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

A cell is placed in a solution where the extracellular glucose concentration is 10 mM and intracellular glucose is 2 mM. The plasma membrane is impermeable to glucose unless a specific carrier protein is present. When the carrier is expressed, glucose enters the cell rapidly; adding an ATP synthesis inhibitor does not change the rate. No vesicles form, and the carrier shows saturation at high extracellular glucose. Which mechanism best explains glucose entry under these conditions?

Simple diffusion through the phospholipid bilayer down the gradient
Facilitated diffusion through a carrier down the concentration gradient
Primary active transport pumping glucose into the cell using ATP
Secondary active transport coupling glucose uptake to ion movement uphill
Endocytosis of extracellular fluid containing dissolved glucose molecules
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AP Biology Quiz

AP Biology Quiz: Membrane Transport

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

What this quiz covers

This quiz focuses on Membrane Transport, 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.

All questions

Question 1

A cell is placed in a solution where the extracellular glucose concentration is 10 mM and intracellular glucose is 2 mM. The plasma membrane is impermeable to glucose unless a specific carrier protein is present. When the carrier is expressed, glucose enters the cell rapidly; adding an ATP synthesis inhibitor does not change the rate. No vesicles form, and the carrier shows saturation at high extracellular glucose. Which mechanism best explains glucose entry under these conditions?

  1. Simple diffusion through the phospholipid bilayer down the gradient
  2. Facilitated diffusion through a carrier down the concentration gradient (correct answer)
  3. Primary active transport pumping glucose into the cell using ATP
  4. Secondary active transport coupling glucose uptake to ion movement uphill
  5. Endocytosis of extracellular fluid containing dissolved glucose molecules
Explanation: This question tests your ability to identify membrane transport mechanisms based on experimental evidence. The glucose moves from high concentration (10 mM) to low concentration (2 mM), which is down its concentration gradient, and the process doesn't require ATP (as shown by the ATP synthesis inhibitor having no effect). Since glucose cannot cross the lipid bilayer directly and requires a specific carrier protein that shows saturation kinetics, this indicates facilitated diffusion through a carrier protein. A common misconception is choosing primary active transport (C) because students often assume that any carrier protein must use ATP, but facilitated diffusion carriers simply provide a pathway for molecules to move down their gradient without energy input. To identify transport mechanisms, check three key factors: direction relative to gradient, ATP requirement, and whether specific proteins are needed.

Question 2

A bacterial cell imports lactose using a membrane symporter that requires a preexisting H+ gradient: extracellular pH is 6.0 and cytosolic pH is 7.5. When a chemical collapses the H+ gradient without affecting ATP levels, lactose uptake stops. Lactose is more concentrated inside than outside during uptake. Which mechanism best explains lactose entry under normal conditions?

  1. Simple diffusion of lactose through the lipid bilayer down its gradient
  2. Facilitated diffusion of lactose through a carrier down its gradient
  3. Secondary active transport coupling lactose import to H+ moving down gradient (correct answer)
  4. Primary active transport of lactose using ATP binding and hydrolysis
  5. Exocytosis exporting H+ to allow lactose to diffuse inward freely
Explanation: This question tests understanding of secondary active transport mechanisms. The lactose symporter uses the pre-existing H+ gradient (lower pH outside means higher H+ concentration outside) to power lactose uptake against its concentration gradient (lactose is more concentrated inside). When the H+ gradient is collapsed, transport stops even though ATP is available, proving the transporter doesn't use ATP directly but rather harnesses the H+ gradient energy. This coupling of lactose uphill movement to H+ downhill movement defines secondary active transport via symport. Students might choose primary active transport (D) because lactose moves uphill, but the dependence on H+ gradient rather than ATP directly distinguishes secondary from primary transport. To identify secondary transport, look for uphill movement of one substance coupled to downhill movement of another.

Question 3

In an epithelial cell, the cytosolic Na+ concentration is 15 mM and extracellular Na+ is 145 mM. A membrane protein exports 3 Na+ from the cytosol to the extracellular fluid each cycle, and transport stops immediately when ATP is removed. The protein continues exporting Na+ even though Na+ is already higher outside than inside. Which mechanism best explains this Na+ movement?

