Review AP Bio Unit 2 to understand how cell structure drives function, from organelle roles and membrane composition to transport mechanisms and the evolutionary origins of compartmentalization. This unit covers 10-13% of the AP Biology exam and builds the cellular foundation for Units 3, 4, and beyond.
Use the topic guides, key terms, and practice questions available for this unit to work through membrane structure, transport types, and endosymbiosis systematically.
Unit 2 asks one central question: how does the structure of a cell, from its organelles to its membrane, allow it to function as a living system? Every topic in this unit connects back to that question, whether you are explaining why ribosomes are universal, why small cells exchange materials more efficiently, or why the Na+/K+ pump requires ATP.
Ribosomes synthesize proteins using mRNA and are found in all life forms, reflecting common ancestry. The endomembrane system, including the rough ER, smooth ER, Golgi complex, lysosomes, vacuoles, and transport vesicles, works as a coordinated network to modify, package, and ship proteins and lipids. Mitochondria and chloroplasts sit outside this system and have their own double membranes and DNA.
The plasma membrane is a phospholipid bilayer described by the fluid mosaic model, with embedded proteins, cholesterol, glycoproteins, and glycolipids. Its hydrophobic interior makes it selectively permeable: small nonpolar molecules like O2 and CO2 cross freely, while ions and large polar molecules need channel or carrier proteins. Transport ranges from passive diffusion down a gradient to active transport against a gradient using ATP, plus bulk transport via endocytosis and exocytosis.
Water moves by osmosis from regions of high water potential to low water potential, described by the equation psi = psi-p + psi-s. Cells regulate water balance through osmoregulatory structures like contractile vacuoles and central vacuoles. Eukaryotic internal membranes compartmentalize reactions, increasing efficiency and surface area, and mitochondria and chloroplasts evolved from endosymbiotic prokaryotes.
From the shape of a phospholipid tail to the double membrane of a mitochondrion, every structural feature in Unit 2 exists because it supports a specific function. The AP Biology exam consistently asks you to explain why a structure enables a process, not just name it. Build the habit of connecting each organelle, membrane component, or transport protein to the job it performs and the conditions it requires.
Ribosomes, the endomembrane system (rough ER, smooth ER, Golgi complex, lysosomes, vacuoles, vesicles), mitochondria, and chloroplasts each have structures matched to specific cellular functions. The secretory pathway moves proteins from ribosomes through the ER and Golgi to their final destinations.
Surface area-to-volume ratio limits cell size. Smaller cells have higher SA:V and exchange materials more efficiently. Membrane folds like microvilli and cristae increase functional surface area. Larger organisms rely on bulk transport systems to compensate for low SA:V.
The plasma membrane is a phospholipid bilayer with embedded proteins, cholesterol, glycoproteins, and glycolipids. The fluid mosaic model describes how all components move laterally. Cholesterol stabilizes fluidity. Hydrophilic heads face aqueous environments; hydrophobic tails face inward.
The hydrophobic interior of the bilayer creates selective permeability. Small nonpolar molecules cross freely; ions and large polar molecules require transport proteins. Cell walls in bacteria, archaea, fungi, and plants add structural support and protection from osmotic lysis.
Passive transport moves molecules down concentration gradients without ATP. Active transport moves molecules against gradients using ATP. Endocytosis and exocytosis use energy to move bulk material into or out of cells via vesicle formation and membrane fusion.
Facilitated diffusion uses channel or carrier proteins to move charged ions and large polar molecules down their concentration gradients without ATP. Aquaporins facilitate rapid water movement. Ion channels can be gated, opening in response to ligands or voltage changes.
Water moves by osmosis from high to low water potential (psi = psi-p + psi-s). Solute potential is calculated with psi-s = -iCRT. Hypotonic, hypertonic, and isotonic conditions predict water movement direction. Contractile vacuoles and central vacuoles are key osmoregulatory structures.
Active transport requires ATP and membrane proteins. The Na+/K+ ATPase pumps 3 Na+ out and 2 K+ in per ATP, maintaining the electrochemical gradient and membrane potential. This gradient drives secondary active transport and is essential for cellular homeostasis.
