilization explains raft liquid ordered existence adaptation temperature dietary changes statin therapy impacts membrane order atherosclerosis pathology relevant physiology pharmacology questions exams and membrane protein function regulation.

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

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Glycolipid and Phospholipid

Which sterol is abundant in plant membranes?

Plant membranes replace cholesterol phytosterols where stigmasterol and sitosterol are prominent structurally related functionally overlapping but distinct regulation. Both derive cycloartenol via CYP51 SMT1 SMT2 sterol methyltransferases, with sitosterol bearing twenty four alpha ethyl group dominating sixty percent free sterols stigmasterol bearing additional trans double bond C22 C23 introduced CYP710A comprising ten to thirty percent campesterol ten percent remainder. In most crops soybean Arabidopsis tomato sitosterol quantitative major but stigmasterol characteristic marker plasma membranes detergent resistant rafts showing inducible increase pathogen challenge avirulent bacteria senescence mediated PLD phospholipase D signaling and ROS tolerance membrane remodelling. Cholesterol minor less than five percent, ergosterol dominates fungi mycobacteria. Phytosterols regulate membrane order permeability ion channels H ATPase activity auxin transport PIN localization brassinosteroid precursor campesterol growth regulation. Ratio sitosterol stigmasterol modulates fluidity freezing tolerance via CBF pathway COR genes. Understanding phytosterol diversity illustrates evolutionary adaptation where cholesterol function fulfilled ethyl sterols explaining dietary plant sterol interference human cholesterol absorption via Niemann Pick C1 Like one transporter NPC1L1 competitor heart health and plant physiology cold adaptation topics in exams.

Ref: Schaller H., Plant Physiol 2003, Plant sterols stigmasterol sitosterol biosynthesis and membrane functions.

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Glycolipid and Phospholipid

Which lipid component is most abundant in lipid rafts?

Detergent resistant membrane fractions shotgun lipidomics consistently show enrichment sphingolipids relative glycerophospholipids quantitative evidence rafts composition bias and lateral organization. Sphingomyelin ceramide cerebrosides gangliosides share sphingosine backbone hydrogen bonding capacity amide hydroxyl groups predominantly saturated very long chains eighteen to twenty four carbons promoting all trans extended conformation high Tm above thirty seven degrees favoring condensation cholesterol into liquid ordered phase where acyl packing tight yet laterally mobile diffusion moderate. Phosphatidylcholine kinked oleoyl chain phosphatidylserine negative charge unsaturated tails favor liquid disordered phase depleted rafts non raft domains. Estimates SM two to three fold gangliosides four fold enrichment rafts versus whole plasma membrane via mass spectrometry imaging super resolution microscopy. Thicker hydrophobic core rafts sorts proteins longer transmembrane helices seventeen versus fifteen amino acids explaining apical sorting polarized epithelial cells. Compositional bias underlies fluorescence microscopy using cholera toxin B GM1 as raft marker and functional assays showing sphingolipid cholesterol depletion blocks signaling sorting mechanisms thoroughly examined in membrane biology and examination questions regarding microdomains composition and functional consequences for protein trafficking.

Ref: Lingwood et al., Nature Chem Biol 2008, Sphingolipid enrichment rafts quantification and sorting.

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Glycolipid and Phospholipid

Which of the following best describes cholesterol’s role in lipid rafts?

Liquid ordered raft stability arises favorable packing between sphingolipid and sterol driven enthalpy entropy compensation. Cholesterol small polar hydroxyl rigid planar tetracycle inserts into voids between long saturated amide linked acyl chains sphingosine backbone maximizing van der Waals contacts permitting hydrogen bond between sphingosine C3 hydroxyl cholesterol hydroxyl and amide carbonyl to amide donor. This promotes condensed complex formation increasing bilayer thickness four angstroms decreasing permeability solutes and creating diffusion distinct from liquid disordered with FRAP coefficients point one micrometer squared per second versus one. Methyl beta cyclodextrin extraction collapses order increasing disorder and lateral mixing. Cholesterol does not thin membranes nor generically increase permeability; low concentrations increase permeability slightly gel phase but fluid phase reduces it significantly. Proteins with saturated GPI double palmitoyl anchors partition into ordered domains enriching signaling kinases Src Lyn. Describing stabilization sphingolipid cholesterol interaction distinguishes physical chemistry rafts from simple protein clustering and explains why raft size regulation affects immune receptor triggering BCR TCR activation and signaling thresholds examined in cell biology immunology questions about domain formation mechanism and biophysics of phase separation.

Ref: Simons & Sampaio, Cold Spring Harb Perspect Biol 2011, Cholesterol sphingolipid stabilization rafts formation.

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Properties of Plasma Membrane

What happens when cholesterol is added to a membrane at high temperature?

Dual effect of cholesterol on membrane order described as condensing and fluidity buffering arises from rigid steroid ring system intercalating among phospholipid acyl chains. At temperatures above phospholipid Tm chains highly disordered with many gauche conformers large area per lipid fluid low order. Cholesterol hydroxyl moiety positioned near phospholipid carbonyl oxygen forms hydrogen bond, rigid tetracyclic rings restrict chain isomerization reducing number gauche kinks order parameter S increases from zero point two to zero point five measured deuterium NMR, lateral diffusion coefficient decreases ten to minus eight to ten to minus nine centimeters squared per second, permeability to solutes ions glucose reduced, bilayer thickness increased zero point three nanometers and bending rigidity increased protecting against lysis while still lateral fluid not gel. At low temperature below Tm cholesterol disrupts crystalline packing inserting between chains increasing disorder fluidity preventing gel formation. At high temperature cholesterol rich liquid-ordered rafts with sphingomyelin thicker ordered slower than liquid-disordered unsaturated phospholipid phase recruiting signaling proteins like Src kinases. Cholesterol therefore decreases fluidity at high temperature buffering against hyperfluidity maintaining integrity for protein function and signal compartmentalization.

Ref: Ohvo-Rekila et al., Cholesterol Interactions and Membrane Fluidity Biphasic Effect, Prog Lipid Res 2002.

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Endocrinology

Which hormone is derived from cholesterol?

Answer: B) Estrogen. For Endocrinology, once you lock onto the key mechanism or definition, Estrogen is the clear fit. If you restate the concept in your own words, Estrogen is the option that correctly names the structure, process, or principle asked for in Endocrinology. So Estrogen is the clean, accurate selection. If a similar stem appears later, start from the same core fact and you will land on the same kind of answer. In class notes, highlight this same phrase next to the related diagram so the wording and the picture reinforce each other. When you practise, cover the choices first, write Estrogen from memory, then reveal the letter — that habit builds real recall for Endocrinology.

Ref: Animal physiology is Unit 7 of the CSIR NET Life Science Syllabus, covering core body systems. It accounts for roughly 10% of the total marks in Sections B and C.

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