AP Biology Quiz: Mechanisms Of Transport
20 questions · exam conditions
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Mechanisms Of TransportQuestion 1 of 20

A drug blocks a specific membrane carrier. After treatment, a polar nutrient no longer enters cells, even though extracellular nutrient concentration remains higher than intracellular. ATP levels in the cell are unchanged. When the drug is removed, nutrient entry resumes and stops at equilibrium. Which mechanism best explains nutrient entry when the carrier is not blocked?

Facilitated diffusion through a carrier transporting the nutrient down its concentration gradient
Primary active transport using ATP hydrolysis to move the nutrient into the cell
Simple diffusion of the polar nutrient through the phospholipid bilayer
Secondary active transport requiring a maintained ion gradient to move nutrient uphill
Exocytosis delivering extracellular nutrient directly into the cytosol
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AP Biology Quiz

AP Biology Quiz: Mechanisms Of Transport

Practice Mechanisms Of Transport in AP Biology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

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

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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.

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Question 1

A drug blocks a specific membrane carrier. After treatment, a polar nutrient no longer enters cells, even though extracellular nutrient concentration remains higher than intracellular. ATP levels in the cell are unchanged. When the drug is removed, nutrient entry resumes and stops at equilibrium. Which mechanism best explains nutrient entry when the carrier is not blocked?

  1. Facilitated diffusion through a carrier transporting the nutrient down its concentration gradient (correct answer)
  2. Primary active transport using ATP hydrolysis to move the nutrient into the cell
  3. Simple diffusion of the polar nutrient through the phospholipid bilayer
  4. Secondary active transport requiring a maintained ion gradient to move nutrient uphill
  5. Exocytosis delivering extracellular nutrient directly into the cytosol

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. Facilitated diffusion is correct because the polar nutrient moves down its gradient via the carrier, without ATP, resuming to equilibrium when unblocked. Blocking stops entry, confirming protein need. Unchanged ATP levels rule out active transport. A tempting distractor is primary active transport, wrong due to the misconception that carrier blocking implies energy involvement, ignoring passive facilitation. To analyze transport, always evaluate if movement is down a gradient (passive) or against it (active) and check for energy requirements.

Question 2

A transporter binds glucose on the extracellular side and releases it into the cytosol. Extracellular glucose is 1 mM and cytosolic glucose is 10 mM, yet glucose still enters the cell. The transport rate decreases sharply when ATP is depleted and resumes when ATP is restored. Blocking Na+^+ gradients has no effect. Which mechanism best explains glucose entry in this experiment?

  1. Facilitated diffusion of glucose down its gradient via a uniporter
  2. Simple diffusion of glucose through the bilayer driven by concentration
  3. Primary active transport of glucose against its gradient using ATP directly (correct answer)
  4. Secondary active transport of glucose coupled to Na+^+ moving inward
  5. Osmosis through aquaporins increasing intracellular glucose concentration

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. The correct answer is primary active transport of glucose against its gradient using ATP directly because glucose moves from low extracellular to high cytosolic concentration, requiring energy from ATP hydrolysis to drive the uphill transport. The sharp decrease in rate upon ATP depletion and resumption with ATP restoration indicate direct energy dependence on ATP. The lack of effect from blocking Na+ gradients rules out coupling to ion movements, confirming the transporter uses ATP itself. A tempting distractor is secondary active transport coupled to Na+ moving inward, but this is incorrect due to the misconception that all against-gradient glucose transport involves Na+; here, Na+ independence points to primary active. To analyze similar problems, always determine if movement is down a gradient (passive) or against (active) and check for energy dependence.

Question 3

A molecule X is uncharged but polar. In cells lacking a specific membrane protein, X does not enter even when extracellular X is high. In cells expressing the protein, X enters until inside and outside concentrations match. Uptake shows a maximum rate at high X concentrations and does not require ATP. Which mechanism best explains transport of X?

  1. Facilitated diffusion via a saturable carrier transporting X down its concentration gradient (correct answer)
  2. Simple diffusion of X through the lipid bilayer driven by concentration differences
  3. Primary active transport of X using ATP hydrolysis to move X into the cell
  4. Secondary active transport of X coupled to K+^+ efflux from the cell
  5. Exocytosis releasing X from intracellular vesicles to the cytosol

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. Facilitated diffusion is correct because polar X moves down its gradient via a protein, showing saturation, without ATP, stopping at equilibrium. No entry without protein distinguishes from simple diffusion. Uncharged nature fits carrier facilitation. A tempting distractor is simple diffusion, wrong due to the misconception that polar molecules cross bilayers easily, ignoring protein need. To analyze transport, always evaluate if movement is down a gradient (passive) or against it (active) and check for energy requirements.