Eukaryotic internal membranes separate incompatible reactions, maintain distinct chemical environments in each organelle, and increase surface area for membrane-bound processes. Lysosomes require low pH for their hydrolases; mitochondrial cristae maximize surface area for ATP synthesis.
Mitochondria and chloroplasts evolved from free-living prokaryotes via endosymbiosis. Evidence includes double membranes, circular DNA, 70S ribosomes, and binary fission-like replication. Prokaryotes lack membrane-bound organelles but have specialized internal regions.
AP Biology Topic 2.10 explained: endosymbiotic theory, evidence for mitochondria and chloroplasts, and prokaryotic vs. eukaryotic compartmentalization, plus practice.
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Review Plasma Membrane with attention to how the concept appears in AP-style source and evidence questions.
Review Cell Size with attention to how the concept appears in AP-style source and evidence questions.
Review Membrane Permeability with attention to how the concept appears in AP-style source and evidence questions.
Review Facilitated Diffusion with attention to how the concept appears in AP-style source and evidence questions.
Each subcellular component has a structure matched to its function. Ribosomes are non-membrane-bound structures made of rRNA and protein that translate mRNA into proteins; they appear in all life forms, which is evidence of common ancestry. The endomembrane system is a coordinated network: rough ER studded with ribosomes synthesizes and folds proteins, smooth ER synthesizes lipids and detoxifies, the Golgi complex modifies and packages products via glycosylation, and vesicles shuttle cargo to lysosomes, vacuoles, or the plasma membrane. Mitochondria and chloroplasts are not part of the endomembrane system and have their own double membranes.
| Organelle | Membrane-bound? | Key function |
|---|---|---|
| Ribosome | No | Protein synthesis from mRNA |
| Rough ER | Yes | Protein synthesis, folding, initial modification |
| Golgi complex | Yes | Modification, sorting, packaging of proteins and lipids |
| Lysosome | Yes | Intracellular digestion of waste and foreign material |
| Mitochondrion | Yes (double) | ATP production via cellular respiration |
A cell's ability to exchange materials with its environment depends on how much membrane surface area it has relative to its volume. As a cell grows, volume increases faster than surface area, so the SA:V ratio drops. A lower SA:V means slower exchange of nutrients, wastes, and gases relative to the cell's metabolic demands. Cells compensate with structural adaptations: membrane folds (cristae in mitochondria, microvilli in intestinal cells) increase functional surface area without increasing overall cell size. Multicellular organisms use bulk transport systems like the circulatory system to overcome SA:V limits at the organismal level.
| Cell size | SA:V ratio | Exchange efficiency |
|---|---|---|
| Small cell | High | Efficient; membrane area meets metabolic demand |
| Large cell | Low | Less efficient; interior farther from membrane |
| Folded membrane (e.g., microvilli) | Effectively higher | Compensates for large size by increasing functional surface |
The plasma membrane is built from a phospholipid bilayer in which hydrophilic phosphate heads face the aqueous environments on each side and hydrophobic fatty acid tails face inward. Cholesterol is embedded among the phospholipids and moderates membrane fluidity: it prevents membranes from becoming too rigid at low temperatures and too fluid at high temperatures. Proteins embedded in or attached to the bilayer serve as channels, carriers, receptors, and structural anchors. Glycoproteins and glycolipids project from the outer surface and function in cell recognition and signaling. The fluid mosaic model captures the key idea that all these components move laterally within the membrane.
Selective permeability results from the hydrophobic interior of the bilayer. Small nonpolar molecules (O2, CO2, N2) cross freely by simple diffusion. Small uncharged polar molecules like H2O and NH3 cross slowly in small amounts. Ions and large polar molecules cannot cross the hydrophobic core and require channel proteins or carrier proteins. Cell walls in bacteria, archaea, fungi, and plants add a structural boundary outside the plasma membrane, providing protection from osmotic lysis and mechanical support. Cell walls are permeable to water and small solutes but restrict the passage of larger molecules.
