1 question, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 1
Cells of a hypothetical seaweed accumulate iodide ions (I⁻) to a concentration many times higher than that in the seawater around them. Which statement best explains why the cells must use metabolic energy, such as energy from ATP, to do this?
Answer and reasoning
AIodide stops moving once its concentrations inside and outside match, so energy restarts it. A student who thinks particles stop moving at equilibrium picks this. Ions keep moving at equilibrium; energy is needed because iodide is being moved against its gradient, not to restart motion.
BMoving iodide from lower to higher concentration is not spontaneous, so it needs energy.Correct Iodide is being moved against its concentration gradient. Net movement in that direction does not happen by diffusion; it requires active transport, in which a membrane protein uses metabolic energy, such as from ATP hydrolysis.
CEnergy is needed just once, to set up the iodide gradient, which then stays in place without it. A student who thinks a gradient persists once built picks this. Iodide leaks back out, so the gradient lasts only while active transport keeps using energy.
DThe cells need iodide, and any substance a cell needs must be taken in using ATP. A student who explains transport by the cell's needs picks this. Cells take in many needed substances passively; energy is required here because iodide moves against its concentration gradient.
In preparation: 0 of 1 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
2.8.A.1 Active transport Fix
Active transport
Movement of molecules or ions across a membrane that requires metabolic energy, such as energy from ATP. It can move a substance from a region of lower concentration to a region of higher concentration, building or maintaining a gradient.
Metabolic energy (ATP)
Energy made available by the cell's metabolism. Hydrolysis of ATP to ADP and inorganic phosphate (Pᵢ) releases energy that membrane proteins can use to move substances against their gradients.
Electrochemical gradient
The combined difference in concentration and in electrical charge across a membrane that drives the movement of an ion. Active transport establishes and maintains these gradients.
Steady state of an ion gradient
A condition in which an ion's concentration inside a cell stays constant because the rate at which it is actively transported in equals the rate at which it leaks out. Ions keep crossing in both directions; stopping the energy supply stops the active part, so the gradient runs down.
Transport protein (pump) in active transport
A membrane protein that couples an energy source, such as ATP hydrolysis, to moving a specific substance across the membrane. Active transport cannot occur without such proteins.
Na⁺/K⁺ pump (Na⁺/K⁺ ATPase)
A membrane protein of animal cells that hydrolyzes ATP and, in each cycle, moves three Na⁺ out of the cell and two K⁺ into it, against their concentration gradients. It maintains the Na⁺ and K⁺ gradients and contributes to the membrane potential.
Membrane potential
The difference in electrical charge (voltage) across a plasma membrane; in a typical animal cell at rest the inside is negative relative to the outside. It depends on the ion gradients maintained by pumps such as the Na⁺/K⁺ ATPase and on the movement of ions through channels.
ATPase
An enzyme that hydrolyzes ATP. Transport ATPases, such as the Na⁺/K⁺ ATPase, use the energy released to move ions across membranes.
Students often think Once a concentration stops changing, or the two sides reach equilibrium, the molecules stop moving across the membrane. In fact No. Molecules and ions keep moving. A constant concentration means the rates of movement in the two directions are equal; for an actively transported ion, pumping in balances leaking out.
Students often think A cell takes in, holds or releases a substance according to whether it needs it, so need explains the direction of transport. In fact No. Movement across membranes is set by mechanisms: concentration and charge gradients, membrane permeability, and the proteins present, including energy-using pumps. A cell's need does not by itself move a substance.
6 more questions. Every wrong answer here is a real mistake students make, and you see why it is wrong as soon as you answer.
Question 1 of 6
Cells of a hypothetical alga were placed in a solution containing ion X, and the concentration of X inside the cells was measured over time. At 30 minutes, a drug that stops the cells from making ATP was added. The graph shows the results; the dashed line shows the concentration of X outside the cells. Which claim is best supported by the data?