  1. Facilitated diffusion of Na+ through a channel down its concentration gradient
  2. Primary active transport of Na+ against its gradient using ATP directly (correct answer)
  3. Osmosis of Na+ through aquaporins driven by water potential differences
  4. Simple diffusion of Na+ through the lipid bilayer down its gradient
  5. Endocytosis of extracellular Na+ followed by vesicle fusion at the membrane
Explanation: This question assesses the skill of analyzing membrane transport mechanisms by evaluating how substances move across cell membranes based on gradients and energy requirements. The correct answer is primary active transport because Na+ is pumped against its gradient from 15 mM inside to 145 mM outside, directly using ATP as shown by transport stopping without it. The continued export despite higher external Na+ confirms movement against the gradient, requiring energy input from ATP hydrolysis. The protein's cycle of exporting 3 Na+ per ATP aligns with primary active transport energetics. A tempting distractor is facilitated diffusion through a channel, which is wrong due to the misconception that all ion movements are passive, ignoring ATP dependence for uphill transport. To distinguish transport types, always check if movement is down or against the gradient and if energy like ATP is directly required.

Question 4

An epithelial cell imports glucose from the extracellular fluid even when extracellular glucose is 1 mM and cytosolic glucose is 10 mM. The transporter requires extracellular Na+; removing Na+ stops glucose uptake. ATP depletion stops uptake over time, and a separate Na+/K+ ATPase is present in the same membrane. No direct ATP binding to the glucose transporter is detected. Which mechanism best explains glucose uptake?

  1. Simple diffusion of glucose through the lipid bilayer from low to high concentration
  2. Facilitated diffusion of glucose down its gradient through a carrier protein
  3. Primary active transport of glucose by ATP hydrolysis at the glucose transporter
  4. Secondary active transport: Na+ moving down its gradient drives glucose uptake (correct answer)
  5. Osmosis of glucose through aquaporins driven by solute concentration differences
Explanation: This question tests the skill of analyzing membrane transport mechanisms. The correct answer D is secondary active transport because glucose is imported against its gradient (from 1 mM external to 10 mM cytosolic), powered by Na+ moving down its gradient, as uptake requires external Na+ and stops without it. The Na+/K+ ATPase indirectly provides energy by maintaining the Na+ gradient via ATP hydrolysis, explaining delayed stoppage upon ATP depletion. No direct ATP binding to the glucose transporter confirms secondary coupling. A tempting distractor is C, primary active transport, but this is incorrect due to no direct ATP interaction with the transporter, stemming from the misconception that all uphill transport directly uses ATP. A transferable strategy is to identify coupled ions and indirect energy sources when movement opposes gradients.

Question 5

A plant cell's vacuolar membrane contains an H+ pump that moves H+ from cytosol (pH 7.2) into the vacuole (pH 5.5). When ATP is supplied, vacuolar pH decreases further; when ATP is removed, the pH gradient dissipates over time through existing leak pathways. Adding an inhibitor of ATP hydrolysis stops further acidification immediately. Which transport mechanism is directly responsible for creating the H+ gradient?

  1. Simple diffusion of H+ through the lipid bilayer down its electrochemical gradient
  2. Facilitated diffusion of H+ through channels from vacuole to cytosol without ATP
  3. Primary active transport using ATP hydrolysis to pump H+ into the vacuole (correct answer)
  4. Secondary active transport using H+ movement against its gradient to power ATP synthesis
  5. Osmosis of H+ driven by water movement into the vacuole through aquaporins
Explanation: This question tests the skill of analyzing membrane transport mechanisms. The correct answer C is primary active transport because H+ is pumped from higher pH cytosol (7.2, lower H+) to lower pH vacuole (5.5, higher H+), against its gradient, directly using ATP hydrolysis as transport stops without ATP or with hydrolysis inhibitors. Energy from ATP powers the pump to acidify the vacuole further. Gradient dissipation without ATP confirms active maintenance. A tempting distractor is D, secondary active transport, but this is incorrect because no coupled ion drives H+ and ATP is directly required, stemming from the misconception that all proton pumps are indirectly powered. A transferable strategy is to test direct ATP effects and gradient opposition to identify primary active transport.

