Plasma Membrane - AP Biology

1. The plasma membrane is a phospholipid bilayer with embedded proteins. The bilayer's hydrophobic interior limits diffusion of polar solutes, while specific transport proteins can facilitate passage. A researcher inserts additional cholesterol molecules between phospholipid tails in a membrane at moderate temperature. The membrane becomes less fluid and shows reduced diffusion of a small nonpolar solute, with no change in transport protein number. Which feature best explains the reduced diffusion of the nonpolar solute?
   Answer: Cholesterol restricts phospholipid tail movement, lowering membrane fluidity and slowing diffusion through the bilayer This question requires analyzing how plasma membrane components affect transport dynamics. The correct answer is A because cholesterol molecules insert between phospholipid tails and restrict their movement, reducing membrane fluidity and making it harder for nonpolar solutes to dissolve into and diffuse through the more rigid bilayer. At moderate temperatures, cholesterol acts as a fluidity buffer, preventing excessive movement of phospholipid tails and creating a more ordered, less permeable membrane structure. Since the nonpolar solute crosses by dissolving in the hydrophobic core, reduced fluidity means slower movement through this more tightly packed environment. Answer D is incorrect because cholesterol doesn't convert the bilayer core into a hydrophilic region—it remains hydrophobic but becomes more ordered and less fluid, which is what reduces diffusion of nonpolar solutes. The principle here is that membrane fluidity directly impacts simple diffusion rates: cholesterol reduces fluidity, thereby slowing diffusion of molecules that travel through the lipid bilayer.
2. A phospholipid bilayer contains embedded transport proteins. The hydrophobic tails create a barrier to charged particles, while some proteins form selective pores lined with polar amino acids. A student compares movement of K+ across membranes with and without a functional K+ channel protein. The concentration of K+ is higher outside than inside. Which feature best explains why K+ moves into the cell only when the channel is functional?
   Answer: A K+ channel provides a hydrophilic pore that reduces interaction with the bilayer core This question assesses the skill of analyzing plasma membrane structure and transport. The functional K+ channel protein forms a hydrophilic pore lined with polar amino acids, allowing charged K+ ions to pass through without interacting with the hydrophobic tails of the bilayer. Without the channel, K+ cannot cross the hydrophobic barrier easily, even with a favorable concentration gradient from outside to inside. This selective pore reduces the energy barrier for ion movement, explaining entry only when the channel is present. A tempting distractor is choice A, which suggests K+ crosses by simple diffusion through tails, stemming from the misconception that charged ions can dissolve in nonpolar environments. When analyzing ion movement, distinguish between simple diffusion and facilitated transport by considering the charge and the role of specific proteins.
3. Researchers create two liposomes. Liposome M contains phospholipids with short fatty acid tails; Liposome N contains phospholipids with longer tails. Both lack transport proteins. The hydrophobic tails form the interior barrier, and small nonpolar molecules cross by simple diffusion. The liposomes are placed in a solution containing a small nonpolar dye, and dye entry is measured over time. Which feature best explains why dye enters Liposome M faster than Liposome N?
   Answer: Shorter tails reduce bilayer thickness, shortening the diffusion path through the core This question assesses the skill of analyzing plasma membrane structure and transport. Shorter fatty acid tails in Liposome M result in a thinner bilayer, shortening the diffusion path through the hydrophobic core for the small nonpolar dye to cross by simple diffusion. Longer tails in Liposome N create a thicker barrier, increasing resistance and slowing dye entry. Both lack transport proteins, so diffusion depends solely on bilayer properties like thickness. A tempting distractor is choice B, which claims longer tails increase fluidity, reflecting the misconception that tail length enhances rather than potentially restricts diffusion by increasing thickness. In experiments comparing lipid structures, evaluate how physical dimensions of the bilayer influence the rate of passive diffusion for nonpolar solutes.
4. A student constructs two liposomes from identical phospholipids. Liposome X has no membrane proteins; Liposome Y includes a transmembrane protein that forms a continuous water-filled pore. When placed in a solution containing a small charged ion, the ion enters Liposome Y rapidly but shows minimal entry into Liposome X. The phospholipid bilayer interior is hydrophobic. Which feature best explains the difference in ion permeability between the two liposomes?