Answer and reasoning
AOnce the gradient of X has been built, it is held without any further ATP use. A student who thinks a gradient persists once built picks this. The data show the opposite: when ATP production stopped at 30 minutes, the inside concentration fell steadily toward the outside level.
BX stopped crossing the membrane once the inside level reached 10 mM. A student who thinks a steady level means no movement picks this. The level stayed at about 10 mM because active uptake balanced leakage; the fall after the drug shows X was still leaking out all along.
CX fell after the drug was added because the cells no longer needed X. A student who explains transport by the cell's needs picks this. X fell because, without ATP, active transport stopped while X kept leaking out down its gradient.
DHolding X above its outside level depends on a continuing supply of ATP.Correct Before the drug, the cells raised X to about ten times the outside concentration and held it there. Once ATP production stopped, X fell back toward 1 mM, so maintaining the gradient, not just building it, requires continued energy input.
A student makes artificial vesicles from phospholipids and inserts a purified membrane protein that, in cells, moves Ca²⁺ against its concentration gradient. The vesicles are placed in a solution containing Ca²⁺ and ATP, and Ca²⁺ accumulates inside them. Which additional set of vesicles is needed to show that the protein is required for this accumulation?
Answer and reasoning
AVesicles lacking the protein, placed in the same Ca²⁺ and ATP solutionCorrect This control differs from the experimental vesicles only in lacking the protein. If Ca²⁺ accumulates only when the protein is present, the protein is required for the accumulation.
BVesicles with no protein, kept in plain water containing no Ca²⁺ or ATP A student who thinks a control means removing everything picks this. These vesicles differ from the experimental set in three ways; with no Ca²⁺ or ATP outside, no Ca²⁺ could accumulate whether or not the protein is required, so this set cannot show the protein's role.
CVesicles holding twice as much protein, given Ca²⁺ and ATP just as before A student who treats any comparison group as a control picks this. More protein is another level of the variable; every set still contains the protein, so its necessity is never tested.
DA repeat batch of the vesicles containing the protein, treated identically A student who confuses replication with a control picks this. A repeat checks consistency, but it still contains the protein, so it cannot show what happens without it.
The model shows one cycle of the Na⁺/K⁺ pump (Na⁺/K⁺ ATPase) in the plasma membrane of an animal cell. In this cell, the inside is negative relative to the outside. Based on the model, which statement best describes how the pump contributes to this membrane potential?
Answer and reasoning
AIt brings K⁺ into the cell, so it makes the inside less negative than it would be. A student who equates a high concentration of a positive ion with positive charge picks this, counting only the K⁺ that enters. The pump moves three Na⁺ out for every two K⁺ in, so each cycle makes the inside more negative, not less.
BIt is the only source of the potential, which would vanish at once if it stopped. A student who thinks the pump alone generates the membrane potential picks this. The pump contributes, but much of the potential comes from ions moving through channels; if the pump stopped, the potential would decline gradually as the gradients ran down.
CIt moves more positive charge out than in, which helps keep the inside negative.Correct Each cycle moves three Na⁺ out and two K⁺ in, so one more positive charge leaves than enters. This adds to the negative charge inside; the pump also maintains the Na⁺ and K⁺ gradients on which the rest of the membrane potential depends.
DIt swaps positive ions for positive ions, so it leaves the charge across it unchanged. A student who thinks swapping like-charged ions cannot change charge picks this. The model shows three Na⁺ out and only two K⁺ in, a net loss of one positive charge from the inside per cycle.
Cells of a hypothetical yeast accumulate nutrient Z by active transport. The graph shows the ratio of the concentration of Z inside the cells to that outside after 30 minutes for four groups: wild-type cells; wild-type cells given a drug that stops ATP production; mutant cells lacking membrane protein T; and mutant cells into which the gene for T has been put back. Error bars show ±2 SE of the mean (n = 6). Which conclusion is best supported by the data?
