- 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?
Answer: Net K+ moves out of the cell by facilitated diffusion
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.
- 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?
Answer: Secondary active transport: Na+ moving down its gradient drives glucose uptake
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.
- A plant cell has 300 mM solute inside its cytoplasm and is placed in a 100 mM solute solution. The plasma membrane is permeable to water but not to the solute. Over several minutes, the cell's volume increases. No ATP-dependent transporters for water are involved. Which direction of net water movement is expected across the membrane?
Answer: Net water moves into the cell by osmosis
This question requires analyzing water movement across a selectively permeable membrane. The plant cell has a higher solute concentration inside (300 mM) than outside (100 mM), making the inside hypertonic relative to the outside. Water moves by osmosis from regions of lower solute concentration (hypotonic) to regions of higher solute concentration (hypertonic), so water will move into the cell. The cell volume increase confirms this net inward water movement. Answer C is incorrect because water never requires active transport—it moves passively by osmosis through aquaporins or the membrane. Remember that water always moves toward the higher solute concentration to equalize concentrations on both sides.
- A cell is placed in a solution containing a small nonpolar molecule X at 50 mM; the cytosol initially contains 0 mM X. The membrane has no known transporters for X. When the external concentration is doubled, the initial rate of X entry doubles. ATP depletion has no effect on X entry, and no saturation is observed. Which mechanism best explains X movement into the cell?
Answer: Simple diffusion through the phospholipid bilayer down X's concentration gradient
This question tests the skill of analyzing membrane transport mechanisms. The correct answer A is simple diffusion because the small nonpolar molecule X moves from high external concentration (50 mM) to low cytosolic (0 mM), down its gradient, without proteins or energy as rate doubles with concentration and ATP depletion has no effect. The lipid bilayer allows direct passage proportional to the gradient, with no saturation observed. Absence of transporters supports unassisted diffusion. A tempting distractor is B, facilitated diffusion, but this is wrong because no saturation occurs and rate is linear, misconceiving that all diffusion through membranes requires proteins. A transferable strategy is to check for protein involvement, saturation, and energy independence to identify simple diffusion.
- 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+?
Answer: Secondary active transport exporting Ca2+ coupled to Na+ moving down gradient
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.
- A red blood cell is moved from a 300 mOsm solution to a 150 mOsm solution. The membrane contains aquaporins that allow rapid water movement but does not allow sucrose to cross. After several minutes, the cell's volume increases. No ATP is required for the volume change. Which outcome is most likely and what transport process best explains it?
Answer: Water enters the cell by osmosis through aquaporins down its water potential gradient
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 through aquaporins because water moves down its water potential gradient from the hypotonic 150 mOsm solution to the hypertonic cell interior, causing volume increase without ATP use. The membrane's impermeability to sucrose creates an osmotic gradient, driving passive water influx to equalize osmolarities. No energy is needed, as this is passive transport driven solely by the concentration difference. A tempting distractor is water exiting by active transport, which is wrong due to the misconception that volume changes always require energy, overlooking passive osmosis in hypotonic conditions. To distinguish transport types, always check if movement is down or against the gradient and if energy like ATP is directly required.
- 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?
Answer: Primary active transport of Ca2+ using ATP hydrolysis
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.
- 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?
Answer: Facilitated diffusion of urea through a channel down its concentration gradient
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.
- In an epithelial cell, intracellular glucose is 20 mM while extracellular glucose is 2 mM. A cotransporter moves glucose into the cell only when Na+ is present outside at high concentration (140 mM outside, 10 mM inside). When ATP is depleted, the Na+ gradient dissipates and glucose uptake stops, even though the cotransporter protein remains intact. Which mechanism best explains glucose uptake when ATP is available?
Answer: Secondary active transport of glucose coupled to Na+ influx
This question requires analyzing secondary active transport mechanisms. Glucose is moving against its concentration gradient (2 mM outside to 20 mM inside), which requires energy. The cotransporter uses the Na+ gradient (140 mM outside, 10 mM inside) as its energy source—Na+ moves down its gradient while glucose moves up its gradient. When ATP is depleted, the Na+ gradient dissipates, stopping glucose uptake even though the transporter remains functional. Facilitated diffusion (A) is incorrect because glucose is moving against its gradient, which passive transport cannot accomplish. Remember that secondary active transport couples uphill movement of one substance to downhill movement of another.
- A neuron maintains intracellular Na+ at 15 mM and extracellular Na+ at 145 mM. After adding a drug that blocks the Na+/K+ ATPase, Na+ gradually accumulates inside the cell even though no Na+ channels are opened intentionally. The membrane contains leak channels that allow Na+ to pass. Which mechanism best explains the direction of Na+ movement after the pump is inhibited?