Question 6

A membrane protein transports Na+ from the cytosol (10 mM) to the extracellular fluid (145 mM). When ATP is present, Na+ efflux occurs; when ATP is removed, Na+ efflux stops immediately. A nonhydrolyzable ATP analog binds the protein but does not support transport. The membrane remains intact, and Na+ channels are blocked to prevent diffusion. Which mechanism best explains Na+ movement across the membrane?

  1. Facilitated diffusion of Na+ through a carrier down its concentration gradient
  2. Primary active transport using ATP hydrolysis to move Na+ against its gradient (correct answer)
  3. Osmosis of Na+ driven by water potential differences across the membrane
  4. Secondary active transport powered by Na+ moving into the cell down its gradient
  5. Simple diffusion of Na+ through the lipid bilayer due to its small size
Explanation: This question tests the skill of analyzing membrane transport mechanisms. The correct answer B is primary active transport because Na+ is pumped from low cytosolic concentration (10 mM) to high extracellular concentration (145 mM), against its gradient, directly powered by ATP hydrolysis as transport stops without ATP and nonhydrolyzable analogs fail. Energy from ATP enables the protein to undergo conformational changes for uphill transport. Blocked channels ensure no passive diffusion interferes, confirming active pumping. A tempting distractor is D, secondary active transport, but this is incorrect because no coupled ion is mentioned and transport requires direct ATP use, arising from the misconception that all anti-gradient transport is indirectly powered. A transferable strategy is to test for direct ATP dependence and gradient direction to distinguish primary from secondary active transport.

Question 7

A cell has intracellular K+ of 140 mM and extracellular K+ of 5 mM. A K+-selective channel is opened, and ATP synthesis is inhibited. Immediately after the channel opens, K+ moves out of the cell, and the rate depends on the concentration difference and channel number. No carrier saturation is observed over the tested range. Which mechanism best explains K+ movement?

  1. Simple diffusion of K+ through the phospholipid bilayer down its gradient
  2. Facilitated diffusion of K+ through an ion channel down its concentration gradient (correct answer)
  3. Primary active transport of K+ requiring ATP to move K+ out of the cell
  4. Secondary active transport of K+ coupled to glucose moving down its gradient
  5. Endocytosis of extracellular K+ into vesicles followed by release to cytosol
Explanation: This question tests the skill of analyzing membrane transport mechanisms. The correct answer B is facilitated diffusion because K+ moves from high intracellular concentration (140 mM) to low extracellular (5 mM), down its gradient, without energy as ATP inhibition does not affect it. The selective ion channel allows passive flow proportional to the concentration difference and channel density. Lack of saturation distinguishes channels from carriers in this passive process. A tempting distractor is C, primary active transport, but this is wrong because no ATP is required and movement is downhill, based on the misconception that all ion movements involve energy. A transferable strategy is to evaluate energy needs and saturation kinetics to differentiate channels from carriers and passive from active transport.

Question 8

In an experiment, a membrane vesicle contains 100 mM sucrose and is placed in 100 mM sucrose solution. The membrane is impermeable to sucrose but permeable to water. Then 50 mM NaCl is added only to the external solution; Na+ and Cl− cannot cross the membrane. The vesicle's volume decreases over several minutes, and ATP is absent. Which mechanism best explains the volume change?

  1. Water leaves the vesicle by osmosis because external solute concentration is higher (correct answer)
  2. Sucrose leaves the vesicle by facilitated diffusion, lowering internal osmolarity
  3. NaCl enters the vesicle by simple diffusion, increasing internal osmolarity
  4. Water is pumped out by primary active transport using ATP hydrolysis
  5. The vesicle loses volume by exocytosis of water-containing internal vesicles
Explanation: This question tests the skill of analyzing membrane transport mechanisms. The correct answer A is osmosis because water leaves the vesicle as the external solution becomes hypertonic after adding 50 mM NaCl (increasing osmolarity), creating a water potential gradient outward, with no energy needed since ATP is absent. The membrane's permeability to water but not solutes drives shrinkage to equalize potentials. Initial equal sucrose concentrations ensure the change is due to added impermeable NaCl. A tempting distractor is B, sucrose leaving, but this is incorrect because the membrane is impermeable to sucrose, based on the misconception that solutes move osmotically like water. A transferable strategy is to calculate effective osmolarities and predict water flow direction in response to impermeable solute differences.