Question 4

A cell produces a large protein that is packaged into secretory vesicles. Over time, the protein appears in the extracellular medium. Microscopy shows vesicles moving to and fusing with the plasma membrane, releasing the protein outside. The process decreases when ATP is depleted. Which mechanism best explains how the protein crosses the plasma membrane?

  1. Simple diffusion of protein through the lipid bilayer down a gradient
  2. Facilitated diffusion of protein through a membrane channel protein pore
  3. Primary active transport through an ATP-driven protein pump in membrane
  4. Exocytosis via vesicle fusion with the plasma membrane releasing contents (correct answer)
  5. Secondary active transport coupled to Na+^+ influx moving protein outward

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. The correct answer is exocytosis via vesicle fusion with the plasma membrane releasing contents because the large protein is packaged into vesicles that fuse with the membrane, releasing it extracellularly in an energy-dependent manner. ATP depletion decreases the process, indicating energy is needed for vesicle transport and fusion, not for direct gradient-driven movement. Microscopy shows vesicle movement and fusion, confirming bulk export rather than diffusion or active pumping through the membrane. A tempting distractor is primary active transport through an ATP-driven pump, but this is wrong due to the misconception that large proteins are pumped like ions; exocytosis is for vesicular release. To analyze similar problems, always determine if movement is down a gradient (passive) or against (active) and check for energy dependence.

Question 5

A polar solute is placed outside cells at high concentration. Net solute entry occurs only when a specific channel protein is open. Entry rate increases with the solute gradient but does not show saturation over the tested range. No ATP is required, and net movement stops when the inside and outside concentrations become equal. Which mechanism best explains the solute's movement?

  1. Primary active transport by an ATPase moving solute against its gradient
  2. Facilitated diffusion through a channel down the solute's electrochemical gradient (correct answer)
  3. Simple diffusion directly through the phospholipid bilayer
  4. Secondary active transport requiring a Na+^+ gradient and cotransport
  5. Exocytosis of solute from intracellular vesicles to the outside

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. Facilitated diffusion is correct because the polar solute moves down its gradient through an open channel protein, without ATP, stopping at equilibrium. The rate increases with gradient but lacks saturation, typical of channels rather than carriers. Protein dependence distinguishes it from simple diffusion. A tempting distractor is simple diffusion, incorrect due to the misconception that polar solutes easily cross bilayers, overlooking the need for protein facilitation. To analyze transport, always evaluate if movement is down a gradient (passive) or against it (active) and check for energy requirements.

Question 6

A cell is exposed to a polar drug at higher concentration outside than inside. The drug enters only when a specific transporter is expressed. Uptake is unaffected by ATP depletion and by collapsing Na+^+ and H+^+ gradients. The uptake rate increases with external drug concentration but plateaus at high concentration. Which mechanism best explains drug entry?

  1. Facilitated diffusion via a saturable carrier protein down the drug gradient (correct answer)
  2. Primary active transport using ATP directly to move drug down its gradient
  3. Secondary active transport coupled to an ion gradient that is not required
  4. Simple diffusion through the lipid bilayer requiring a specific transporter
  5. Exocytosis releasing drug from intracellular vesicles into the cytosol

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. The correct answer is facilitated diffusion via a saturable carrier protein down the drug gradient because the polar drug moves passively from high external to low internal concentration through a specific transporter, with plateauing kinetics indicating saturation. Unaffected by ATP depletion or ion gradient collapse confirms passivity and no secondary coupling. Requires the transporter for entry. A tempting distractor is simple diffusion through the lipid bilayer, but this is incorrect due to the misconception that polar drugs diffuse freely without proteins; carriers are needed for facilitation. To analyze similar problems, always determine if movement is down a gradient (passive) or against (active) and check for energy dependence.

Question 7

In an experiment, cells are placed in a solution containing a polar sugar analog at a higher concentration outside than inside. Uptake of the sugar analog increases rapidly and then plateaus as external concentration continues to rise. When a membrane protein inhibitor is added, uptake drops to near zero. When cellular ATP is depleted, uptake rate is unchanged. The sugar analog does not cross a pure phospholipid bilayer at a measurable rate. Which mechanism best explains the sugar analog entering the cells under normal conditions?

