Lipids and Biological Membranes (5D)
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MCAT Chemical and Physical Foundations of Biological Systems › Lipids and Biological Membranes (5D)
Investigators reconstituted a planar phospholipid bilayer from purified lipids and monitored lateral diffusion of a fluorescent lipid analog (a proxy for membrane fluidity) in a chamber containing 150 mM NaCl. When temperature was increased from 20°C to 37°C, diffusion increased markedly. The group then repeated the experiment with 30 mol% cholesterol incorporated into the bilayer. Which change is most consistent with cholesterol’s role in membrane structure across this temperature shift?
Cholesterol increases diffusion at 37°C by disrupting hydrophobic interactions between acyl chains
Cholesterol increases diffusion equally at both temperatures by acting as a membrane-spanning channel
Cholesterol eliminates diffusion by forming covalent crosslinks between phospholipids
Cholesterol reduces the temperature-dependent increase in diffusion by constraining phospholipid motion at 37°C
Explanation
This question tests understanding of lipids and their role in biological membranes. Cholesterol modulates membrane fluidity by interacting with phospholipid acyl chains, reducing fluidity at high temperatures and preventing gel phase at low temperatures. In this scenario, increasing temperature from 20°C to 37°C enhances fluidity in pure phospholipid bilayers, but adding cholesterol buffers this change. The correct answer (B) follows because cholesterol constrains phospholipid motion at 37°C, reducing the temperature-dependent increase in diffusion. A common distractor (A) fails by incorrectly stating cholesterol increases diffusion at 37°C, ignoring its rigidifying effect at higher temperatures. When evaluating cholesterol's impact, consider how it maintains intermediate fluidity across temperature ranges. Always verify temperature contexts, as cholesterol's effects are phase-dependent.
A group compared two synthetic vesicle membranes at 25°C: Membrane 1 contained phosphatidylcholine with mostly saturated 16:0 acyl chains; Membrane 2 contained phosphatidylcholine with mostly monounsaturated 18:1 acyl chains. Both had identical headgroups and no cholesterol. Which change is most expected when moving from Membrane 1 to Membrane 2 under these conditions?
No change in fluidity because headgroup identity fully determines bilayer dynamics
Decreased membrane fluidity due to tighter packing from cis double bonds
Increased rigidity because unsaturated chains form additional hydrogen bonds in the bilayer core
Increased membrane fluidity due to reduced van der Waals packing between kinked acyl chains
Explanation
This question tests understanding of lipids and their role in biological membranes. Unsaturated acyl chains introduce kinks that disrupt tight packing, increasing membrane fluidity compared to saturated chains. In this comparison, Membrane 2 with monounsaturated 18:1 chains should exhibit greater fluidity than Membrane 1 with saturated 16:0 chains at 25°C. The correct answer (B) reflects how reduced van der Waals packing between kinked chains enhances fluidity. A common distractor (A) fails by wrongly claiming cis double bonds cause tighter packing and decreased fluidity. When assessing acyl chain effects, evaluate how unsaturation influences packing and transition temperatures. Consider chain length and saturation together for overall bilayer dynamics.
Researchers measured passive permeability of giant unilamellar vesicles to glycerol at 30°C. Vesicles were identical except for lipid composition: one set was enriched in sphingomyelin and cholesterol; the other set was enriched in polyunsaturated phosphatidylcholine and lacked cholesterol. Which membrane is most expected to show lower glycerol permeability, and why?
Sphingomyelin/cholesterol vesicles, because tighter packing reduces transient free volume for solute passage
Sphingomyelin/cholesterol vesicles, because cholesterol forms protein-like pores that trap glycerol
Both vesicles, because lipid composition does not affect permeability without transport proteins
Polyunsaturated vesicles, because double bonds create a more hydrophobic barrier to polar solutes
Explanation
This question tests understanding of lipids and their role in biological membranes. Membrane permeability to small solutes like glycerol depends on lipid packing, with ordered domains reducing transient defects for passage. Here, sphingomyelin/cholesterol vesicles form tighter, more ordered structures compared to polyunsaturated ones, lowering permeability. The correct answer (A) follows because tighter packing reduces free volume for solute diffusion. A common distractor (B) fails by incorrectly stating double bonds create a more hydrophobic barrier, overlooking fluidity's role in permeability. When comparing membrane compositions, assess how cholesterol and saturation affect order and defect formation. Remember, proteins are not required for passive permeability in lipid bilayers.