Question 9

A membrane carrier transports amino acid Y. Outside concentration is 5 mM and inside is 5 mM at the start. When outside is increased to 50 mM, Y enters the cell, and the rate rises then plateaus as outside concentration continues to increase. ATP depletion does not change the initial influx rate, but a competitive inhibitor reduces influx at all concentrations. Which mechanism best explains Y entry?

  1. Facilitated diffusion through a carrier moving Y down its concentration gradient with saturation (correct answer)
  2. Simple diffusion through the lipid bilayer with no protein involvement or saturation
  3. Primary active transport requiring ATP hydrolysis to import Y regardless of gradients
  4. Secondary active transport using Y export to drive Na+ import into the cell
  5. Osmosis of Y through aquaporins driven by differences in water potential
Explanation: This question tests the skill of analyzing membrane transport mechanisms. The correct answer A is facilitated diffusion because amino acid Y moves from high external (50 mM) to equal or lower internal concentration, down its gradient, without energy as ATP depletion does not affect influx. The carrier protein binds Y, showing saturation as rate plateaus at high concentrations, and competitive inhibition confirms protein mediation. Equal starting concentrations ensure gradient-driven entry upon increase. A tempting distractor is C, primary active transport, but this is wrong because no ATP is required, arising from the misconception that saturation always indicates active processes. A transferable strategy is to analyze kinetics like saturation and energy needs to distinguish facilitated diffusion from active transport.

Question 10

A cell maintains Ca2+ at 0.0001 mM in the cytosol and 1.0 mM outside. A membrane pump moves Ca2+ from cytosol to extracellular fluid. When ATP is depleted, Ca2+ export rapidly decreases, and cytosolic Ca2+ rises. The lipid bilayer is not permeable to Ca2+. Which mechanism best explains Ca2+ export under normal conditions?

  1. Facilitated diffusion of Ca2+ down its concentration gradient
  2. Simple diffusion of Ca2+ through the phospholipid bilayer
  3. Primary active transport of Ca2+ using ATP hydrolysis (correct answer)
  4. Osmosis-driven Ca2+ movement through aquaporins
  5. Secondary active transport of Ca2+ coupled to ATP synthesis
Explanation: This question tests understanding of active transport against concentration gradients. The Ca2+ gradient shows much higher concentration outside (1.0 mM) than inside (0.0001 mM), yet the pump moves Ca2+ from inside to outside—against its concentration gradient. Moving ions against their gradient requires energy input, and the rapid decrease in Ca2+ export when ATP is depleted confirms this is primary active transport using ATP hydrolysis. Facilitated diffusion (A) is incorrect because it would move Ca2+ into the cell down its gradient, not out against it. The strategy is to identify when transport goes against the concentration gradient—this always requires energy input.

Question 11

A membrane separates two solutions: side 1 contains 0.6 M sucrose and side 2 contains 0.2 M sucrose. The membrane is permeable to water but not to sucrose, and no ATP is involved. Over time, the volume on side 1 increases while side 2 decreases. Which direction of net water movement and transport type best explain the change?

  1. Water moves from side 2 to side 1 by osmosis through the membrane (correct answer)
  2. Water moves from side 1 to side 2 by active transport using ATP
  3. Sucrose moves from side 1 to side 2 by facilitated diffusion through channels
  4. Sucrose moves from side 2 to side 1 by simple diffusion through the bilayer
  5. Water moves from side 1 to side 2 by endocytosis of extracellular fluid
Explanation: This question assesses the skill of analyzing membrane transport mechanisms by evaluating how substances move across cell membranes based on gradients and energy requirements. The correct answer is osmosis from side 2 to side 1 because water moves down its potential gradient toward higher solute concentration (0.6 M vs. 0.2 M), increasing side 1 volume without ATP. The membrane's impermeability to sucrose establishes the osmotic gradient, driving passive water flow to dilute the concentrated side. No energy involvement confirms passive transport driven by concentration differences. A tempting distractor is water moving by active transport, which is wrong due to the misconception that volume changes need energy, ignoring passive osmosis across semipermeable membranes. To distinguish transport types, always check if movement is down or against the gradient and if energy like ATP is directly required.

