Mechanisms of Transport
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AP Biology › Mechanisms of Transport
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
Secondary active transport requiring a maintained ion gradient to move nutrient uphill
Exocytosis delivering extracellular nutrient directly into the cytosol
Simple diffusion of the polar nutrient through the phospholipid bilayer
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.
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?
Osmosis through aquaporins increasing intracellular glucose concentration
Secondary active transport of glucose coupled to Na$^+$ moving inward
Facilitated diffusion of glucose down its gradient via a uniporter
Primary active transport of glucose against its gradient using ATP directly
Simple diffusion of glucose through the bilayer driven by 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.
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?
Simple diffusion of X through the lipid bilayer driven by concentration differences
Facilitated diffusion via a saturable carrier transporting X down its concentration gradient
Secondary active transport of X coupled to K$^+$ efflux from the cell
Primary active transport of X using ATP hydrolysis to move X into the cell
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.
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?
Secondary active transport coupled to Na$^+$ influx moving protein outward
Primary active transport through an ATP-driven protein pump in membrane
Simple diffusion of protein through the lipid bilayer down a gradient
Facilitated diffusion of protein through a membrane channel protein pore
Exocytosis via vesicle fusion with the plasma membrane releasing contents
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.
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?
Exocytosis of solute from intracellular vesicles to the outside
Facilitated diffusion through a channel down the solute’s electrochemical gradient
Primary active transport by an ATPase moving solute against its gradient
Secondary active transport requiring a Na$^+$ gradient and cotransport
Simple diffusion directly through the phospholipid bilayer
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.
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?
Primary active transport using ATP directly to move drug down its gradient
Exocytosis releasing drug from intracellular vesicles into the cytosol
Facilitated diffusion via a saturable carrier protein down the drug gradient
Secondary active transport coupled to an ion gradient that is not required
Simple diffusion through the lipid bilayer requiring a specific transporter
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.
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?
Primary active transport by an ATP-driven pump moving sugar against gradient
Simple diffusion directly through the phospholipid bilayer down its gradient
Secondary active transport coupled to ion movement up the sugar gradient
Facilitated diffusion through a specific membrane carrier down its gradient
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.
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?
Primary active transport of H$^+$ driven directly by ATP hydrolysis
Secondary active transport using glucose movement to power H$^+$ export
Simple diffusion of H$^+$ through the hydrophobic lipid interior
Osmosis of H$^+$ through aquaporins with water movement
Facilitated diffusion of H$^+$ down its electrochemical gradient through a channel
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.
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?
Secondary active transport exchanging Na$^+$ for glucose in the same direction
Exocytosis releasing Na$^+$ stored in vesicles into the cytosol
Facilitated diffusion through a gated ion channel down electrochemical gradient
Primary active transport of Na$^+$ using ATP hydrolysis by the channel
Simple diffusion of Na$^+$ through the hydrophobic bilayer without proteins
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.
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?
Facilitated diffusion of Na$^+$ through the antiporter down its gradient
Primary active transport of Na$^+$ by direct ATP hydrolysis at the antiporter
Secondary active transport using the H$^+$ gradient to drive Na$^+$ export
Simple diffusion of Na$^+$ through the lipid bilayer without proteins
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.