Membrane Transport
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AP Biology › Membrane Transport
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?
Secondary active transport coupling glucose uptake to ion movement uphill
Primary active transport pumping glucose into the cell using ATP
Endocytosis of extracellular fluid containing dissolved glucose molecules
Simple diffusion through the phospholipid bilayer down the gradient
Facilitated diffusion through a carrier down the concentration gradient
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.
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?
Primary active transport of lactose using ATP binding and hydrolysis
Exocytosis exporting H+ to allow lactose to diffuse inward freely
Secondary active transport coupling lactose import to H+ moving down gradient
Simple diffusion of lactose through the lipid bilayer down its gradient
Facilitated diffusion of lactose through a carrier down its gradient
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.
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?
Endocytosis of extracellular Na+ followed by vesicle fusion at the membrane
Simple diffusion of Na+ through the lipid bilayer down its gradient
Osmosis of Na+ through aquaporins driven by water potential differences
Facilitated diffusion of Na+ through a channel down its concentration gradient
Primary active transport of Na+ against its gradient using ATP directly
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.
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?
Facilitated diffusion of glucose down its gradient through a carrier protein
Primary active transport of glucose by ATP hydrolysis at the glucose transporter
Osmosis of glucose through aquaporins driven by solute concentration differences
Simple diffusion of glucose through the lipid bilayer from low to high concentration
Secondary active transport: Na+ moving down its gradient drives glucose uptake
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.
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?
Facilitated diffusion of H+ through channels from vacuole to cytosol without ATP
Osmosis of H+ driven by water movement into the vacuole through aquaporins
Secondary active transport using H+ movement against its gradient to power ATP synthesis
Primary active transport using ATP hydrolysis to pump H+ into the vacuole
Simple diffusion of H+ through the lipid bilayer down its electrochemical gradient
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.
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?
Primary active transport using ATP hydrolysis to move Na+ against its gradient
Secondary active transport powered by Na+ moving into the cell down its gradient
Simple diffusion of Na+ through the lipid bilayer due to its small size
Osmosis of Na+ driven by water potential differences across the membrane
Facilitated diffusion of Na+ through a carrier down its concentration gradient
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.
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?
Primary active transport of K+ requiring ATP to move K+ out of the cell
Endocytosis of extracellular K+ into vesicles followed by release to cytosol
Secondary active transport of K+ coupled to glucose moving down its gradient
Facilitated diffusion of K+ through an ion channel down its concentration gradient
Simple diffusion of K+ through the phospholipid bilayer down its gradient
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.
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?
The vesicle loses volume by exocytosis of water-containing internal vesicles
NaCl enters the vesicle by simple diffusion, increasing internal osmolarity
Sucrose leaves the vesicle by facilitated diffusion, lowering internal osmolarity
Water is pumped out by primary active transport using ATP hydrolysis
Water leaves the vesicle by osmosis because external solute concentration is higher
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.
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?
Simple diffusion through the lipid bilayer with no protein involvement or saturation
Secondary active transport using Y export to drive Na+ import into the cell
Primary active transport requiring ATP hydrolysis to import Y regardless of gradients
Facilitated diffusion through a carrier moving Y down its concentration gradient with saturation
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.
A cell is placed in a solution where Na+ concentration is 140 mM outside and 10 mM inside. A membrane protein allows Na+ to cross, and Na+ movement increases when the protein is present but stops when the protein is denatured. When ATP production is blocked, Na+ still moves into the cell at the same rate. Which mechanism best explains Na+ entry into the cell?
Which transport mechanism best explains Na+ movement under these conditions?
Secondary active transport of Na+ coupled to glucose export
Primary active transport of Na+ using ATP hydrolysis
Osmosis of water that carries Na+ through aquaporins
Simple diffusion of Na+ directly through the lipid bilayer
Facilitated diffusion of Na+ down its concentration gradient
Explanation
This question tests your ability to analyze membrane transport mechanisms based on experimental evidence. The Na+ concentration gradient (140 mM outside, 10 mM inside) creates a driving force for Na+ to move into the cell down its concentration gradient. The fact that Na+ movement requires a protein (stops when denatured) but continues when ATP is blocked indicates the protein facilitates passive transport rather than active transport. Simple diffusion (B) is incorrect because Na+ is charged and cannot cross the lipid bilayer directly—it requires a protein channel or carrier. The key strategy is to check whether transport depends on ATP: if movement continues without ATP but follows the concentration gradient, it's facilitated diffusion.