Question 12

A cell is placed in a solution containing 200 mM glucose outside and 5 mM glucose inside. The membrane is impermeable to glucose unless a specific carrier protein is present. When the carrier is expressed, glucose enters the cell rapidly, and uptake stops if the carrier is inhibited. Cellular ATP levels do not change during uptake, and glucose continues to move until concentrations equalize across the membrane. Which mechanism best explains glucose entry under these conditions?

  1. Simple diffusion directly through the phospholipid bilayer down its gradient
  2. Active transport by a pump moving glucose against its gradient using ATP
  3. Endocytosis of extracellular fluid containing glucose into intracellular vesicles
  4. Facilitated diffusion through a carrier protein down the glucose gradient (correct answer)
  5. Osmosis of glucose through aquaporins driven by water potential differences
Explanation: This question assesses the skill of analyzing membrane transport mechanisms by evaluating how substances move across cell membranes based on gradients and energy requirements. The correct answer is facilitated diffusion through a carrier protein because glucose moves down its concentration gradient from 200 mM outside to 5 mM inside, requiring no energy input as evidenced by unchanged ATP levels. The presence of a specific carrier protein indicates that the impermeable membrane needs facilitation for glucose passage, and movement continues until equilibrium, consistent with passive transport. No ATP involvement rules out active transport, aligning with the energy logic of passive diffusion down a gradient. A tempting distractor is active transport by a pump, which is wrong due to the misconception that all carrier-mediated transport requires energy, ignoring that facilitated diffusion is passive. To distinguish transport types, always check if movement is down or against the gradient and if energy like ATP is directly required.

Question 13

A membrane is permeable to urea only when a urea channel is open. Urea is 20 mM outside the cell and 2 mM inside. When the channel opens, urea enters until the concentrations become equal, and the movement is unchanged by ATP depletion. Which mechanism best explains urea entry when the channel is open?

  1. Primary active transport of urea into the cell using ATP to move uphill
  2. Facilitated diffusion of urea through a channel down its concentration gradient (correct answer)
  3. Osmosis of urea through aquaporins driven by water potential differences
  4. Simple diffusion of urea through the lipid bilayer independent of proteins
  5. Endocytosis of urea in vesicles formed from the plasma membrane
Explanation: This question assesses the skill of analyzing membrane transport mechanisms by evaluating how substances move across cell membranes based on gradients and energy requirements. The correct answer is facilitated diffusion through a channel because urea moves down its gradient from 20 mM outside to 2 mM inside until equilibrium, unchanged by ATP depletion, indicating passive transport. The channel's requirement for permeability shows protein facilitation, but no energy is needed for downhill movement. Equalization of concentrations confirms gradient-driven passive flow. A tempting distractor is primary active transport, which is wrong due to the misconception that channel opening implies energy use, overlooking passive diffusion down gradients. To distinguish transport types, always check if movement is down or against the gradient and if energy like ATP is directly required.

Question 14

A membrane protein binds lactose on the extracellular side and releases it into the cytosol. Lactose is 1 mM outside and 10 mM inside, yet lactose still accumulates inside. Transport stops when ATP is removed, and lactose accumulation continues even when the concentration difference increases. Which mechanism best explains lactose uptake?

  1. Facilitated diffusion of lactose down its gradient through a carrier protein
  2. Simple diffusion of lactose through the phospholipid bilayer down its gradient
  3. Primary active transport importing lactose against its gradient using ATP directly (correct answer)
  4. Osmosis of lactose through aquaporins driven by differences in water potential
  5. Endocytosis of lactose followed by vesicle transport to the cytosol
Explanation: This question assesses the skill of analyzing membrane transport mechanisms by evaluating how substances move across cell membranes based on gradients and energy requirements. The correct answer is primary active transport because lactose accumulates against its gradient from 1 mM outside to 10 mM inside, stopping without ATP, indicating direct energy use for uphill transport. Continued uptake despite increasing internal concentration requires ATP hydrolysis to oppose the gradient. The protein's binding and release mechanism with ATP dependence fits primary active energetics. A tempting distractor is facilitated diffusion, which is wrong due to the misconception that carrier proteins are always passive, overlooking ATP needs for against-gradient movement. To distinguish transport types, always check if movement is down or against the gradient and if energy like ATP is directly required.

