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AP Biology Unit 2, Cells, covers cell structure and function across 10 topics, worth 10-13% of the AP exam, with the cell membrane at the center of how cells control what moves in and out. You'll compare prokaryotic and eukaryotic cells, map out organelles like the nucleus and mitochondria, and get into the plasma membrane in real detail. Cell transport is a big chunk of this unit, from simple diffusion to facilitated transport, tonicity, and osmoregulation. AP Bio Unit 2 also covers how compartmentalization evolved and why it matters for keeping cellular processes organized.
AP Biology Unit 2, Cell Structure and Function, is about how cells are built and how their membranes control what gets in and out. The single biggest idea is the plasma membrane as a selectively permeable barrier that maintains homeostasis, the cell's stable internal environment. This unit is worth 10-13% of the AP exam, and it covers organelles, surface area-to-volume ratios, every type of membrane transport, tonicity and water potential, and the endosymbiotic origins of compartmentalized cells.
| Transport type | Energy needed? | Direction vs. gradient | Helper proteins? | Example |
|---|---|---|---|---|
| Simple diffusion | No | High to low | No | O₂, CO₂ crossing the membrane |
| Osmosis | No | High to low water potential | No (or aquaporins) | Water entering a cell |
| Facilitated diffusion | No | High to low | Yes (channel/carrier) | Na⁺, K⁺ through channels |
| Active transport | Yes (ATP) | Low to high (against) | Yes (pump) | Na⁺/K⁺ pump |
| Endocytosis | Yes | Into cell, bulk | Membrane folds in | Cell taking in large particles |
| Exocytosis | Yes | Out of cell, bulk | Vesicle fusion | Secreting large molecules |
This unit anchors the course's themes of structure determines function and homeostasis. The membrane isn't just a wall; its molecular design is exactly what lets a cell control its insides, and almost every later process depends on that control.
Unit 2 is 10-13% of the AP exam, so it's a reliable source of points. Expect multiple-choice questions that ask you to predict water movement given tonicity, identify organelle functions, or explain why a molecule can or can't cross the membrane based on its structure and charge.
On the free-response side, water potential is a favorite for calculation and graphing. You might solve for ψ, plug values into ψₛ = −iCRT, then explain which direction water moves and why. Conceptual FRQs ask you to explain how structure leads to function (for example, how the hydrophobic interior creates selective permeability) or describe and justify how a cell maintains homeostasis under hypertonic or hypotonic conditions. SA:V questions often pair a calculation with an explanation of why small cells exchange materials more efficiently. Across question types, you're doing more than recall: you connect a structure to what it does, predict an outcome, and justify it with a clear cause-and-effect chain.
AP Bio Unit 2 covers 10 topics focused on cell structure and the cell membrane. The full topic list includes: Cell Structure and Function (2.1), Cell Size (2.2), Plasma Membrane (2.3), Membrane Permeability (2.4), Membrane Transport (2.5), Facilitated Diffusion (2.6), Tonicity and Osmoregulation (2.7), Mechanisms of Transport (2.8), Cell Compartmentalization (2.9), and Origins of Cell Compartmentalization (2.10). Together these topics build from comparing prokaryotic and eukaryotic cells and their organelles all the way through how cells move materials across membranes. See AP Bio Unit 2 for matched study resources.
AP Bio Unit 2 makes up 10-13% of the AP exam, making it one of the foundational units you'll want to know well. The unit covers cell structure and function, the cell membrane and plasma membrane, membrane transport, and how eukaryotic cell organelles are compartmentalized. That's a solid chunk of multiple-choice questions and a common source of free-response prompts.
The AP Bio Unit 2 progress check includes both MCQ and FRQ parts that draw from all 10 topics in the unit. The MCQ section tests your ability to interpret diagrams of cell structure, identify organelles, and reason through cell transport scenarios like facilitated diffusion and tonicity. The FRQ part typically asks you to explain how the plasma membrane regulates what enters and exits a cell, or to predict what happens to a cell placed in a hypertonic or hypotonic solution. The best way to prep for the progress check is to practice with questions matched to each topic. You can find those at AP Bio Unit 2.
AP Bio Unit 2 FRQs most often come from membrane transport, tonicity and osmoregulation, and cell compartmentalization, so those are the topics to prioritize. College Board free-response questions in this unit typically ask you to explain a mechanism (like how facilitated diffusion works), analyze experimental data about cell transport, or justify why a cell's structure supports a specific function. To practice effectively, write out full explanations using precise vocabulary like "plasma membrane," "concentration gradient," and "selectively permeable," then check your reasoning against the scoring guidelines. You'll find practice FRQs organized by topic at AP Bio Unit 2.
The best place to find AP Bio Unit 2 practice questions, including multiple-choice and practice test sets, is AP Bio Unit 2. That page organizes MCQ and FRQ practice by topic, so you can target cell structure, plasma membrane permeability, cell transport, or any of the other 10 topics in the unit. Working through topic-specific MCQs before doing a full practice test helps you spot exactly where your understanding of organelles or membrane mechanisms needs work.
Start AP Bio Unit 2 by building a clear picture of the cell membrane and how it controls what moves in and out of a cell, since that concept threads through almost every topic in the unit. Here's a study plan that works: 1. **Compare cell types first.** Sketch out the differences between prokaryotic and eukaryotic cells and label key organelles. Knowing structure before function makes everything else click. 2. **Learn transport in order.** Go from simple diffusion to facilitated diffusion to active transport. Each builds on the last, and the AP exam loves asking you to distinguish between them. 3. **Do tonicity problems with visuals.** Draw a cell in hypertonic, hypotonic, and isotonic solutions. Predict what happens, then check. This topic shows up in both MCQ and FRQ. 4. **Practice explaining mechanisms out loud.** Unit 2 FRQs reward precise language. Say "the plasma membrane is selectively permeable" instead of "the membrane lets things through." 5. **Use topic-by-topic practice.** Hit each of the 10 topics with focused questions at AP Bio Unit 2 before doing a timed mixed set. Unit 2 is 10-13% of the exam, so the time you put in here pays off across the whole test.