   Answer: The pore provides a hydrophilic route that bypasses the hydrophobic core, increasing ion entry This question tests understanding of plasma membrane structure and transport through protein channels. The correct answer is B because the transmembrane protein creates a water-filled pore that provides a continuous hydrophilic pathway across the membrane, allowing charged ions to move through without encountering the hydrophobic bilayer interior that would otherwise exclude them. The comparison between identical liposomes with and without the pore protein clearly demonstrates that ions require a hydrophilic route to cross efficiently. Answer A is incorrect because it claims the bilayer interior is hydrophilic, which represents a fundamental misconception - the phospholipid tails create a hydrophobic core that repels charged particles. When analyzing ion permeability, remember that charged particles need hydrophilic pathways to cross hydrophobic barriers.
5. A scientist alters a membrane by removing most transmembrane proteins while leaving the phospholipid bilayer intact. The bilayer has hydrophilic heads facing outward and a hydrophobic interior formed by fatty acid tails. After removal, the scientist measures transport rates of CO2 (small, nonpolar) and Ca2+ (charged). CO2 transport changes little, but Ca2+ transport decreases sharply. Which feature best explains the different effects on CO2 versus Ca2+ movement?
   Answer: CO2 diffuses through the hydrophobic core, while Ca2+ depends on protein pathways This question assesses the skill of analyzing plasma membrane structure and transport. CO2, being small and nonpolar, can diffuse directly through the hydrophobic core of the intact bilayer, so its transport remains largely unaffected by protein removal. In contrast, charged Ca2+ relies on transmembrane proteins like channels for a hydrophilic pathway, and removing them sharply decreases its transport. The phospholipid heads and tails maintain the bilayer's selective permeability, highlighting the differential dependence on proteins. A tempting distractor is choice A, which reverses the mechanisms by saying CO2 needs channels and Ca2+ diffuses, arising from the misconception of confusing polar and nonpolar transport requirements. When proteins are manipulated, compare solute properties to predict impacts on simple versus facilitated diffusion across membranes.
6. A cell's plasma membrane is a phospholipid bilayer with embedded proteins. The bilayer's hydrophobic core limits diffusion of polar solutes, while certain transmembrane proteins form selective channels lined with polar amino acids. A mutation replaces several polar amino acids lining a glucose channel with nonpolar amino acids, without changing the channel's overall size. After the change, glucose transport through the channel decreases markedly. Which feature best explains the reduced glucose transport?
   Answer: A less polar channel lining reduces favorable interactions with glucose during passage This question tests understanding of plasma membrane structure and transport selectivity. The correct answer is A because polar amino acids lining the channel create a hydrophilic environment that stabilizes polar solutes like glucose during passage, while nonpolar amino acids cannot provide these favorable interactions. The mutation replacing polar with nonpolar amino acids reduces glucose transport despite maintaining channel size, demonstrating the importance of chemical compatibility. Answer C is incorrect because nonpolar amino acids are uncharged and cannot repel molecules—only charged amino acids create electrostatic forces, reflecting a misconception about amino acid properties. When analyzing channel selectivity, match the chemical properties of the channel lining (polar/nonpolar) with the transported solute for optimal transport.
7. Two membranes have identical phospholipid composition, but differ in cholesterol content. Cholesterol fits among phospholipid tails and can restrict their movement at moderate temperatures. Small nonpolar molecules cross the bilayer mainly by simple diffusion through the hydrophobic core, while polar solutes require transport proteins. In an assay at 25°C, Membrane A has high cholesterol and Membrane B has low cholesterol. Which feature best explains a lower diffusion rate of nonpolar molecules across Membrane A?
   Answer: High cholesterol reduces tail movement, decreasing permeability of the hydrophobic core This question assesses the skill of analyzing plasma membrane structure and transport. High cholesterol in Membrane A restricts the movement of phospholipid tails at 25°C, reducing membrane fluidity and making the hydrophobic core less permeable to small nonpolar molecules that cross by simple diffusion. In contrast, low cholesterol in Membrane B allows greater tail flexibility, facilitating easier diffusion through the core. This explains the lower diffusion rate in Membrane A, as cholesterol's stabilizing effect packs the bilayer more tightly. A tempting distractor is choice A, which incorrectly states high cholesterol increases spacing, reflecting the misconception that cholesterol always enhances rather than reduces fluidity at moderate temperatures. For questions on membrane composition, consider environmental factors like temperature when assessing how components like cholesterol influence fluidity and permeability.