  1. Simple diffusion directly through the phospholipid bilayer down its gradient
  2. Facilitated diffusion through a specific membrane carrier down its gradient (correct answer)
  3. Primary active transport by an ATP-driven pump moving sugar against gradient
  4. Secondary active transport coupled to ion movement up the sugar gradient
  5. Osmosis through aquaporins because sugar increases water potential differences

Explanation: This question assesses the skill of analyzing mechanisms of membrane transport. The sugar analog is polar and cannot cross a pure phospholipid bilayer, indicating it requires a membrane protein for entry, and uptake plateaus with increasing external concentration, suggesting saturation of a carrier, characteristic of facilitated diffusion. This process moves the analog down its concentration gradient from higher outside to lower inside without requiring energy, as evidenced by unchanged uptake rates upon ATP depletion. The drop in uptake with a membrane protein inhibitor further confirms involvement of a specific carrier protein facilitating passive transport. A tempting distractor is choice C, primary active transport, which is incorrect because it assumes ATP is needed to move against a gradient, misconstruing the lack of ATP dependence and the downhill movement as energy-requiring. To identify transport types, compare uptake kinetics, energy dependence, and permeability in artificial bilayers across experimental conditions.

Question 8

A toxin blocks ATP production in cells. Shortly after, an ATP-dependent proton pump in the plasma membrane stops moving H+^+ out of the cytosol, even though the H+^+ concentration is higher outside than inside. When ATP is restored, the pump resumes transporting H+^+ outward. Which mechanism best explains H+^+ movement by this pump when ATP is present?

  1. Facilitated diffusion of H+^+ down its electrochemical gradient through a channel
  2. Primary active transport of H+^+ driven directly by ATP hydrolysis (correct answer)
  3. Simple diffusion of H+^+ through the hydrophobic lipid interior
  4. Secondary active transport using glucose movement to power H+^+ export
  5. Osmosis of H+^+ through aquaporins with water movement

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. Primary active transport is correct because the pump uses ATP hydrolysis directly to move H+ against its gradient, stopping without ATP despite favorable gradient. Resumption with ATP confirms direct energy use. Phosphorylation or ATP sites would support this. A tempting distractor is facilitated diffusion, incorrect due to the misconception that uphill movement can be passive, overlooking energy needs. To analyze transport, always evaluate if movement is down a gradient (passive) or against it (active) and check for energy requirements.

Question 9

A membrane protein opens in response to a voltage change and allows Na+^+ to cross. When open, Na+^+ moves rapidly into the cell from high extracellular concentration to lower cytosolic concentration. The movement stops when the channel is blocked and is unaffected by ATP depletion. Which mechanism best explains Na+^+ movement through this protein?

  1. Facilitated diffusion through a gated ion channel down electrochemical gradient (correct answer)
  2. Primary active transport of Na+^+ using ATP hydrolysis by the channel
  3. Simple diffusion of Na+^+ through the hydrophobic bilayer without proteins
  4. Secondary active transport exchanging Na+^+ for glucose in the same direction
  5. Exocytosis releasing Na+^+ stored in vesicles into the cytosol

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. The correct answer is facilitated diffusion through a gated ion channel down electrochemical gradient because Na+ moves passively from high extracellular to low cytosolic concentration when the voltage-gated channel opens, without energy. Unaffected by ATP depletion and stops when blocked, confirming channel-mediated passive flow. Rapid movement supports ion channel kinetics. A tempting distractor is simple diffusion through the bilayer, but this is incorrect due to the misconception that charged Na+ crosses hydrophobic lipids without proteins; channels are required. To analyze similar problems, always determine if movement is down a gradient (passive) or against (active) and check for energy dependence.

Question 10

A membrane contains an antiporter that moves H+^+ into the cell while moving Na+^+ out. Na+^+ export continues even when extracellular Na+^+ is higher than intracellular Na+^+, but it stops if the H+^+ gradient is eliminated. The antiporter does not bind ATP. Another protein uses ATP to pump H+^+ out, maintaining higher extracellular H+^+. Which process best explains Na+^+ export by the antiporter?

  1. Secondary active transport using the H+^+ gradient to drive Na+^+ export (correct answer)
  2. Primary active transport of Na+^+ by direct ATP hydrolysis at the antiporter
  3. Facilitated diffusion of Na+^+ through the antiporter down its gradient
  4. Simple diffusion of Na+^+ through the lipid bilayer without proteins
  5. Exocytosis exporting Na+^+ in vesicles that fuse with the membrane

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. Secondary active transport is correct because the antiporter uses the H+ gradient (from ATP pump) to drive Na+ out against its gradient, without direct ATP. Stopping without H+ gradient confirms coupled energy. Directionality supports antiport mechanism. A tempting distractor is primary active transport, incorrect due to the misconception that all antiporters use ATP directly, overlooking ion gradient powering. To analyze transport, always evaluate if movement is down a gradient (passive) or against it (active) and check for energy requirements.