In a cold-shock experiment, cultured mammalian cells were shifted from 37°C to 10°C for 30 minutes. A membrane probe reported decreased fluidity. The investigators then supplemented the culture medium with cholesterol and repeated the temperature shift. Which outcome is most consistent with cholesterol’s effect at low temperature?
Fluidity is unchanged because cholesterol partitions exclusively into the aqueous phase at 10°C
Fluidity increases relative to unsupplemented cells because cholesterol disrupts close packing at low temperature
Fluidity decreases further because cholesterol always rigidifies membranes regardless of temperature
Fluidity increases because cholesterol catalyzes phospholipid desaturation during the 30-minute exposure
Explanation
This question tests understanding of lipids and their role in biological membranes. At low temperatures, cholesterol increases fluidity by disrupting gel-phase packing of acyl chains. In this cold-shock from 37°C to 10°C, cholesterol supplementation counters the decrease in fluidity. The correct answer (B) reflects cholesterol's role in preventing close packing at low temperatures. A common distractor (A) fails by assuming cholesterol always rigidifies membranes, ignoring its fluidizing effect below transition temperatures. When analyzing temperature shifts, consider cholesterol's buffering across phases. Evaluate lipid composition's influence on adaptability to environmental changes.
A lab created two liposomes with identical phospholipid headgroups but different mean acyl chain lengths: Liposome X had predominantly 14-carbon chains; Liposome Y had predominantly 20-carbon chains. At the same temperature and without cholesterol, which change is most expected for Liposome Y relative to Liposome X?
Higher permeability to ions because longer chains increase dielectric constant of the bilayer core
No change because chain length affects only membrane surface charge
Higher fluidity because longer chains reduce van der Waals interactions
Lower fluidity because longer chains increase hydrophobic contact and packing interactions
Explanation
This question tests understanding of lipids and their role in biological membranes. Longer acyl chains increase hydrophobic interactions, leading to tighter packing and lower fluidity. Here, Liposome Y with 20-carbon chains should have lower fluidity than Liposome X with 14-carbon chains. The correct answer (B) follows because extended chains enhance van der Waals contacts and rigidity. A common distractor (A) fails by wrongly suggesting longer chains reduce interactions and increase fluidity. When comparing chain lengths, assess impacts on bilayer thickness and transition temperatures. Consider how this affects overall membrane stability and permeability.
To probe lipid raft formation, researchers used a fluorescent marker that preferentially partitions into ordered membrane domains. In cells at 37°C, the marker showed punctate clustering that increased when sphingolipid content was experimentally elevated, with cholesterol unchanged. Which interpretation is most consistent with lipid raft behavior?
Elevated sphingolipids prevent microdomain formation because all lipids mix ideally in bilayers
Ordered domains increase because sphingolipids convert the bilayer into a covalently crosslinked gel
Elevated sphingolipids promote more ordered microdomains by enabling tighter packing with existing cholesterol
Clustering must be caused by actin polymerization because lipids cannot laterally segregate
Explanation
This question tests understanding of lipids and their role in biological membranes. Lipid rafts are ordered microdomains enriched in sphingolipids and cholesterol, promoting phase separation. Elevating sphingolipids here enhances clustering of ordered domains without changing cholesterol. The correct answer (A) reflects how sphingolipids enable tighter packing in rafts. A common distractor (B) fails by claiming lipids mix ideally, ignoring raft formation. When studying microdomains, examine lipid interactions driving segregation. Consider how composition influences domain stability and function.
A permeability assay compared passive diffusion of O$_2$ across two artificial bilayers at 25°C. Bilayer A contained mostly saturated phospholipids; Bilayer B contained mostly unsaturated phospholipids. No proteins were present. Which result is most expected?