Answer: Facilitated diffusion of Na+ into the cell through leak channels
This question tests understanding of ion movement when active transport is disrupted. The Na+/K+ ATPase normally maintains low intracellular Na+ (15 mM) against the high extracellular concentration (145 mM), but when blocked, Na+ can only move through leak channels. With the pump inhibited, Na+ moves down its concentration gradient from outside (high) to inside (low) through these leak channels by facilitated diffusion, causing the observed accumulation. This passive movement requires no ATP and depends only on the existing concentration gradient and channel availability. A common error is choosing primary active transport (A), but this would move Na+ against its gradient and requires the very ATP-dependent pump that was blocked. To determine ion movement direction, identify the concentration gradient and whether energy-requiring pumps are functional.
- 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?
Answer: Facilitated diffusion of K+ through a channel down its concentration gradient
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.
- 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?
Answer: Water leaves the vesicle by osmosis because external solute concentration is higher
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.
- 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?
Answer: Primary active transport using ATP hydrolysis to move Na+ against its gradient
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.
- A red blood cell is moved from 300 mOsm solution to 150 mOsm solution. The membrane is permeable to water but not to the major solutes in either solution. Aquaporins are present, and cellular ATP levels are unchanged during the first minute. After transfer, the cell's volume increases measurably. Which outcome is most likely during the first minute, and what transport explains it?
Answer: Water enters the cell by osmosis down its water potential gradient through aquaporins
This question tests the skill of analyzing membrane transport mechanisms. The correct answer A is osmosis because water moves into the cell from the hypotonic external solution (150 mOsm) to the hypertonic cytosol (300 mOsm), down the water potential gradient, without energy input as ATP levels remain unchanged. Aquaporins facilitate rapid water passage across the impermeable solute barrier, leading to cell swelling. The membrane's impermeability to solutes ensures water movement drives the volume change, not solute transport. A tempting distractor is B, water exiting the cell, but this is wrong because the external solution is hypotonic, misconceiving the direction of osmotic flow in response to solute gradients. A transferable strategy is to compare osmolarities and determine water movement direction based on water potential differences while considering membrane permeability.
- 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?
Answer: Primary active transport using ATP hydrolysis to pump H+ into the vacuole
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.
- A cell contains 1 mM urea inside and is placed in 10 mM urea outside. Urea crosses the membrane slowly without proteins, but when a specific urea channel is inserted, the rate increases greatly. The rate does not require ATP and increases with the urea concentration difference. Which mechanism best explains urea movement when the channel is present?
Answer: Facilitated diffusion of urea down its concentration gradient
This question analyzes facilitated diffusion of small polar molecules. Urea moves down its concentration gradient (10 mM outside to 1 mM inside) much faster when the channel is present, indicating protein-facilitated transport. The lack of ATP requirement and the dependence on concentration difference confirm this is passive transport through a channel. Simple diffusion (B) is incorrect because while urea can cross slowly without proteins, the question asks about transport when the channel is present, which greatly increases the rate. The key is recognizing that channels facilitate passive movement down concentration gradients without requiring energy.
- A membrane protein transports H+ from the cytosol (pH 7.2) to the extracellular space (pH 6.2). When ATP is depleted, H+ export decreases substantially. The H+ gradient corresponds to higher H+ concentration outside than inside, yet export continues when ATP is present. Which mechanism best explains H+ movement under normal conditions?
Answer: Primary active transport of H+ using ATP hydrolysis
This question examines active transport of protons against their gradient. The pH values indicate H+ concentration is higher outside (pH 6.2 = more H+) than inside (pH 7.2 = less H+), yet the pump exports H+ from inside to outside—against the concentration gradient. This uphill transport requires energy, and the substantial decrease when ATP is depleted confirms primary active transport. Facilitated diffusion (A) is incorrect because H+ would move into the cell down its gradient, not out against it. The strategy is to convert pH to H+ concentration: lower pH means higher H+ concentration, then determine if transport follows or opposes the gradient.
- A freshwater protist has an internal solute concentration of 300 mOsm and is suddenly transferred to a medium of 50 mOsm. The membrane is permeable to water but not to the major intracellular solutes. Within minutes, the cell volume increases. If no pumps or channels change activity during this interval, which outcome is most likely for water movement across the membrane?
Answer: Water moves into the cell by osmosis, requiring no ATP input
This question requires analyzing water movement across membranes in response to osmotic gradients. The protist has a higher internal solute concentration (300 mOsm) than the external medium (50 mOsm), creating a hypotonic external environment where water concentration is higher outside than inside. Water moves by osmosis from high water concentration (low solute) to low water concentration (high solute), so water enters the cell, causing the volume increase observed. This passive process requires no ATP input, only the concentration gradient and membrane permeability to water. Students often incorrectly choose water moving out (B) by confusing solute concentration with water concentration—remember that high solute means low water concentration. When analyzing osmosis problems, always identify where water concentration is highest and remember water moves toward higher solute concentrations.
- 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?
Answer: Facilitated diffusion through a carrier protein down the glucose gradient
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.
- 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?
Answer: Primary active transport exporting Ca2+ against its gradient using ATP
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.
