AP Biology: Unit 2, Membrane Transport & Osmosis (Drill 4)

Question 1. Based on the data, what is the approximate solute concentration of the potato core cells?

• A) 0.0 M, because potato cores in distilled water gained mass, indicating the potato has no solutes.
• B) Between 0.2 M and 0.4 M, because the potato core shows near-zero mass change at 0.4 M, indicating osmotic equilibrium near that concentration. ✓
• C) Greater than 1.0 M, because potato cores lost mass at all sucrose concentrations tested.
• D) Exactly 0.6 M, because the potato cores lost the greatest amount of mass at that particular sucrose concentration in the data table in the data set described.

Explanation: At osmotic equilibrium, the solute concentration inside the cell equals the solute concentration of the surrounding solution, producing zero net water movement and zero mass change. The potato core shows +0.3% mass change at 0.4 M -- essentially zero -- indicating the internal solute concentration is approximately 0.4 M. A is incorrect -- gaining mass in distilled water indicates the potato cells have a higher solute concentration than pure water, not zero solutes. C is incorrect -- the potato gains mass at 0.2 M and is near equilibrium at 0.4 M, placing internal concentration near 0.4 M, not above 1.0 M. D is incorrect -- 0.6 M produces a mass loss of 3.8%, meaning the external solution is already more concentrated than cell contents at that point.

Question 2. In Experiment 1, the dialysis bag filled with 0.0 M sucrose submerged in distilled water shows no mass change. A student claims this proves the dialysis membrane is impermeable to water at low solute concentrations. Which reasoning best refutes this claim?

• A) The claim is correct -- dialysis membranes only become permeable to water once the surrounding solute concentration rises above a minimum threshold.
• B) The claim is incorrect -- water moves bidirectionally across the membrane at equal rates because water potential is identical on both sides, producing no net mass change despite membrane permeability. ✓
• C) The claim is incorrect -- water cannot cross dialysis tubing under any conditions because it lacks aquaporin channels.
• D) The claim is partially correct -- water permeability is low at 0.0 M but increases proportionally with solute concentration gradient.

Explanation: The absence of net mass change does not indicate impermeability -- it indicates equal water potential on both sides of the membrane. Water moves freely and bidirectionally across a permeable membrane, but when water potential is identical on both sides (both 0.0 M), there is no net directional movement and mass does not change. A is incorrect -- membrane permeability is a physical property independent of solute concentration. C is incorrect -- dialysis tubing is permeable to water; aquaporins are membrane proteins in living cells, not a feature of dialysis tubing. D is incorrect -- permeability does not scale with concentration gradient; the gradient drives net flux, not permeability itself.

Question 3. A student calculates that between treatments C and D in Experiment 2, the rate of mass loss is approximately 4.1 percentage points per 0.2 M increase in sucrose concentration. Assuming the relationship remains approximately linear over this interval, what percent mass change would be predicted for a potato core submerged in 1.2 M sucrose?

• A) -13.6%
• B) -11.6%
• C) -15.7% ✓
• D) -9.4%

Explanation: The observed mass change at 1.0 M is -11.6%. Extending at the stated rate of 4.1 percentage points per 0.2 M increase: -11.6 + (-4.1) = -15.7% at 1.2 M. A (-13.6%) adds only 2.0 percentage points rather than 4.1. B (-11.6%) is the value at 1.0 M with no extrapolation applied. D (-9.4%) moves in the wrong direction, subtracting from the loss rather than extending it.

Question 4. A researcher proposes that the mass gain observed in dialysis bags (Experiment 1) and the mass loss observed in potato cores at high sucrose concentrations (Experiment 2) are both best explained by the same underlying mechanism. Which explanation best supports this claim?

• A) Both results are caused by active transport of sucrose across the membrane against its concentration gradient, requiring ATP in each experiment.
• B) Both results are caused by osmosis -- net water movement across a selectively permeable membrane driven by differences in water potential -- with water moving into the bag in Experiment 1 and out of potato cells in Experiment 2. ✓
• C) Both results occur because dialysis tubing and cell membranes are fully permeable to all solutes, allowing equilibration.
• D) Both results reflect facilitated diffusion of water through aquaporin channels, which operate only when solute concentrations exceed 0.4 M.

Explanation: In Experiment 1, the dialysis bag contains a higher solute concentration than the surrounding distilled water, so water moves into the bag by osmosis, increasing mass. In Experiment 2, when external sucrose concentration exceeds internal cell concentration, water moves out of potato cells by osmosis, decreasing mass. Both are driven by net water movement across a selectively permeable membrane down a water potential gradient. A is incorrect -- active transport requires energy and carrier proteins; dialysis tubing has neither. C is incorrect -- dialysis tubing and cell membranes are selectively permeable, not freely permeable to all solutes. D is incorrect -- aquaporins facilitate water movement but are not required for osmosis, and their activity is not concentration-gated at 0.4 M.

Question 5. A student claims that cells in the potato core at treatment F (1.0 M sucrose) are plasmolyzed. Which evidence from the data is most consistent with this claim, and what would directly confirm it?

• A) The -11.6% mass loss is consistent with significant water loss from cells; direct confirmation would require microscopic observation of the cell membrane pulling away from the cell wall. ✓
• B) The -11.6% mass loss is consistent with plasmolysis; direct confirmation would require measuring the sucrose concentration inside individual potato cells.
• C) The mass loss alone confirms plasmolysis because any negative mass change in plant cells indicates the plasma membrane has separated from the cell wall.
• D) The data are inconsistent with plasmolysis because plasmolysis occurs only in animal cells exposed to hypertonic solutions, not in plant tissue.

Explanation: Plasmolysis occurs in plant cells when water loss is severe enough that the plasma membrane pulls away from the cell wall. The 11.6% mass loss at 1.0 M sucrose is consistent with significant osmotic water loss that could produce plasmolysis, but mass change alone cannot confirm membrane separation from the wall -- microscopic observation is required. B is incorrect -- measuring internal sucrose concentration would confirm osmotic conditions but not membrane separation. C is incorrect -- mass loss indicates water movement but not necessarily membrane-wall separation, which is a specific structural event. D is incorrect -- plasmolysis is a plant cell phenomenon; animal cells lack cell walls and undergo crenation instead.