Bilayer B has lower O$_2$ permeability because double bonds increase bilayer thickness
Both have identical O$_2$ permeability because nonpolar solutes require channels
Bilayer B has higher O$_2$ permeability because increased fluidity increases transient gaps for small nonpolar solutes
Bilayer A has higher O$_2$ permeability because saturated chains create larger free-volume defects
Explanation
This question tests understanding of lipids and their role in biological membranes. Unsaturated phospholipids increase fluidity and permeability to nonpolar solutes like O2 by creating packing defects. Bilayer B with unsaturated chains should permit higher O2 diffusion than saturated Bilayer A. The correct answer (B) follows because greater fluidity enhances transient gaps for solutes. A common distractor (A) fails by incorrectly attributing defects to saturated chains. When evaluating permeability, consider fluidity's role in defect formation. Assess solute polarity alongside membrane composition for accurate predictions.
A team observed that a bacterial strain grown at 15°C maintained near-constant membrane fluidity compared with the same strain grown at 37°C. Lipid analysis showed a higher fraction of unsaturated fatty acyl chains at 15°C. Which change is most consistent with the principle underlying this observation?
Increased unsaturation decreases fluidity at low temperature by increasing van der Waals attractions
The observation requires membrane proteins; lipid composition alone cannot affect fluidity
Increased unsaturation counteracts cold-induced rigidification by increasing hydrogen bonding in the bilayer core
Increased unsaturation counteracts cold-induced rigidification by reducing acyl-chain packing
Explanation
This question tests understanding of lipids and their role in biological membranes. Bacteria adjust membrane fluidity via acyl chain unsaturation to maintain homeostasis across temperatures. Higher unsaturation at 15°C prevents rigidification compared to 37°C. The correct answer (A) reflects how unsaturation reduces packing to counteract cold effects. A common distractor (C) fails by claiming unsaturation decreases fluidity, reversing the principle. When analyzing adaptations, evaluate unsaturation's impact on transition temperatures. Consider environmental factors influencing lipid biosynthesis.
In a red blood cell ghost preparation, investigators increased temperature gradually and tracked hemolysis in hypotonic buffer as an indirect readout of membrane integrity. Cells with experimentally reduced cholesterol content lysed at lower temperatures than controls. Which explanation is most consistent with cholesterol’s role?
Lower cholesterol increases temperature sensitivity because cholesterol is the primary source of membrane surface charge
Lower cholesterol has no effect because osmotic lysis depends only on aquaporin expression
Lower cholesterol prevents lysis because cholesterol always decreases fluidity at all temperatures
Lower cholesterol increases temperature sensitivity by allowing excessive fluidity at higher temperatures, weakening the barrier
Explanation
This question tests understanding of lipids and their role in biological membranes. Cholesterol stabilizes membranes at high temperatures by reducing fluidity and maintaining integrity. Reduced cholesterol increases sensitivity to heat-induced lysis in hypotonic conditions. The correct answer (A) follows because lower cholesterol allows excessive fluidity, weakening the barrier. A common distractor (C) fails by stating cholesterol always decreases fluidity, overlooking context. When assessing stability, consider cholesterol's role in fluidity modulation. Evaluate osmotic and thermal stresses together for membrane responses.
Cells were treated with a short-chain alcohol that partitions into the bilayer and disrupts acyl-chain packing. Fluorescence anisotropy measurements indicated increased membrane fluidity at constant temperature. Which downstream physical change in the membrane is most expected?
No change in permeability because only membrane proteins determine solute flux
Increased permeability to small polar molecules due to increased transient free volume
Decreased lateral diffusion of lipids due to stronger van der Waals interactions
Elimination of the hydrophobic core because alcohol converts the bilayer into a micelle
Explanation
This question tests understanding of lipids and their role in biological membranes. Short-chain alcohols increase fluidity by disrupting acyl-chain packing, enhancing permeability to polar molecules. Treatment here leads to greater transient free volume. The correct answer (B) follows because increased fluidity facilitates solute passage. A common distractor (A) fails by predicting decreased diffusion, opposite to fluidity effects. When studying perturbations, assess impacts on packing and defects. Evaluate downstream consequences for permeability and function.