Question 15

A cell's cytosol contains 1 mM Ca2+, while the extracellular fluid contains 2 mM Ca2+. A specific Ca2+ transporter moves Ca2+ from the cytosol to the extracellular space, and transport stops when ATP is absent. Ca2+ continues to be exported even though cytosolic Ca2+ is lower than extracellular Ca2+. Which mechanism best explains Ca2+ export?

  1. Facilitated diffusion of Ca2+ outward through a channel down its gradient
  2. Simple diffusion of Ca2+ outward directly through the lipid bilayer
  3. Primary active transport exporting Ca2+ against its gradient using ATP (correct answer)
  4. Osmosis of Ca2+ outward through aquaporins driven by water movement
  5. Exocytosis of Ca2+ from vesicles that fuse with the plasma membrane
Explanation: This question assesses the skill of analyzing membrane transport mechanisms by evaluating how substances move across cell membranes based on gradients and energy requirements. The correct answer is primary active transport because Ca2+ is exported against its gradient from 1 mM inside to 2 mM outside, stopping without ATP, indicating direct energy use. Continued export despite the gradient requires ATP hydrolysis to drive uphill movement. The transporter's specificity and ATP dependence fit primary active transport logic. A tempting distractor is facilitated diffusion outward, which is wrong due to the misconception that ions always follow gradients passively, ignoring ATP needs for against-gradient transport. To distinguish transport types, always check if movement is down or against the gradient and if energy like ATP is directly required.

Question 16

A cell has 2 mM K+ outside and 140 mM K+ inside. When a selective K+ channel opens, K+ leaves the cell, and the net movement slows as the concentration difference decreases. Blocking ATP production does not change the initial rate of K+ movement through the open channel. Which mechanism best explains K+ movement when the channel is open?

  1. Primary active transport moving K+ out using ATP to oppose its gradient
  2. Facilitated diffusion of K+ through a channel down its concentration gradient (correct answer)
  3. Simple diffusion of K+ through the phospholipid bilayer down its gradient
  4. Secondary active transport symporting K+ out coupled to glucose influx
  5. Exocytosis of K+-containing vesicles to release K+ outside the cell
Explanation: This question assesses the skill of analyzing membrane transport mechanisms by evaluating how substances move across cell membranes based on gradients and energy requirements. The correct answer is facilitated diffusion through a channel because K+ moves down its gradient from 140 mM inside to 2 mM outside, with no ATP effect indicating passive transport. The selective channel enables rapid efflux, and slowing as gradients equalize follows passive diffusion logic without energy input. ATP blocking not affecting the rate confirms no direct energy requirement for this downhill movement. A tempting distractor is primary active transport, which is wrong due to the misconception that all ion pumps require ATP, overlooking passive channel-mediated flow down gradients. To distinguish transport types, always check if movement is down or against the gradient and if energy like ATP is directly required.

Question 17

A nonpolar gas (O2) is at higher concentration outside a cell than inside. The cell membrane contains no specific O2 transport proteins. When extracellular O2 is increased, intracellular O2 rises proportionally, and the process is unaffected by ATP depletion. Which mechanism best explains O2 entry into the cell?

  1. Simple diffusion of O2 through the phospholipid bilayer down its gradient (correct answer)
  2. Facilitated diffusion of O2 through a carrier protein that binds O2
  3. Primary active transport pumping O2 into the cell using ATP hydrolysis
  4. Osmosis of O2 through aquaporins driven by water potential differences
  5. Endocytosis of O2-containing fluid followed by vesicle release into cytosol
Explanation: This question assesses the skill of analyzing membrane transport mechanisms by evaluating how substances move across cell membranes based on gradients and energy requirements. The correct answer is simple diffusion through the bilayer because nonpolar O2 moves down its gradient without proteins, unaffected by ATP, indicating passive transport. The proportional rise in intracellular O2 with extracellular increase follows diffusion kinetics driven by concentration differences. No specific transporters align with direct bilayer passage for small nonpolar molecules. A tempting distractor is facilitated diffusion through a carrier, which is wrong due to the misconception that all gases need proteins, overlooking simple diffusion for nonpolar substances. To distinguish transport types, always check if movement is down or against the gradient and if energy like ATP is directly required.