8. The plasma membrane contains a phospholipid bilayer with a hydrophobic interior that restricts diffusion of ions. Embedded proteins can form channels that allow specific ions to move down their concentration gradients without crossing the hydrophobic core. In a lab, cells are placed in a solution with a higher concentration of $\mathrm{Na^+}$ outside than inside. When a sodium channel protein is added to the membrane, $\mathrm{Na^+}$ enters rapidly; without the channel, little $\mathrm{Na^+}$ enters. Which feature best explains why $\mathrm{Na^+}$ requires the channel to cross efficiently?
   Answer: The bilayer interior is hydrophobic, which disfavors passage of charged ions This question tests understanding of plasma membrane structure and transport selectivity. The correct answer is A because the phospholipid bilayer's hydrophobic interior repels charged particles like Na+ ions, preventing them from passing through without assistance from protein channels that provide a hydrophilic route. The experiment demonstrates that Na+ enters rapidly only when sodium channels are present, confirming that the hydrophobic barrier blocks direct ion passage. Answer D is incorrect because ions cannot diffuse through hydrophobic lipid tails at all—they require the hydrophilic environment of protein channels, reflecting a misconception about ion-lipid interactions. When analyzing ion transport, remember that charged particles are incompatible with the nonpolar bilayer core and always require protein-mediated pathways.
9. Two cell membranes contain identical transport proteins but differ in phospholipid tail composition. Membrane Q has a higher proportion of saturated fatty acids; Membrane R has a higher proportion of unsaturated fatty acids. At the same temperature, lateral movement of membrane proteins is slower in Membrane Q than in Membrane R. Which membrane feature best explains the reduced protein mobility in Membrane Q?
   Answer: More saturated tails pack tightly, decreasing membrane fluidity and limiting protein diffusion This question assesses the skill of analyzing plasma membrane structure and transport. The reduced protein mobility in Membrane Q results from its higher proportion of saturated fatty acids, which pack tightly and decrease overall membrane fluidity. This gel-like state limits lateral diffusion of embedded proteins compared to Membrane R's more fluid, unsaturated tails that allow easier movement. Both membranes have identical proteins, so tail saturation directly affects the bilayer's physical properties at the same temperature. A tempting distractor is choice D, which mistakenly claims saturated tails create larger gaps for faster movement, reversing the actual effect of reduced fluidity on protein diffusion. When comparing protein mobility, examine how fatty acid saturation influences membrane fluidity and lateral dynamics.
10. A phospholipid bilayer forms a hydrophobic barrier to most polar molecules. Some embedded proteins function as aquaporins, providing a hydrophilic channel for water. Two cell membranes have the same phospholipid composition, but Membrane 1 contains many aquaporins and Membrane 2 contains few. When placed in the same hypotonic solution, cells with Membrane 1 change volume more rapidly than cells with Membrane 2. Which feature best explains the faster volume change in cells with Membrane 1?
    Answer: Aquaporins provide a hydrophilic pathway that increases water permeability across the hydrophobic bilayer This question tests your ability to analyze plasma membrane structure and specialized transport proteins. The correct answer is A because aquaporins are channel proteins that provide a hydrophilic pathway specifically for water molecules to cross the otherwise hydrophobic interior of the phospholipid bilayer. Water is polar and faces a significant barrier crossing the nonpolar bilayer core, but aquaporins create a water-friendly route that dramatically increases membrane permeability to water. The faster volume change in cells with Membrane 1 occurs because more aquaporins allow more water to enter the cell rapidly in response to the hypotonic (lower solute concentration) environment. Answer B is incorrect because aquaporins don't chemically convert water—they simply provide a physical pathway, and water remains a polar molecule throughout its passage. The key concept is that specialized transport proteins like aquaporins overcome the bilayer's selective permeability by providing appropriate chemical environments for specific molecules to cross.
