3 questions, one for each idea where we can. Answer them, then see which ideas to fix.
Question 1 of 3
A freshwater protist lives in pond water that is hypotonic to its cytoplasm. The protist has a contractile vacuole that repeatedly fills and then empties to the outside of the cell. Which statement best explains why the protist needs its contractile vacuole?
Answer and reasoning
AWater keeps leaving by osmosis, and the vacuole stores water to replace the loss. A student who thinks water moves toward the lower solute concentration picks this. Water moves toward the higher solute concentration, which here is the cytoplasm, so water enters the protist rather than leaving it.
BSalts keep leaving the cell by osmosis, and the vacuole takes in salt to replace the loss. A student who thinks osmosis is the movement of solute picks this: salt would spread from the more concentrated cytoplasm into the dilute pond. Osmosis is the movement of water; in a hypotonic pond, water enters the cell, and the contractile vacuole expels it to the outside.
CWater cannot cross the cell membrane, so the vacuole brings in all the water it needs. A student who thinks the membrane is impermeable to water picks this. Water crosses the membrane by osmosis; the vacuole removes the water that enters this way, rather than bringing water in.
DOsmosis brings water into the cell steadily, and the vacuole expels the excess water.Correct The pond water has a lower solute concentration (higher water potential) than the cytoplasm, so water continually enters the cell by osmosis. The contractile vacuole collects this water and pumps it out, so the cell does not swell and burst.
Cells near the tip of a young root of a hypothetical plant grow by taking up water, which enlarges each cell. The root is moved from pure water into a sucrose solution whose water potential is equal to that of the growing cells. Which prediction about the growth of these cells immediately after the transfer is best supported?
Answer and reasoning
AGrowth stops, as water molecules stop crossing the cell membranes at equal potentials. A student who thinks molecules stop moving at equilibrium picks this. Water molecules keep crossing the membranes in both directions; growth stops because there is no net gain of water, not because movement ceases.
BGrowth continues as before, as the cells take in whatever water they need to grow. A student who thinks cells take in water because they need it picks this. Osmosis follows water potential; with equal water potentials there is no net water uptake, whatever the cells need.
CGrowth by water uptake stops, as there is no longer net movement of water into the cells.Correct With equal water potentials inside and outside, water crosses the membranes equally in both directions, so the cells gain no net water and cannot enlarge by water uptake. Growth depends on continued net movement of water into the cells.
DThe cells shrink, as any solution containing sugar draws water out of the cells placed in it. A student who thinks any solute-containing solution dehydrates cells picks this. This solution has the same water potential as the cells, so it causes no net water movement in either direction.
The table gives measurements for a cell from a hypothetical plant. Using ψ = ψP + ψS and ψS = −iCRT, where R = 0.0831 liter bars/mole K, what is the water potential of the cell?
B−5.0 bars A student who thinks water potential always equals solute potential picks this. −5.0 bars is ψS alone; this cell has a pressure potential of +2.0 bars, which must be added.
C+7.0 bars A student who thinks solute raises water potential picks this, treating ψS as +5.0 bars and adding ψP to get +7.0 bars. Solute potential is negative: ψS = −iCRT.
D−8.0 bars A student who thinks sucrose splits into ions uses i = 2, giving ψS = −10.0 bars and ψ = −8.0 bars. Sucrose does not ionize, so i = 1.0.
Working T = 27 °C + 273 = 300 K. Sucrose does not ionize, so i = 1.0. ψS = −iCRT = −(1.0)(0.20 mol/L)(0.0831 L·bars/mol·K)(300 K) = −4.99 bars ≈ −5.0 bars. ψ = ψP + ψS = +2.0 bars + (−5.0 bars) = −3.0 bars.
In preparation: 0 of 3 sections compiled and reviewed. The rest show key terms and common mistakes from our question bank until they are.
2.7.A.1 Hypertonic, hypotonic and isotonic Fix
Hypertonic, hypotonic and isotonic
Terms that compare the solute concentration of two solutions, such as the external environment and a cell's interior. A hypertonic solution has the higher solute concentration, a hypotonic solution the lower, and isotonic solutions have equal solute concentrations. Each term describes one solution relative to another, not a solution on its own.