Question 11

Cells are placed in a medium with high extracellular K+^+ and low intracellular K+^+. K+^+ enters only when a specific K+^+ channel is open, and net influx stops as intracellular K+^+ rises. No ATP is required for K+^+ entry, and K+^+ can move out if the gradient reverses. Which mechanism best explains K+^+ entry under these conditions?

  1. Facilitated diffusion of K+^+ through an ion channel down its electrochemical gradient (correct answer)
  2. Primary active transport of K+^+ using ATP to move K+^+ into the cell
  3. Simple diffusion of K+^+ through the hydrophobic lipid bilayer
  4. Secondary active transport of K+^+ coupled to glucose transport into the cell
  5. Osmosis of K+^+ through aquaporins with water movement

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. Facilitated diffusion is correct because K+ moves down its electrochemical gradient through an open channel, without ATP, stopping at equilibrium or reversing with gradient. Channel dependence confirms facilitation. Ion nature requires protein aid. A tempting distractor is simple diffusion, wrong due to the misconception that charged ions diffuse through bilayers, ignoring hydrophobicity barriers. To analyze transport, always evaluate if movement is down a gradient (passive) or against it (active) and check for energy requirements.

Question 12

A cell imports phosphate (Pi) using a cotransporter that moves Pi into the cell only when H+^+ also moves in. Pi accumulates inside the cell above its extracellular concentration. When the H+^+ gradient is dissipated, Pi uptake stops even though ATP is present in the cytosol. Which mechanism best explains Pi uptake?

  1. Simple diffusion of phosphate through the bilayer down its concentration gradient
  2. Facilitated diffusion of phosphate through a channel down its gradient
  3. Primary active transport of phosphate by direct ATP hydrolysis at cotransporter
  4. Secondary active transport driven by H+^+ moving down its gradient (correct answer)
  5. Endocytosis internalizing phosphate in vesicles that then fuse with cytosol

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. The correct answer is secondary active transport driven by H+ moving down its gradient because Pi accumulates against its gradient, powered by H+ influx through the cotransporter, not direct ATP. Uptake stops when H+ gradient dissipates, despite ATP presence, confirming secondary dependence on the ion gradient. The cotransporter requires both ions for movement. A tempting distractor is primary active transport by ATP hydrolysis, but this is wrong due to the misconception that all uphill transport directly uses ATP; H+ dependence indicates secondary. To analyze similar problems, always determine if movement is down a gradient (passive) or against (active) and check for energy dependence.

Question 13

A cell in an isotonic solution shows no net change in volume. When a drug blocks aquaporins, water movement across the membrane slows, but the final cell volume remains unchanged. ATP depletion does not alter the direction of water movement. Which mechanism best explains water movement across the membrane in both conditions?

  1. Osmosis, a passive process that can occur through bilayer or aquaporins (correct answer)
  2. Primary active transport pumping water to maintain equal cell volume
  3. Secondary active transport coupling water movement to Na+^+ transport
  4. Facilitated diffusion of solutes that pulls water against its gradient
  5. Endocytosis of extracellular water to balance osmotic conditions

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. The correct answer is osmosis, a passive process that can occur through bilayer or aquaporins because water moves down its gradient to maintain equilibrium in isotonic conditions, with no net volume change. Aquaporin blocking slows but doesn't alter final volume, and ATP depletion doesn't change direction, confirming passivity. The process balances osmotic pressures without energy. A tempting distractor is primary active transport pumping water, but this is wrong due to the misconception that equilibrium requires active intervention; osmosis is passive. To analyze similar problems, always determine if movement is down a gradient (passive) or against (active) and check for energy dependence.

Question 14

A membrane contains a cotransporter that moves Na+^+ and an amino acid into the cell in the same direction. Amino acid uptake occurs even when its intracellular concentration is higher than extracellular. Uptake stops if the Na+^+ gradient is eliminated, even though ATP is still present. ATP is not used by the cotransporter itself, but another membrane protein uses ATP to maintain low intracellular Na+^+. Which process best explains amino acid uptake?