11. A student compares diffusion of ethanol and Na+ across a pure phospholipid bilayer lacking proteins. Ethanol crosses slowly but measurably, while Na+ shows almost no movement. The bilayer has polar phosphate heads facing water and a hydrophobic interior formed by fatty acid tails. The student then adds a membrane with embedded ion channels and observes rapid Na+ movement. Which membrane feature best explains why Na+ requires channels for efficient transport?
    Answer: The hydrophobic core excludes charged ions, so Na+ needs a hydrophilic channel pathway This question assesses the skill of analyzing plasma membrane structure and transport. Na+ requires channels for efficient transport because the hydrophobic core of the phospholipid bilayer excludes charged ions, preventing their diffusion without a hydrophilic pathway. Ethanol, being small and less polar, can cross slowly through the lipid interior, but Na+ shows almost no movement until ion channels are added, allowing rapid passage. The polar heads face water, but the nonpolar tails create a barrier for ions, which is bypassed by channels. A tempting distractor is choice D, which incorrectly attributes slow Na+ diffusion solely to size, overlooking the key role of charge incompatibility with the hydrophobic core. When examining ion transport, evaluate how charge interacts with the bilayer's hydrophobicity and the necessity of protein channels.
12. A plasma membrane is a phospholipid bilayer in which hydrophilic heads face the aqueous environments and hydrophobic tails form a core that limits passage of ions and most polar solutes. Some molecules cross by simple diffusion through the bilayer, while others require embedded transport proteins. A researcher replaces many membrane phospholipids containing unsaturated fatty acid tails with phospholipids containing saturated tails, leaving the number of transport proteins unchanged. After the replacement, fewer small nonpolar molecules enter the cell per unit time. Which change would most likely account for the decreased diffusion of small nonpolar molecules?
    Answer: Saturated tails pack more tightly, reducing bilayer fluidity and diffusion rate This question requires analyzing plasma membrane structure and transport properties. The correct answer is A because saturated fatty acid tails are straight and pack together more tightly than kinked unsaturated tails, reducing membrane fluidity and slowing the diffusion of molecules through the lipid bilayer. The experiment shows that replacing unsaturated with saturated tails decreases nonpolar molecule entry while keeping transport proteins constant, indicating the change affects simple diffusion through lipids. Answer B is incorrect because fatty acid tail saturation doesn't change membrane charge—the tails remain nonpolar regardless of saturation, reflecting a misconception about chemical properties. To solve membrane fluidity problems, remember that saturated tails pack tightly (less fluid) while unsaturated tails with kinks pack loosely (more fluid), affecting diffusion rates.
13. The plasma membrane is composed of a phospholipid bilayer with hydrophobic tails that form a nonpolar interior. Some phospholipids have unsaturated fatty acid tails that introduce kinks, reducing tight packing. A researcher compares two artificial membranes: Membrane X has mostly saturated tails, and Membrane Y has more unsaturated tails. Both lack transport proteins. When exposed to the same small nonpolar solute, Membrane Y shows faster diffusion of the solute. Which feature best explains the higher permeability of Membrane Y?
    Answer: Unsaturated tails pack less tightly, increasing bilayer fluidity and diffusion of nonpolar solutes This question requires analyzing how plasma membrane lipid composition affects transport properties. The correct answer is A because unsaturated fatty acid tails have kinks that prevent tight packing of phospholipids, creating a more fluid bilayer with more space between molecules for nonpolar solutes to move through. The experiment shows that Membrane Y, with more unsaturated tails, allows faster diffusion of nonpolar solutes compared to Membrane X with mostly saturated (straight) tails that pack tightly together. This increased fluidity and spacing in the hydrophobic core makes it easier for nonpolar molecules to dissolve into and diffuse through the membrane. Answer D is incorrect because unsaturated tails don't convert the bilayer interior into a hydrophilic region—the interior remains hydrophobic regardless of saturation, and this hydrophobic nature is what allows nonpolar solutes to cross. When comparing membrane permeability, remember that lipid saturation affects packing and fluidity: unsaturated = more fluid = faster diffusion for molecules that cross through the lipid bilayer.
14. Cells are placed in a solution containing a high concentration of glucose. The plasma membrane is a phospholipid bilayer with a hydrophobic interior and embedded transport proteins. Glucose is polar and does not readily pass through the hydrophobic core by simple diffusion. In one group, a membrane protein that spans the bilayer and has a glucose-binding site is abundant; in another group, that protein is absent. Which feature best explains why glucose enters cells more rapidly when the protein is present?