| Molecule type | Example | Crosses bilayer freely? | Reason |
|---|---|---|---|
| Small nonpolar | O2, CO2 | Yes | Compatible with hydrophobic interior |
| Small polar uncharged | H2O, NH3 | Slowly/in small amounts | Polar but small enough to pass occasionally |
| Large polar | Glucose | No | Too polar and large for hydrophobic core |
| Ions | Na+, K+ | No | Charge repelled by hydrophobic interior |
Passive transport moves molecules from high to low concentration without ATP input. Simple diffusion applies to small nonpolar molecules. Facilitated diffusion uses channel proteins or carrier proteins to move ions and large polar molecules down their concentration gradient. Channel proteins form hydrophilic pores; ion channels are often gated, opening in response to specific stimuli. Carrier proteins bind a specific solute and change shape to move it across. Aquaporins are specialized channel proteins that allow rapid water movement across membranes. Endocytosis and exocytosis are bulk transport processes that require energy: endocytosis folds the plasma membrane inward to engulf material, while exocytosis fuses internal vesicles with the plasma membrane to release contents.
Osmosis is the movement of water across a selectively permeable membrane from a region of higher water potential to lower water potential. Water potential (psi) equals pressure potential (psi-p) plus solute potential (psi-s). Solute potential is calculated as psi-s = -iCRT, where i is the ionization constant, C is molar concentration, R is 0.0831 L-bar/mol-K, and T is temperature in Kelvin. Adding solutes lowers solute potential and therefore lowers water potential, drawing water in. A hypotonic external solution causes water to enter the cell; a hypertonic solution causes water to leave. Organisms maintain water balance through osmoregulatory structures: contractile vacuoles in protists pump out excess water, and the central vacuole in plant cells maintains turgor pressure.
| Tonicity of external solution | Water movement | Effect on animal cell | Effect on plant cell |
|---|---|---|---|
| Hypotonic | Into cell | Swells, may lyse | Swells, turgor pressure increases |
| Isotonic | No net movement | No change | No change |
| Hypertonic | Out of cell | Shrinks (crenation) | Plasmolysis (membrane pulls from wall) |
Active transport moves molecules against their concentration gradient and requires ATP. Membrane proteins are essential: they bind the solute and use energy from ATP hydrolysis to change conformation and move the solute across. The Na+/K+ ATPase is the primary example: it pumps 3 Na+ out of the cell and 2 K+ in per ATP hydrolyzed, maintaining the electrochemical gradient across the plasma membrane. This gradient is used by other transport processes (secondary active transport) and is critical for membrane potential. Without active transport, concentration gradients established by passive transport would eventually equilibrate.
Eukaryotic cells use internal membranes to partition the cell into specialized compartments. This compartmentalization keeps competing reactions separate, allows each organelle to maintain unique chemical conditions (such as the low pH inside lysosomes), and increases the surface area available for membrane-bound reactions. Prokaryotes lack membrane-bound organelles but still organize functions in specialized internal regions. Endosymbiotic theory explains that mitochondria and chloroplasts evolved from free-living prokaryotes engulfed by a host cell. Evidence includes their double membranes, circular DNA, 70S ribosomes similar to bacteria, and binary fission-like division.
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | Absent (nucleoid region) | Present, membrane-bound |
| Membrane-bound organelles | Absent | Present (ER, Golgi, mitochondria, etc.) |
| Ribosomes | 70S | 80S cytoplasmic; 70S in mitochondria/chloroplasts |
| Internal membranes | Limited (some invaginations) | Extensive endomembrane system |
| DNA | Circular, in cytoplasm | Linear in nucleus; circular in mitochondria/chloroplasts |
Try stimulus-based AP practice questions and written prompts after you review the notes.
