AP Biology Quiz: Plasma Membrane

What this quiz covers

This quiz focuses on Plasma Membrane, giving you a quick way to practice the rules, question types, and explanations that matter most for AP Biology.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. More saturated tails pack tightly, decreasing membrane fluidity and limiting protein diffusion (correct answer)
2. More saturated tails increase membrane permeability, causing proteins to leak out of the bilayer
3. More saturated tails increase negative charge, forming ionic bonds that immobilize proteins
4. More saturated tails create larger gaps, allowing proteins to move faster through the bilayer
5. More saturated tails shorten phospholipids, forcing proteins to unfold and slow transport

Explanation: 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.

Question 2

1. K+ crosses by simple diffusion because charged ions pass through hydrophobic tails easily
2. A K+ channel provides a hydrophilic pore that reduces interaction with the bilayer core (correct answer)
3. Phospholipid heads bind K+ and flip to the inner leaflet to deliver the ion
4. K+ enters only when the membrane becomes more saturated, increasing fluidity
5. K+ enters only when the channel makes the membrane thicker, increasing permeability

Explanation: 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.

Question 3

1. Carrier proteins create a hydrophilic route by binding the solute and changing conformation (correct answer)
2. Amino acids diffuse rapidly through the hydrophobic core because they are polar
3. Phospholipid heads rotate inward, exposing water to the core and moving amino acids
4. Carrier proteins increase membrane thickness, which increases permeability to polar solutes
5. Embedded proteins convert polar solutes into nonpolar solutes so they cross the bilayer

Explanation: This question assesses the skill of analyzing plasma membrane structure and transport. Carrier proteins bind the polar amino acid and change conformation to provide a hydrophilic route across the hydrophobic interior, facilitating movement even without a concentration gradient. This mechanism increases transport rate when the carrier is present, as the bilayer's core otherwise repels polar solutes. The unchanged hydrophobic interior emphasizes the carrier's role in overcoming this barrier. A tempting distractor is choice B, which incorrectly states amino acids diffuse rapidly through the core due to polarity, based on the misconception that polar molecules can cross nonpolar regions without assistance. For facilitated transport questions, examine how protein conformational changes enable solute movement independent of gradients.

Question 4

1. Cholesterol reduces tight packing of phospholipid tails at low temperature, maintaining fluidity (correct answer)
2. Cholesterol increases the polarity of the bilayer interior, dissolving nonpolar molecules faster
3. Cholesterol forms open channels that allow nonpolar molecules to pass through the membrane
4. Cholesterol binds to nonpolar molecules and carries them across by active transport
5. Cholesterol increases phospholipid head charge, pulling nonpolar molecules across the surface

Explanation: 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.

Question 5

1. Oxygen is nonpolar and can dissolve in the hydrophobic core, allowing simple diffusion (correct answer)
2. Sucrose is polar and therefore diffuses rapidly through the hydrophobic tails
3. Oxygen requires carrier proteins because it is nonpolar and cannot cross the bilayer
4. Sucrose crosses faster because large molecules move more quickly through membranes
5. Oxygen enters faster because phospholipid heads bind oxygen and pull it inward

Explanation: 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.

Question 6

1. Decrease the number of aquaporins so fewer water molecules can cross
2. Increase aquaporin abundance to provide more hydrophilic channels for water (correct answer)
3. Replace phospholipids with proteins so the membrane becomes fully hydrophilic
4. Increase the number of carbohydrate chains on the outer surface of the membrane
5. Increase phospholipid head polarity to prevent water from entering the bilayer

Explanation: This question assesses the skill of analyzing plasma membrane structure and transport. Increasing aquaporin abundance in Membrane Y would provide more hydrophilic channels, allowing polar water molecules to cross the hydrophobic interior more rapidly via facilitated diffusion. Membrane X already has many aquaporins, enabling faster water movement, while Membrane Y's few aquaporins limit the rate, especially under a hypertonic gradient driving water loss. This change directly addresses the barrier posed by the hydrophobic tails, which slow simple diffusion of water. A tempting distractor is choice A, which suggests decreasing aquaporins, based on the misconception that fewer channels would somehow enhance water flow rather than restrict it. When tackling osmosis-related questions, focus on how protein channels modulate the permeability of polar molecules across lipid bilayers.

Question 7

1. A higher density of specific carrier proteins increases facilitated diffusion of glucose (correct answer)
2. More carrier proteins increase phospholipid tail saturation, raising glucose solubility
3. Fewer carriers in Cell Y increase membrane fluidity, preventing glucose from entering
4. Glucose crosses faster in Cell X because polar molecules diffuse best through lipid cores
5. Carrier proteins in Cell X reduce extracellular glucose concentration, increasing diffusion force

Explanation: 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.