Answer and reasoning
APutting T back restores uptake exactly to the wild-type level, as those two error bars do overlap. A student who thinks overlapping error bars prove two means are equal picks this. Overlap means no difference has been shown, not that the means are exactly the same.
BEvery mutant cell held less Z than every wild-type cell, as the two error bars do not overlap. A student who reads ±2 SE bars as the range of individual values picks this. The bars describe the precision of the means; they say nothing about every individual cell.
CThe drug lowered uptake less than the lack of T did, as the drug group's mean bar is higher. A student who treats any difference in means as real picks this. The drug group's bar (1.2) and the mutant's (1.0) have overlapping ±2 SE error bars, so the data do not show that their effects differ.
DCells lacking T accumulate much less Z than wild-type cells, as their error bars are far apart.Correct The mutant lacking T has a mean ratio of about 1.0 (no accumulation) against 12.0 for wild type, and the ±2 SE bars are far from overlapping, so the difference is likely real. Restoring T restores accumulation, supporting the claim that protein T is needed for active transport of Z.
Working Read each mean ± 2 SE: wild type 12.0 (10.5–13.5); wild type + drug 1.2 (0.8–1.6); mutant lacking T 1.0 (0.7–1.3); mutant + T gene 11.0 (9.2–12.8). Wild type versus mutant lacking T: the intervals are far apart, so the difference is likely significant. Drug versus mutant (0.8–1.6 and 0.7–1.3) and wild type versus mutant + T (10.5–13.5 and 9.2–12.8) overlap, so no difference is shown for those pairs; overlap does not show that the means are equal.
In a hypothetical animal cell, a drug blocks the Na⁺/K⁺ pump (Na⁺/K⁺ ATPase) but has no direct effect on any ion channel. Which prediction about the cell during the following hour is best supported?
Answer and reasoning
AThe membrane potential gradually shrinks, as Na⁺ and K⁺ leak through channels and their gradients run down.Correct The pump maintains the Na⁺ and K⁺ gradients against continual leakage. With the pump blocked, Na⁺ leaks in and K⁺ leaks out, the gradients gradually decline, and the membrane potential, which depends on them, becomes smaller.
BThe membrane potential drops to zero at once, as the pump by itself produces all of the potential. A student who thinks the pump alone generates the membrane potential picks this. The potential depends mostly on ions moving through channels down existing gradients, so it declines gradually as the gradients run down.
CThe Na⁺ and K⁺ gradients stay as they are, as gradients need energy just to be built, not kept. A student who thinks gradients persist once built picks this. Ions keep leaking through channels, so without the pump the gradients run down.
DIon channels take over, moving Na⁺ out and K⁺ in against their gradients as the pump did. A student who thinks channels can move ions against their gradients picks this. Channels allow only passive movement down gradients; moving Na⁺ out and K⁺ in requires an energy-using pump.
Cells of a hypothetical alga take up ion X from pond water in which X is less concentrated than it is in the cells. Which of the following must the cells have in order to keep taking up X?
Answer and reasoning
AOpen channel proteins through which X can simply diffuse into the cell A student who thinks channels can move ions whichever way the cell requires picks this. Through an open channel, X would move down its gradient, out of the cell.
BMembrane proteins that use metabolic energy to move X into the cellCorrect X is moving from lower to higher concentration, against its gradient, so this is active transport. Active transport requires membrane proteins that use metabolic energy, such as from ATP.
CATP that pushes X directly across the phospholipid bilayer itself A student who thinks ions can cross the phospholipid bilayer directly picks this, adding energy to move X against its gradient. The hydrophobic interior blocks ions; active transport needs a membrane protein that couples ATP use to moving X.
DA need for X, which draws X into the cell across the plasma membrane A student who explains transport by the cell's needs picks this. Need does not move ions; moving X against its gradient requires energy-using membrane proteins.
Compiled from the AP Biology Course and Exam Description (effective Fall 2025) and our question bank · Specialist review in progress. How these pages are made · Free, no account