| Term | Definition |
|---|---|
| Endomembrane System | Network of membrane-bound organelles including the nuclear envelope, ER, Golgi complex, lysosomes, vacuoles, and vesicles that work together to modify, package, and transport proteins and lipids within eukaryotic cells. |
| Fluid-Mosaic Model | Model of membrane structure in which phospholipids, proteins, cholesterol, glycoproteins, and glycolipids move laterally within a dynamic bilayer, enabling selective permeability and cell communication. |
| Phospholipid Bilayer | Double layer of amphipathic phospholipids with hydrophilic heads facing aqueous environments and hydrophobic tails facing inward, forming the structural core of all cell membranes. |
| Selective Permeability | Property of the plasma membrane that allows small nonpolar molecules to cross freely while blocking ions and large polar molecules, which require transport proteins. |
| Surface area-to-volume ratio | Ratio of membrane surface area to internal volume; smaller cells have higher ratios and exchange materials more efficiently. Decreases as cell size increases, limiting maximum cell size. |
| Aquaporins | Channel proteins that allow rapid facilitated movement of water across membranes; critical for osmosis and osmoregulation in cells and organisms. |
| Active Transport | Movement of molecules against a concentration gradient across a membrane using ATP and specialized membrane transport proteins. |
| Na⁺/K⁺ATPase | Membrane pump that uses one ATP to move 3 Na+ out of and 2 K+ into the cell, maintaining the electrochemical gradient and membrane potential. |
| Water Potential | Measure of the tendency of water to move across a membrane; calculated as psi = psi-p + psi-s. Water moves from higher to lower water potential. |
| Osmoregulation | Mechanisms used by cells and organisms to maintain water and solute balance, including contractile vacuoles in protists and the central vacuole in plant cells. |
| Endosymbiosis | Evolutionary process in which a host cell engulfed free-living prokaryotes, giving rise to mitochondria (from alphaproteobacteria) and chloroplasts (from cyanobacteria) in eukaryotic cells. |
| Endocytosis | Energy-requiring bulk transport process in which the plasma membrane folds inward to form a vesicle that brings large molecules or particles into the cell. |
| Ribosome | Non-membrane-bound structure of rRNA and protein that synthesizes proteins by translating mRNA; present in all known life forms, reflecting common ancestry. |
| turgor pressure | Pressure exerted by water against the plant cell wall, maintained by the central vacuole; essential for cell rigidity and lost when cells are placed in hypertonic solutions. |
Water moves toward the region of lower water potential (higher solute concentration), not toward the region of higher water concentration. Students often say water moves toward water, which reverses the correct direction. Always anchor osmosis to water potential, not solute movement.
Facilitated diffusion uses channel or carrier proteins but requires no ATP. The defining feature of active transport is energy input to move molecules against a gradient. Using a protein does not automatically mean energy is required.
Mitochondria and chloroplasts are not part of the endomembrane system. The endomembrane system includes the nuclear envelope, ER, Golgi complex, lysosomes, vacuoles, vesicles, and the plasma membrane. Mixing these up leads to errors in secretory pathway questions.
Larger cells have lower SA:V ratios, not higher. Students sometimes state that larger cells are more efficient at exchange because they have more total surface area. Total surface area is not the relevant measure; the ratio to volume is what determines exchange efficiency.
Endosymbiotic theory applies specifically to mitochondria and chloroplasts, not to all membrane-bound organelles. The ER and Golgi complex evolved through membrane invagination, not endosymbiosis. Citing double membranes as evidence only works for organelles with prokaryotic ancestors.
AP Biology frequently asks you to explain how a structural feature of a membrane, organelle, or cell enables a specific function. For Unit 2, practice explaining why the hydrophobic bilayer interior creates selective permeability, why cristae increase ATP synthesis surface area, and why smaller cells exchange materials more efficiently. These questions require you to connect a described structure to a mechanism, not just name the organelle.
Unit 2 includes two quantitative skills the exam tests directly: calculating or comparing SA:V ratios for cells of different sizes, and using the water potential equations (psi = psi-p + psi-s and psi-s = -iCRT) to predict water movement. Expect to interpret data tables or experimental setups where you must determine which way water moves between compartments or explain why a cell changes volume.
The exam may present experimental data and ask you to evaluate whether it supports a claim about membrane transport or evolutionary origin. For transport, you might analyze data showing how inhibiting the Na+/K+ pump affects cell volume or membrane potential. For endosymbiosis, you might evaluate whether a described feature (such as circular DNA or 70S ribosomes in an organelle) supports or challenges the endosymbiotic model.
Open the individual guides for Unit 2 when you want a closer review of one topic.
browse guidesPractice free-response reasoning and compare your answer with scoring guidance.
practice FRQsWatch past review streams filtered to Unit 2 when you want a video walkthrough.
open videosUse unit cheatsheets for a quick visual review after you work through the notes.
open cheatsheetsEstimate your broader AP score goal after you review the course and exam format.
open calculatorAP 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.