  1. Secondary active transport using the Na+^+ electrochemical gradient to drive amino acid uptake (correct answer)
  2. Facilitated diffusion of amino acids down their concentration gradient
  3. Primary active transport of amino acids by direct ATP hydrolysis at the cotransporter
  4. Simple diffusion of amino acids through the phospholipid bilayer
  5. Osmosis of amino acids through aquaporin channels

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. Secondary active transport is correct because the cotransporter uses the Na+ electrochemical gradient (maintained by ATP elsewhere) to drive amino acid uptake against its gradient, without directly using ATP. Uptake stops when the Na+ gradient is eliminated, showing dependence on this stored energy rather than direct hydrolysis. Amino acid accumulation above extracellular levels confirms active transport powered indirectly. A tempting distractor is primary active transport, incorrect due to the misconception that all uphill transport directly uses ATP, overlooking coupled ion gradients. To analyze transport, always evaluate if movement is down a gradient (passive) or against it (active) and check for energy requirements.

Question 15

An ion pump in the plasma membrane exports Ca2+^{2+} from the cytosol to the extracellular space. Cytosolic Ca2+^{2+} is kept lower than extracellular Ca2+^{2+}. When ATP is removed, Ca2+^{2+} export stops and cytosolic Ca2+^{2+} rises. The pump protein binds ATP and Ca2+^{2+} during transport. Which mechanism best explains Ca2+^{2+} export?

  1. Simple diffusion of Ca2+^{2+} through the lipid bilayer down gradient
  2. Facilitated diffusion through a Ca2+^{2+} channel down electrochemical gradient
  3. Primary active transport of Ca2+^{2+} using ATP hydrolysis by a pump (correct answer)
  4. Secondary active transport driven by Ca2+^{2+} moving down its gradient
  5. Exocytosis releasing Ca2+^{2+} stored in vesicles to the outside medium

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. The correct answer is primary active transport of Ca2+ using ATP hydrolysis by a pump because Ca2+ is exported against its gradient from low cytosolic to high extracellular concentration, directly powered by ATP binding and hydrolysis. ATP removal stops export and causes cytosolic Ca2+ rise, confirming direct energy dependence. The pump's binding of ATP and Ca2+ during transport indicates it's a primary active mechanism maintaining low internal Ca2+. A tempting distractor is facilitated diffusion through a channel, but this is wrong due to the misconception that against-gradient movement can be passive; active transport is required for uphill movement. To analyze similar problems, always determine if movement is down a gradient (passive) or against (active) and check for energy dependence.

Question 16

A cell is exposed to a lipid-soluble steroid at higher concentration outside than inside. The steroid appears in the cytosol rapidly, and uptake is unaffected by inhibitors of ATP production and by blocking membrane proteins. No vesicles are observed. Which mechanism best explains steroid entry into the cell?

  1. Simple diffusion through the phospholipid bilayer down concentration gradient (correct answer)
  2. Facilitated diffusion through a steroid-specific carrier protein requiring binding
  3. Primary active transport moving steroid inward against gradient using ATP
  4. Secondary active transport coupled to Na+^+ influx providing energy
  5. Endocytosis internalizing steroid in vesicles formed from plasma membrane

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. The correct answer is simple diffusion through the phospholipid bilayer down concentration gradient because the lipid-soluble steroid crosses passively without proteins or energy, equilibrating from high external to low internal concentration. Unaffected by ATP inhibitors or protein blockers, and no vesicles, confirms direct bilayer passage due to its nonpolar nature. Rapid cytosolic appearance supports simple diffusion. A tempting distractor is facilitated diffusion through a carrier, but this is wrong due to the misconception that all small molecules need proteins; lipid-soluble ones diffuse simply. To analyze similar problems, always determine if movement is down a gradient (passive) or against (active) and check for energy dependence.

Question 17

Cells are placed in a solution where the extracellular solute concentration is higher than the cytosol. Over time, cell volume decreases. Blocking aquaporins slows the rate of volume change but does not prevent it. No ATP is required for the volume change, and the direction reverses when cells are moved to a more dilute solution. Which mechanism best explains the change in cell volume?

  1. Osmosis causing net water movement out of cells down a water potential gradient (correct answer)
  2. Primary active transport pumping water out of the cell using ATP
  3. Facilitated diffusion moving solute out of the cell through carriers
  4. Secondary active transport exporting water coupled to Na+^+ export
  5. Exocytosis removing cytosolic water by vesicle fusion with the membrane

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. Osmosis is correct because water moves passively out down its potential gradient due to higher extracellular solute, causing shrinkage without ATP. Direction reversal in dilute solution confirms gradient dependence. Aquaporin blocking slows but doesn't prevent it, showing bilayer diffusion. A tempting distractor is primary active transport, wrong due to the misconception that volume changes require energy, ignoring passive water flow. To analyze transport, always evaluate if movement is down a gradient (passive) or against it (active) and check for energy requirements.