    Answer: A transmembrane carrier provides a hydrophilic pathway for polar glucose to cross This question assesses the skill of analyzing plasma membrane structure and transport. The transmembrane carrier protein provides a hydrophilic pathway for polar glucose molecules, binding them and undergoing a conformational change to shuttle them across the hydrophobic core of the bilayer. Without this protein, glucose cannot easily pass through the hydrophobic interior by simple diffusion due to its polarity, resulting in slower entry. The abundance of this protein in one group explains the faster glucose transport observed, as it facilitates movement down the concentration gradient. A tempting distractor is choice A, which wrongly suggests glucose dissolves in the hydrophobic core, stemming from the misconception that polar molecules can readily interact with nonpolar regions. For similar problems, identify whether the solute is polar or nonpolar and evaluate the role of specific proteins in overcoming the bilayer's hydrophobic barrier.
15. Two membranes have the same phospholipid bilayer composition but differ in cholesterol content. Membrane A has high cholesterol; Membrane B has low cholesterol. At a low temperature, small nonpolar molecules diffuse more readily through Membrane A than through Membrane B. Cholesterol is embedded among phospholipid tails. Which membrane property change best explains the observation?
    Answer: Cholesterol reduces tight packing of phospholipid tails at low temperature, maintaining fluidity This question assesses the skill of analyzing plasma membrane structure and transport. At low temperatures, Membrane A with high cholesterol allows better diffusion of small nonpolar molecules by reducing tight packing of phospholipid tails, thus maintaining membrane fluidity. Cholesterol embeds among the tails and prevents crystallization-like solidification that would occur in low-cholesterol Membrane B, increasing permeability. Both membranes have the same phospholipids, so cholesterol's moderating effect on fluidity explains the difference. A tempting distractor is choice C, which falsely claims cholesterol forms open channels, confusing its structural role with that of transport proteins. To assess permeability at varying temperatures, analyze how cholesterol influences bilayer fluidity and packing.
16. Two artificial membranes are made from phospholipids. Membrane 1 is composed mostly of saturated fatty acid tails; Membrane 2 is composed mostly of unsaturated tails with cis double bonds. Both membranes lack transport proteins. A small, nonpolar gas diffuses through Membrane 2 faster than through Membrane 1 at the same temperature. Phospholipid heads face outward toward water, and tails face inward. Which membrane feature best explains the higher permeability of Membrane 2?
    Answer: Unsaturated tails create kinks that reduce packing, increasing bilayer fluidity and diffusion rate This question requires analyzing plasma membrane structure and transport properties based on fatty acid saturation. The correct answer is A because unsaturated fatty acids contain cis double bonds that create kinks in the tails, preventing tight packing and increasing membrane fluidity, which allows nonpolar gases to diffuse more readily through the loosely packed hydrophobic interior. The experimental setup specifically compares membranes differing only in tail saturation, isolating this variable's effect on permeability. Answer C is incorrect because it suggests unsaturated tails increase hydrogen bonding with water, which represents a misconception - the hydrophobic tails actually repel water and cannot form hydrogen bonds due to their nonpolar nature. To predict membrane permeability, evaluate how structural features affect packing density and fluidity in the hydrophobic core.
17. A membrane's permeability depends on phospholipid arrangement and embedded proteins. The bilayer's interior is hydrophobic, while many transport proteins contain hydrophilic amino acids lining a passageway. In a lab, a mutation alters a transmembrane protein so that nonpolar amino acids replace several hydrophilic residues lining its pore. The total number of proteins in the membrane remains the same, and the bilayer is unchanged. After the change, transport of a polar solute through this protein decreases. Which change best explains the reduced transport?
