AP Biology Unit 2 Practice Test: Cells MCQs (Free PDF)

Here's a free AP Biology Unit 2 practice test: 20 AP-style MCQs on cells, seven data-based stimulus sets, and a full rationale for every answer, all aligned to the 2025 CED.

Below, three mistakes I see students make on Unit 2 and how to avoid them, plus one worked example so you can see the reasoning.

Unit 2 is 10-13% of the exam (the full 8-unit breakdown sets out the weightings for the rest), and it's where most of the transport, tonicity and organelle-function questions live, so the MCQs get stimulus-dense fast: centrifugation fractionation tables, SA:V ratio tables, permeability data, potato-cylinder osmosis and organelle-abundance comparisons across cell types.

The AP Bio MCQ section is 60 questions in 90 minutes, and most of them anchor to a stimulus (see our MCQ strategy guide for the general playbook). Unit 2 leans especially hard on tabular data, because so much of cell biology is about comparing conditions: cell A vs cell B, hypertonic vs hypotonic, with inhibitor vs without, cell size vs SA:V ratio.

The stimulus formats to expect are:

• Potato-cylinder mass-change tables (osmosis)
• Red-blood-cell tonicity data
• Centrifugation fractionation with marker enzymes
• Glucose-uptake rate tables that compare facilitated diffusion with and without an inhibitor
• Organelle-abundance percentages across different cell types

The practice pack mirrors that mix. 14 of the 20 questions sit on a data-table stimulus (most stimuli support two or three questions in a row, matching the real paper), and every question is tagged to a Unit 2 topic and a named Science Practice.

Every year I see students walk into Unit 2 MCQs making the same three moves. Catch yourself doing any of them and your score jumps a couple of marks with no extra content revision.

Students see a transport protein in the diagram and immediately pick 'active transport'. That's wrong roughly half the time. Channels and carrier proteins can support facilitated diffusion, which is passive: it moves substances down their gradient without ATP.

Active transport is defined by two things at once, it needs energy AND it moves substances against the gradient (the Na+/K+ ATPase is the canonical example). This is one of the top Unit 2 traps.

The fix is to check the direction of movement relative to the gradient before you check for a protein. If it's downhill, it's passive, even if a protein is involved.

Tonicity terms are relative, not absolute. A solution isn't 'hypertonic' full stop; it's hypertonic to something. Students routinely write 'the cell is in a hypertonic solution' without specifying which side has more nonpenetrating solute, so on MCQs where the passage inverts the usual 'cell in a beaker' framing they swap water-movement direction. The fix: always identify both sides (the cell and the environment), then draw an arrow showing which side has more solute. Water moves toward the higher-solute side.

When asked why eukaryotic cells have internal membranes (endomembrane system, mitochondrial cristae, thylakoid stacks), students default to 'so things stay separate'. That's only half of it.

The bigger answer is that folded internal membranes dramatically increase the surface area available for membrane-bound reactions. More cristae means more electron transport chain complexes; more thylakoid membrane means more photosystems.

The same reasoning explains why cells stay small: volume grows faster than surface area, so a cell that gets too large can't exchange materials fast enough.

Here's a question straight from the Unit 2 pack. The stimulus: students placed identical potato cylinders (initial mass 5.0 g) into sucrose solutions of different molarities for 90 minutes, then blotted and reweighed each cylinder.

The stem: 'Based on Table 5, what is the best estimate of the sucrose concentration that is isotonic to the potato cells?'

• (A) 0.0 M
• (B) 0.2 M
• (C) 0.3 M
• (D) 0.6 M

Answer: C. In an isotonic solution, water enters and leaves the cell at the same rate, so mass does not change. Interpolating between 0.2 M (+4.0%) and 0.4 M (−4.0%), the zero-crossing sits at about 0.3 M sucrose. In 0.0 and 0.2 M the cylinders gained mass (hypotonic external solutions), and in 0.6 M they lost mass (hypertonic), so those solutions do not match the potato cells osmotically.

Every Unit 2 tonicity MCQ shows a cell in a solution. Before you look at the answer choices, write 'high' or 'low' on the cell side and the solution side based on nonpenetrating solute. Water moves to the high-solute side, so the cell either shrinks (hypertonic external solution) or swells (hypotonic external solution). This is the single highest-value 10-second habit for the unit. It prevents the classic mistake of picking the answer that describes solute movement when the question is asking about water.

Endomembrane-system MCQs usually give you a molecule (typically a secreted protein or a lipid) and ask which organelles it visits. The reliable answer is always:

1. Rough ER (protein synthesis on ribosomes)
2. Transport vesicle
3. Golgi apparatus (modification, sorting)
4. Secretory vesicle
5. Plasma membrane (exocytosis)

For lipids, swap in smooth ER for synthesis. Memorize this pathway once, then any question that names a protein defect ('has no sugar modifications' → Golgi) or a lipid function ('detoxification' → smooth ER) becomes a lookup.

Any MCQ that asks why cells stay small, why bacteria divide instead of growing, why a large cell has trouble exchanging materials, or why intestinal cells have microvilli is really asking about surface-area-to-volume ratio.

Volume scales with the cube of radius; surface area scales with the square. Doubling a cell's size cuts its SA:V ratio in half, which slows diffusion in and waste diffusion out. Recognizing the SA:V hook lets you skip past three of four answer choices and go straight to the one that names surface area or diffusion rate.

Three-pass method.

• First, an untimed pass: no clock, about 90 seconds per question. The goal is diagnostic, to see which stimulus types trip you up.
• Second, a timed pass one week later: 30 minutes for 20 questions, which matches the AP exam pace of about 1.5 minutes per MCQ.
• Third, a review pass: check the answer key at the back of the PDF, revise the weak topics on Cognito's AP Biology notes, then retry the missed questions a week after that.

If you want a fuller sit-a-paper session, our mock exam guide covers pacing for the full three hours. The gap between passes is where the retention comes from.