Question 18

A transporter exchanges one Ca2+^{2+} from the cytosol to the extracellular space while moving three Na+^+ into the cytosol. Ca2+^{2+} export continues when cytosolic Ca2+^{2+} is already lower than extracellular Ca2+^{2+}, but it stops if the Na+^+ gradient is eliminated. The exchanger does not hydrolyze ATP, but an ATP-dependent Na+$/K^+$/K^+pumpmaintainstheNa pump maintains the Na^+gradient.WhichprocessbestexplainsCa gradient. Which process best explains Ca^{2+}$ export?

  1. Secondary active transport using the Na+^+ gradient to drive Ca2+^{2+} export (correct answer)
  2. Facilitated diffusion of Ca2+^{2+} down its concentration gradient through a carrier
  3. Primary active transport of Ca2+^{2+} by direct ATP hydrolysis in the exchanger
  4. Simple diffusion of Ca2+^{2+} through the lipid bilayer
  5. Osmosis of Ca2+^{2+} with water through aquaporins

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. Secondary active transport is correct because the exchanger uses the Na+ gradient (from Na+/K+ pump) to drive Ca2+ out against its gradient, without direct ATP. Stopping without Na+ gradient confirms indirect energy. Stoichiometry supports coupling. A tempting distractor is primary active transport, incorrect due to the misconception that all exchangers hydrolyze ATP, overlooking gradient powering. To analyze transport, always evaluate if movement is down a gradient (passive) or against it (active) and check for energy requirements.

Question 19

In a model membrane system, adding cholesterol decreases permeability to small polar molecules but has little effect on permeability to a small nonpolar molecule. When the nonpolar molecule is added at higher concentration outside, it still equilibrates across the membrane without proteins or ATP. Which mechanism best explains the nonpolar molecule's movement?

  1. Simple diffusion through the lipid bilayer down the concentration gradient (correct answer)
  2. Facilitated diffusion requiring a specific carrier protein for nonpolar solute
  3. Primary active transport using ATP to move the nonpolar solute into membrane
  4. Secondary active transport coupled to Na+^+ influx providing driving force
  5. Endocytosis internalizing the nonpolar molecule within membrane vesicles

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. The correct answer is simple diffusion through the lipid bilayer down the concentration gradient because the small nonpolar molecule crosses passively without proteins or ATP, equilibrating concentrations despite membrane changes. Cholesterol affects polar permeability but not nonpolar, supporting direct bilayer passage. No vesicles or energy involvement confirms simplicity. A tempting distractor is facilitated diffusion requiring a carrier, but this is incorrect due to the misconception that nonpolar molecules need proteins; they diffuse simply through lipids. To analyze similar problems, always determine if movement is down a gradient (passive) or against (active) and check for energy dependence.

Question 20

A membrane antiporter exchanges Na+^+ moving into the cell with H+^+ moving out. In the experiment, Na+^+ is high outside and low inside; H+^+ becomes lower inside than outside after transport proceeds. The antiporter stops when the Na+^+ gradient is eliminated, even though ATP is still present. Which mechanism best explains H+^+ export in this system?

  1. Secondary active transport driven by Na+^+ moving down its gradient (correct answer)
  2. Primary active transport exporting H+^+ by directly hydrolyzing ATP
  3. Facilitated diffusion exporting H+^+ down its gradient through a channel
  4. Simple diffusion of H+^+ through the lipid bilayer without proteins
  5. Exocytosis releasing H+^+ by vesicle fusion with the plasma membrane

Explanation: This question assesses the skill of analyzing membrane transport mechanisms. The correct answer is secondary active transport driven by Na+ moving down its gradient because H+ is exported against its gradient, powered by Na+ influx through the antiporter, not direct ATP use. Stopping when Na+ gradient is eliminated, despite ATP presence, confirms dependence on the ion gradient for energy. The antiporter exchanges ions, lowering internal H+ below external levels. A tempting distractor is primary active transport exporting H+ by hydrolyzing ATP, but this is incorrect due to the misconception that all uphill export is primary; Na+ dependence indicates secondary. To analyze similar problems, always determine if movement is down a gradient (passive) or against (active) and check for energy dependence.