Question 8

1. The bilayer interior is hydrophilic, so ions enter X but not Y due to protein crowding
2. The pore provides a hydrophilic route that bypasses the hydrophobic core, increasing ion entry (correct answer)
3. The pore decreases membrane thickness, making the bilayer tails shorter and more charged
4. The pore converts ions into nonpolar molecules, allowing them to dissolve in the tails
5. The pore blocks diffusion by binding ions permanently, preventing movement into the vesicle

Explanation: 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.

Question 9

1. Unsaturated tails pack less tightly, increasing bilayer fluidity and diffusion of nonpolar solutes (correct answer)
2. Unsaturated tails increase the membrane's overall charge, attracting nonpolar solutes into the bilayer
3. Saturated tails create hydrophilic pores that specifically block nonpolar solutes
4. Unsaturated tails convert the bilayer interior into a hydrophilic region that excludes nonpolar solutes
5. Saturated tails bind nonpolar solutes and shuttle them across without proteins

Explanation: 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.

Question 10

1. Aquaporins provide hydrophilic channels that allow rapid water passage across the bilayer (correct answer)
2. Aquaporins make the phospholipid tails more polar, increasing water solubility in the core
3. Aquaporins convert water into ions, which diffuse through the hydrophobic interior
4. Aquaporins decrease the number of phospholipids, shortening the diffusion distance for water
5. Aquaporins increase membrane surface charge, pulling water across by electrostatic attraction

Explanation: This question assesses the skill of analyzing plasma membrane structure and transport. The faster swelling of aquaporin-containing liposomes in a hypotonic solution results from aquaporins forming hydrophilic channels that enable rapid water passage through the otherwise hydrophobic bilayer core. Without aquaporins, water crosses slowly due to the nonpolar fatty acid tails repelling polar water molecules, but aquaporins provide a polar pathway that bypasses this barrier. Both sets have identical phospholipids, so the difference is solely due to these embedded proteins facilitating osmosis. A tempting distractor is choice B, which wrongly claims aquaporins make tails more polar to increase water solubility, confusing the role of proteins with altering lipid properties. When studying osmosis across membranes, identify how specific proteins like aquaporins enhance transport of polar molecules like water.

Question 11

1. Shorter tails reduce bilayer thickness, shortening the diffusion path through the core (correct answer)
2. Longer tails increase membrane fluidity, allowing faster diffusion of nonpolar dye
3. Short tails create a charged interior that attracts nonpolar dye molecules
4. Long tails increase head polarity, which speeds diffusion through the bilayer
5. Short tails increase the number of embedded proteins that transport dye molecules

Explanation: 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.

Question 12

1. Na+ will diffuse through the hydrophobic core because ions are small
2. Na+ permeability will decrease because the blocked channel no longer provides a pathway (correct answer)
3. Na+ permeability will increase because blocked channels destabilize phospholipid tails
4. Na+ permeability will be unchanged because phospholipid heads transport ions
5. Na+ permeability will increase because Na+ binds to the channel exterior and crosses

Explanation: This question assesses the skill of analyzing plasma membrane structure and transport. Blocking the Na+-specific channel protein prevents Na+ from using its hydrophilic pore to cross the hydrophobic bilayer interior, thereby decreasing Na+ permeability. The drug blocks the channel without removing the protein, so the pathway for ions is obstructed, reducing entry despite the high external NaCl concentration. The hydrophobic tails naturally barrier ions, making channels essential for their transport, and blocking them limits diffusion. A tempting distractor is choice C, which claims blocked channels increase permeability by destabilizing tails, arising from the misconception that obstruction enhances rather than impedes ion flow. In ion transport scenarios, evaluate how alterations to channel function affect the membrane's selective permeability to charged particles.

Question 13

1. Glucose dissolves in the hydrophobic core and diffuses faster when proteins are nearby
2. A transmembrane carrier provides a hydrophilic pathway for polar glucose to cross (correct answer)
3. Phospholipid heads repel glucose, so more transport occurs when heads are removed
4. Cholesterol forms pores that specifically allow glucose to pass through the bilayer
5. Saturated fatty acid tails bind glucose and shuttle it across the membrane

Explanation: 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.

Question 14

1. Decrease the number of phospholipid molecules to create gaps in the bilayer
2. Increase the number of transmembrane transport proteins that provide hydrophilic passageways (correct answer)
3. Replace fatty acid tails with additional phosphate groups to make the core nonpolar
4. Add more peripheral proteins to the membrane surface to attract glucose
5. Increase membrane carbohydrate content to convert glucose into a nonpolar solute

Explanation: This question requires analyzing plasma membrane structure and transport to determine how to increase glucose entry. The correct answer is B because glucose is a polar molecule that cannot easily cross the hydrophobic core of the phospholipid bilayer and requires transmembrane transport proteins to provide a hydrophilic pathway. The data shows that cell type 1, which has more transmembrane proteins, accumulates glucose rapidly while cell type 2, with fewer such proteins, shows minimal glucose entry despite having similar phospholipid composition. Adding more transmembrane transport proteins would create additional hydrophilic routes for glucose to cross the membrane. Answer C is incorrect because replacing fatty acid tails with phosphate groups would destroy the bilayer structure—phosphate groups are hydrophilic, not nonpolar, and the membrane needs its hydrophobic core to function. When comparing transport between cells, always check both the membrane's lipid composition and the presence of transport proteins that facilitate movement of polar solutes.