Osmosis
The net movement of water across a selectively permeable membrane from a region of higher water potential to a region of lower water potential (from hypotonic to hypertonic, if pressure is equal).
Water potential (ψ)
A measure, in bars, of the tendency of water to move from one region to another. ψ = ψP + ψS. Water moves by osmosis from higher (less negative) to lower (more negative) water potential. Pure water in an open container has ψ = 0 bars.
Pressure potential (ψP)
The part of water potential due to physical pressure. In a turgid plant cell, the cell wall pushes back on the expanding cell, so ψP is positive and raises the cell's water potential. In an open container ψP = 0.
Contractile vacuole
An organelle of many freshwater protists that collects water entering the cell by osmosis and expels it from the cell, preventing the cell from swelling and bursting in a hypotonic environment.
Central vacuole and turgor pressure
The large vacuole of a plant cell stores water and solutes. When water enters it by osmosis, the cell presses against its wall, producing turgor pressure (positive ψP) that supports the plant.
Students often think Water moves by osmosis toward the side with the lower solute concentration, as if water and solute both spread from where they are more concentrated. In fact No. Water moves toward the region with the higher solute concentration, because dissolved solutes lower water potential and water moves from higher to lower water potential (when pressure is equal on both sides).
Students often think Osmosis is the movement of dissolved solute across a membrane until the concentrations on both sides are equal; if the solute cannot cross, nothing happens. In fact No. Osmosis is the movement of water across a selectively permeable membrane. Solutes that cannot cross the membrane stay where they are; water moves.
2.7.B.1 Growth and homeostasis depend on membrane transport Fix
Growth and homeostasis depend on membrane transport
Cells grow and keep their internal conditions stable only because molecules such as water, ions and nutrients move continually across their membranes; if net movement of a needed substance stops, processes that depend on it stop too.
Students often think Cells and organisms take in the water they need, so their uptake continues whenever they need water, regardless of water potential. In fact No. Water moves by osmosis according to differences in water potential, whether or not the cell 'needs' water. A cell whose water potential equals that of its surroundings gains no net water, however much it would benefit from more.
2.7.B.2 Solute potential (ψS) Fix
Solute potential (ψS)
The part of water potential due to dissolved solutes: ψS = −iCRT. Adding solute makes ψS more negative and so lowers water potential. Pure water has ψS = 0.
Ionization constant (i)
The number of particles each formula unit of a solute produces in solution: 1.0 for sucrose, which does not ionize, and 2.0 for NaCl, which separates into Na⁺ and Cl⁻.
Osmoregulation
The control of water balance and of internal solute composition and water potential. Organisms that osmoregulate keep their internal solute concentration within a range even when the concentration of their surroundings changes.
Students often think Every substance that dissolves breaks into ions, so sucrose, like salt, has an ionization constant of 2. In fact No. Sucrose dissolves as whole molecules, so i = 1.0. Salts such as NaCl separate into ions (i = 2.0 for NaCl).
Students often think Osmoregulation means bringing the body's solute concentration into line with the environment, so the species whose blood matches the water best is the best osmoregulator. In fact No. An organism whose body fluids simply follow the concentration of its surroundings is not regulating its overall solute concentration. Osmoregulation means controlling internal solute concentration and water potential, often keeping them different from the surroundings.
4 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 4
The model shows a U-tube divided at the bottom by a membrane that is permeable to water but not to sucrose. At the start, the liquid levels on the two sides are equal and the temperature is the same throughout. Which statement best describes the net movement of water in the model?
Answer and reasoning
AWater moves from side B to side A, as side B has the higher water potential of the two. A student who thinks more solute means a higher water potential picks this. Solute lowers water potential (ψS = −iCRT), so side B, with more sucrose, has the lower water potential.
BWater moves from side B to side A, toward the side with the lower sucrose concentration. A student who thinks water moves toward the lower solute concentration picks this. Water moves toward the higher solute concentration (lower water potential), which is side B.
CWater moves from side A to side B, as side A has the higher water potential.Correct Side A has less solute, so its solute potential, and therefore its water potential, is higher (less negative) than side B's. Water moves by osmosis from higher to lower water potential, from side A to side B, so the level on side B rises.