Question 18

A cell is placed in a solution with equal solute concentration inside and outside (isotonic). The membrane contains aquaporins and is impermeable to the solute. Over time, the cell's volume remains constant, and ATP depletion has no effect on water movement rates. Which statement best describes net water movement across the membrane?

  1. Net water movement is into the cell by osmosis because solute cannot cross
  2. Net water movement is out of the cell by active transport requiring ATP
  3. Net water movement is zero because osmosis occurs equally in both directions (correct answer)
  4. Net water movement is into the cell by facilitated diffusion of water carriers
  5. Net water movement is zero because water cannot cross without solute transport
Explanation: This question assesses the skill of analyzing membrane transport mechanisms by evaluating how substances move across cell membranes based on gradients and energy requirements. The correct answer is net zero water movement because in isotonic conditions, equal osmolarities create no water potential gradient, resulting in balanced osmosis without net flow or volume change. Aquaporins allow bidirectional water passage, but no gradient means equal rates in both directions, unaffected by ATP. Impermeability to solute maintains the balance without energy input. A tempting distractor is net water into the cell by osmosis, which is wrong due to the misconception that solute impermeability always drives influx, ignoring isotonic equilibrium. To distinguish transport types, always check if movement is down or against the gradient and if energy like ATP is directly required.

Question 19

A neuron's membrane has a K+ channel that opens, allowing K+ to move. At the moment the channel opens, K+ concentration is 140 mM inside and 5 mM outside. ATP depletion does not change the immediate direction of K+ movement through the open channel. Which outcome is most likely when the channel first opens?

  1. Net K+ moves out of the cell by facilitated diffusion (correct answer)
  2. Net K+ moves into the cell by facilitated diffusion
  3. Net K+ moves out of the cell by primary active transport
  4. Net K+ moves into the cell by secondary active transport
  5. No net K+ movement occurs because channels require ATP
Explanation: This question tests understanding of ion movement through channels. The K+ gradient shows much higher concentration inside (140 mM) than outside (5 mM), creating a driving force for K+ to move out of the cell. When the channel opens, K+ will move down its concentration gradient from high to low concentration, and this passive movement doesn't require ATP. Answer B (K+ moves in) is incorrect because it goes against the concentration gradient—K+ would need active transport to move from 5 mM to 140 mM. Remember that ions always move down their concentration gradient through open channels unless actively transported.

Question 20

A cell expresses a membrane antiporter that exchanges Ca2+ out of the cell while Na+ moves into the cell. Intracellular Na+ is 12 mM and extracellular Na+ is 140 mM; intracellular Ca2+ is 0.0001 mM and extracellular Ca2+ is 2 mM. When extracellular Na+ is reduced to 5 mM, Ca2+ export by the antiporter decreases sharply without changing ATP levels. Which mechanism best explains Ca2+ export under normal extracellular Na+?

  1. Primary active transport exporting Ca2+ directly using ATP hydrolysis
  2. Facilitated diffusion exporting Ca2+ down its concentration gradient
  3. Secondary active transport exporting Ca2+ coupled to Na+ moving down gradient (correct answer)
  4. Simple diffusion exporting Ca2+ through the lipid bilayer without proteins
  5. Osmosis exporting Ca2+ because water movement carries ions outward
Explanation: This question tests understanding of secondary active transport via antiport. The antiporter exports Ca2+ against its steep concentration gradient (from 0.0001 mM inside to 2 mM outside) by coupling this uphill movement to Na+ moving down its gradient (from 140 mM outside to 12 mM inside). When external Na+ is reduced below internal levels, the Na+ gradient reverses and Ca2+ export decreases, proving the mechanism depends on the Na+ gradient rather than ATP directly. This coupling of Ca2+ uphill movement to Na+ downhill movement in opposite directions defines secondary active transport via antiport. Students might choose primary active transport (A) because Ca2+ moves uphill, but the dependence on Na+ gradient rather than ATP identifies this as secondary transport. To recognize antiporters, look for coupled movement of substances in opposite directions where one moves uphill using another's downhill gradient.