Question 15

1. A less polar channel lining reduces favorable interactions with glucose during passage (correct answer)
2. A less polar channel lining increases the membrane's surface tension, blocking diffusion
3. Nonpolar amino acids add charge to the channel, repelling glucose molecules away
4. Nonpolar amino acids convert glucose into a lipid, trapping it within the bilayer core
5. Nonpolar amino acids increase phospholipid head size, preventing channels from opening

Explanation: 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.

Question 16

1. Ions require hydrophilic pathways through transmembrane proteins to cross the hydrophobic bilayer core (correct answer)
2. Nonpolar molecules require channel proteins because they cannot dissolve in membrane lipids
3. Channel proteins increase bilayer thickness, which prevents nonpolar molecules from diffusing
4. Phospholipid heads form covalent bonds with ions, pulling them through without proteins
5. Blocking channels increases membrane fluidity, which selectively traps ions inside the bilayer

Explanation: This question tests your understanding of plasma membrane structure and selective transport mechanisms. The correct answer is A because ions are charged particles that cannot dissolve in or pass through the hydrophobic interior of the phospholipid bilayer, so they require hydrophilic pathways provided by channel proteins to cross the membrane. The experiment demonstrates this principle: when channels are blocked, ion movement decreases sharply because ions lose their only route across the membrane, while nonpolar molecules continue to cross unchanged because they can dissolve directly in the hydrophobic core without needing protein channels. Answer B is incorrect because it reverses the actual transport requirements—nonpolar molecules don't need channel proteins since they can dissolve in membrane lipids, while ions do need channels. The fundamental principle is that a solute's polarity determines its transport mechanism: polar/charged solutes need protein-mediated pathways, while nonpolar solutes can use direct diffusion through the lipid bilayer.

Question 17

1. Tighter packing of hydrophobic tails reduces the rate at which nonpolar solutes dissolve and move through the bilayer (correct answer)
2. Cooling converts phospholipids into ions, increasing electrostatic repulsion of nonpolar solutes
3. Lower temperature increases the number of embedded proteins, which physically blocks diffusion
4. Cooling makes phosphate heads nonpolar, preventing nonpolar solutes from entering the membrane
5. Lower temperature causes nonpolar solutes to become charged, so they cannot cross the bilayer

Explanation: This question tests your understanding of how plasma membrane fluidity affects transport. The correct answer is A because cooling causes phospholipid tails to pack more tightly together, reducing the membrane's fluidity and making it more difficult for nonpolar solutes to dissolve into and move through the hydrophobic bilayer core. At lower temperatures, the fatty acid tails have less kinetic energy and movement, creating a more rigid structure with less space between phospholipids for solutes to slip through. Since the nonpolar solute relies on dissolving in and diffusing through the lipid bilayer (not through proteins), this tighter packing directly slows its movement. Answer B is incorrect because cooling doesn't convert phospholipids into ions—phospholipids remain unchanged in their chemical structure, only their physical arrangement becomes more ordered. The key principle is that membrane fluidity directly affects the rate of simple diffusion through the bilayer: more fluid membranes allow faster diffusion, while less fluid membranes slow it down.

Question 18

1. High cholesterol increases spacing between phospholipids, raising nonpolar diffusion
2. High cholesterol reduces tail movement, decreasing permeability of the hydrophobic core (correct answer)
3. Cholesterol forms hydrophilic channels that exclude nonpolar molecules from crossing
4. Low cholesterol causes phospholipid heads to face inward, blocking diffusion
5. Low cholesterol increases membrane thickness, which decreases resistance to diffusion

Explanation: 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.

Question 19

1. Aquaporins provide a hydrophilic pathway that increases water permeability across the hydrophobic bilayer (correct answer)
2. Aquaporins convert water into a nonpolar molecule that diffuses through phospholipid tails
3. Aquaporins increase the number of phosphate heads, making the membrane interior more hydrophobic
4. Aquaporins block diffusion by filling the bilayer with protein, reducing water movement
5. Aquaporins remove the phospholipid bilayer, allowing water to cross through open gaps

Explanation: 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.

Question 20

1. Saturated tails pack more tightly, reducing bilayer fluidity and diffusion rate (correct answer)
2. Saturated tails increase membrane charge, repelling nonpolar molecules from entry
3. Saturated tails create more protein channels, increasing selective permeability overall
4. Saturated tails make the membrane more hydrophilic, trapping nonpolar molecules outside
5. Saturated tails shorten the bilayer, forcing nonpolar molecules to use transport proteins

Explanation: 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.