AP Biology Unit 2 FRQ Practice: Free Response Questions

Free Response Question · Unit 2 · Tonicity & Osmosis in Plant Cells

A student designs an experiment to determine the solute concentration inside potato cells. She cuts five identical cylindrical cores from a fresh potato, weighs each, and places one core in each of five sucrose solutions at varying concentrations. After 24 hours at room temperature, she removes each core, blots it dry, and reweighs it. Her results are shown below.

Sucrose concentration (M)	Percent change in mass (%)
0.0	+18
0.2	+8
0.4	0
0.6	−10
1.0	−22

Estimate the approximate solute concentration inside the potato cells. Justify your answer using the data.

✓ Model answer (earns the point)

The internal solute concentration of the potato cells is approximately 0.4 M. At this concentration, the core showed no change in mass, meaning there was no net movement of water into or out of the cells. This indicates that the 0.4 M sucrose solution is isotonic to the potato cells, so the solute concentration inside the cells must also be about 0.4 M.

Why it scores: States the specific concentration (0.4 M), connects "no mass change" to "no net water movement," AND uses the term isotonic correctly. Just saying "the cells are at 0.4 M" without the reasoning would not earn full credit.

Predict what would happen to the mass of a potato core placed in a 1.5 M sucrose solution. Explain the cellular mechanism, including the role of tonicity.

✓ Model answer (earns the point)

The mass of the potato core would decrease substantially (more than the 22% loss seen at 1.0 M). A 1.5 M sucrose solution is hypertonic to the potato cells (which are 0.4 M internally), meaning the solute concentration outside is much higher than inside. As a result, water moves out of the cells by osmosis — from the area of higher water concentration (inside) to the area of lower water concentration (outside). The cells lose water, the plasma membrane pulls away from the cell wall (plasmolysis), and the overall mass of the core drops.

Why it scores: Makes a specific prediction (mass decreases, even more than at 1.0 M), names hypertonic, describes the direction of water movement, and uses the term osmosis. Bonus points for mentioning plasmolysis explicitly.

Explain why a plant cell can survive in a hypotonic environment without bursting, but a typical animal cell cannot.

✓ Model answer (earns the point)

In a hypotonic environment, water moves into the cell by osmosis because the solute concentration is lower outside than inside. A plant cell has a rigid cell wall made of cellulose outside the plasma membrane. As water enters, the cell swells and presses against the wall, creating turgor pressure. The wall is strong enough to resist this pressure, so the cell becomes turgid but does not burst — this is actually the normal, healthy state for a plant cell. An animal cell lacks a cell wall, so nothing resists the increasing internal pressure. Without that structural support, water continues to enter and the cell ultimately lyses (bursts).

Why it scores: Names the cell wall as the key structural difference, describes turgor pressure as the plant cell's response, contrasts with lysis in the animal cell, and uses the structure–function logic (rigid wall = resistance to pressure). Vague answers like "plant cells are stronger" would not earn the point.

How to score points on AP Biology FRQs

Lead with the answer, then justify. AP graders look for a clear claim followed by the biology that supports it. Don't bury your conclusion.
Name specific biology, not vague concepts. "Plasma membrane pulls away from the cell wall (plasmolysis)" scores higher than "the cell shrinks." "Hypertonic" beats "the outside has more stuff."
Answer the verb in the prompt. "Identify" needs a name only. "Explain" needs a name PLUS a reason. "Predict and justify" needs a prediction PLUS the biology that supports it.
Use the data when it's provided. If there's a table or graph in the stimulus, reference specific values. Cite the data point that supports your claim.
Connect cause to effect. Don't just state what happens — explain WHY. "Water moves out because the solution is hypertonic" is a complete causal chain.