DNo net movement of water occurs, as the sucrose cannot cross the membrane at all. A student who thinks osmosis is the movement of solute picks this. Osmosis is the movement of water; because sucrose cannot cross, water moves toward side B, where the solute concentration is higher.
Two species of hypothetical crab, P and Q, were each kept in water of different solute concentrations, and the solute concentration of their blood was then measured. The graph shows the results. Which claim is best supported by the data?
Answer and reasoning
AQ osmoregulates better than P, since Q's blood matches the water's concentration at each level. A student who thinks osmoregulation means matching the surroundings picks this. Q's blood simply follows the water; P controls its internal concentration over a wide range, which is osmoregulation.
BP osmoregulates, since its blood concentration stays nearly steady as the water's changes.Correct Across water concentrations from 100 to 700 mOsm/L, P's blood stays between about 600 and 635 mOsm/L, so P controls its internal solute concentration: it osmoregulates. Q's blood stays just above the water's concentration at every level, so Q's blood follows its surroundings.
CIn dilute water, P loses water to its surroundings, since its blood holds more solute. A student who thinks water moves toward the lower solute concentration picks this. P's blood is more concentrated than dilute water, so water tends to enter P by osmosis, not leave it.
DNeither species osmoregulates, since each one's blood concentration changes to some degree. A student who thinks regulation means no change at all picks this. P's blood changes by only about 35 mOsm/L while the water changes by 600 mOsm/L, which shows regulation within a range.
Root cells of a hypothetical salt-marsh plant accumulate extra solutes, which keeps the water potential of the root cells below that of the salty soil water around them. A mutation stops the root cells from accumulating these extra solutes; without them, the solute concentration of the root cells would be lower than that of the soil water. Which prediction about the mutant plant's root cells is best supported?
Answer and reasoning
AThe root cells take in water faster, as water moves into the cells with less solute. A student who thinks water moves toward the lower solute concentration picks this. Water moves toward the higher solute concentration, which is now the soil water.
BThe root cells take in more water, as fewer solutes give them a lower water potential. A student who thinks solutes raise water potential picks this. Fewer solutes make the solute potential less negative, so the root cells' water potential rises, not falls.
CThe root cells keep taking in water as before, since the plant still needs water to live. A student who thinks organisms take in water because they need it picks this. The direction of osmosis depends on water potential, not need; the mutant cells now have the higher water potential.
DThe root cells lose water to the soil, as their water potential is now the higher of the two.Correct With a lower solute concentration than the soil water, the root cells have a higher (less negative) solute potential, and any positive pressure potential raises their water potential further. Water moves from higher to lower water potential, so it tends to leave the root cells.
A student fills bags of dialysis tubing, which is permeable to water but not to sucrose, with 0.2 M, 0.4 M or 0.6 M sucrose solution and places each bag in a beaker of distilled water. The percent change in mass of each bag is measured after 30 minutes. To show that the mass changes are caused by the sucrose concentration inside the bags, and not by the tubing itself absorbing water, which additional set-up is needed?
Answer and reasoning
AAn empty bag of tubing that is kept dry outside any beaker A student who thinks a control means doing nothing picks this. A dry, empty bag differs from the others in several ways (no liquid inside, no water outside), so it cannot show how much water the tubing absorbs in the experiment.
BA bag of distilled water placed in a beaker of distilled waterCorrect This control is identical to the other set-ups except that it contains no sucrose, so there is no water potential difference across the tubing. Any change in its mass shows how much comes from the tubing itself, separate from osmosis.
CA bag of 1.0 M sucrose solution placed in distilled water A student who treats an extra level of the independent variable as a control picks this. A 1.0 M bag is another experimental group; it still has a sucrose difference across the tubing, so it cannot separate the tubing's effect from osmosis.
DThe same three bags, weighed again after a further 30 minutes in water A student who thinks repeating measurements acts as a control picks this. Weighing the same bags again adds more data on the same treatments, but every bag still contains sucrose, so the tubing's own water uptake is never measured separately.
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