    Answer: The pore becomes less favorable for polar solutes because its lining is more hydrophobic This question assesses your ability to analyze how plasma membrane protein structure affects transport function. The correct answer is A because replacing hydrophilic amino acids with nonpolar ones in the channel's pore makes the pathway less favorable for polar solutes, which need to interact with hydrophilic residues to move through the protein. The mutation changes the chemical environment inside the transport protein from hydrophilic (water-friendly) to more hydrophobic (water-repelling), creating an unfavorable environment for polar solutes that reduces their ability to pass through. This demonstrates the importance of matching the chemical properties of the transport pathway with the solute being transported. Answer C is incorrect because proteins don't become lipids through amino acid substitutions—they remain proteins but with altered properties, and this wouldn't increase diffusion of polar solutes through the bilayer core. When analyzing transport proteins, remember that the amino acids lining the pore must be chemically compatible with the solutes they transport: hydrophilic for polar/charged solutes, hydrophobic for nonpolar ones.
18. A cell membrane is a phospholipid bilayer with a hydrophobic interior and embedded proteins. Researchers compare transport of two solutes: oxygen (small and nonpolar) and sucrose (large and polar). The membrane contains no sucrose-specific transport proteins. Both solutes are present at higher concentration outside the cell than inside. Which feature best explains why oxygen enters the cell more readily than sucrose?
    Answer: Oxygen is nonpolar and can dissolve in the hydrophobic core, allowing simple diffusion This question assesses the skill of analyzing plasma membrane structure and transport. Oxygen, being small and nonpolar, can readily dissolve in the hydrophobic interior of the phospholipid bilayer and cross by simple diffusion down its concentration gradient. Sucrose, however, is large and polar, making it unable to pass easily through the hydrophobic core without specific transport proteins, which are absent in this membrane. This difference in polarity and the lack of sucrose transporters explain why oxygen enters more readily despite both having favorable gradients. A tempting distractor is choice B, which claims sucrose diffuses rapidly due to its polarity, based on the misconception that polar molecules interact favorably with hydrophobic regions. To solve transport comparison questions, classify solutes by size and polarity to predict their diffusion mechanisms across bilayers.
19. A phospholipid bilayer has hydrophilic heads facing outward and hydrophobic tails forming the membrane's interior. Some transmembrane proteins act as channels with a hydrophilic pore. A toxin binds to the external surface of a specific channel protein and physically blocks its pore without altering the bilayer. After toxin exposure, a particular ion's movement across the membrane decreases, but movement of other ions and small nonpolar molecules is unchanged. Which feature best explains the specificity of the toxin's effect?
    Answer: Channel proteins have selective pores that allow only certain ions to pass, so blocking one channel reduces only that ion's transport This question tests your understanding of plasma membrane channel specificity and selective transport. The correct answer is A because different channel proteins have structurally distinct pores with specific sizes, shapes, and charge distributions that allow only certain ions to pass through—this selectivity filter is what makes each channel specific for particular ions. When the toxin blocks one specific channel type, only the transport of ions that use that particular channel decreases, while other ions that use different channels and nonpolar molecules that diffuse through the lipid bilayer remain unaffected. This demonstrates the highly specific nature of protein-mediated transport across membranes. Answer E is incorrect because membrane proteins don't all form identical pores—each transport protein has evolved to be selective for specific solutes based on the structure of its channel. The key insight is that membrane transport specificity comes from the precise structure of individual transport proteins, not from the general properties of the lipid bilayer.
20. Two cells have plasma membranes with identical phospholipid composition and thickness. Cell X has many transmembrane carrier proteins specific for glucose; Cell Y has far fewer of these carriers. Both cells are placed in a solution with glucose concentration higher outside than inside. Glucose enters Cell X faster than Cell Y, even though glucose is polar. Which membrane feature best explains the higher glucose uptake rate?
    Answer: A higher density of specific carrier proteins increases facilitated diffusion of glucose This question assesses the skill of analyzing plasma membrane structure and transport. Glucose enters Cell X faster because it has a higher density of specific carrier proteins that facilitate diffusion of the polar glucose molecules down their concentration gradient. Despite identical phospholipid composition, the abundance of carriers in Cell X provides more pathways for glucose, which cannot easily cross the hydrophobic bilayer without assistance. This explains the higher uptake rate even though glucose is polar and the gradient is the same for both cells. A tempting distractor is choice B, which mistakenly states more carriers increase tail saturation to raise glucose solubility, ignoring that saturation affects fluidity, not polar solute solubility in lipids. To compare transport rates, consider how the number of specific transport proteins influences facilitated diffusion of polar molecules.

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