---
title: "AP Bio Unit 2 Review: Cell Structure and Function | Fiveable"
description: "AP Biology Unit 2 covers Cell Structure and Function, Cell Size, Plasma Membrane, and Membrane Permeability. Study guides, practice questions, and key terms."
canonical: "https://fiveable.me/ap-bio/unit-2"
type: "unit"
subject: "AP Biology"
unit: "Unit 2 – Cell Structure and Function"
---

# AP Bio Unit 2 Review: Cell Structure and Function | Fiveable

## Overview

Unit 2 covers how cells are built and how they manage materials. You will learn organelle structure and function, why cell size is constrained by surface area-to-volume ratio, how the phospholipid bilayer controls what crosses the membrane, the difference between passive and active transport, how water moves by osmosis, and how eukaryotic compartmentalization evolved through endosymbiosis.

## AP CED Alignment

This unit hub is organized around AP Course and Exam Description topics, skills, and exam task types when they are available in the source data.
- 2.1: Cell Structure and Function
- 2.2: Cell Size
- 2.3: Plasma Membrane
- 2.4: Membrane Permeability
- 2.5: Membrane Transport
- 2.6: Facilitated Diffusion
- 2.7: Tonicity and Osmoregulation
- 2.8: Mechanisms of Transport
- 2.9: Cell Compartmentalization
- 2.10: Origins of Cell Compartmentalization
- guide: Origins of Cell Compartmentalization Review
- 2.1: Organelles and the endomembrane system
- 2.2: Cell size and surface area-to-volume ratio
- 2.3: Plasma membrane structure and the fluid mosaic model
- 2.4: Membrane permeability and cell walls
- 2.5-2.6: Passive transport and facilitated diffusion
- 2.7: Tonicity, osmosis, and water potential
- 2.8: Active transport and the Na+/K+ pump
- 2.9-2.10: Cell compartmentalization and endosymbiosis
- Science Practice 2 - Visual Representations
- Science Practice 5 - Statistical Tests and Data Analysis
- Science Practice 4 - Representing and Describing Data
- FRQ 6 – Analyze Data (Short)
- FRQ 2 – Interpreting and Evaluating Experimental Results with Graphing (Long)
- FRQ 5 – Analyze Model or Visual Representation (Short)

## Topics

- [2.1: Cell Structure and Function](/ap-bio/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9): 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.
- [2.2: Cell Size](/ap-bio/unit-2/cell-structure-function/study-guide/znjrRPCY6596o2nWt05n): 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.
- [2.3: Plasma Membrane](/ap-bio/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG): 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.
- [2.4: Membrane Permeability](/ap-bio/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi): 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.
- [2.5: Membrane Transport](/ap-bio/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ): 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.
- [2.6: Facilitated Diffusion](/ap-bio/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P): 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.
- [2.7: Tonicity and Osmoregulation](/ap-bio/unit-2/tonicity-osmoregulation/study-guide/i3qUckt9PGfT4pQlHq5B): 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.
- [2.8: Mechanisms of Transport](/ap-bio/unit-2/mechanisms-of-transport/study-guide/CCENUydXNVb7K4P7ikp7): 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.
- [2.9: Cell Compartmentalization](/ap-bio/unit-2/mechanisms-transport/study-guide/sMlGCA9J4sGVE7S24xBM): 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.
- [2.10: Origins of Cell Compartmentalization](/ap-bio/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu): 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.
- [guide: Origins of Cell Compartmentalization Review](/ap-bio/unit-2/origins-of-compartmentalization/study-guide/GC84zJJwIDYFWwSzFPNy): AP Biology Topic 2.10 explained: endosymbiotic theory, evidence for mitochondria and chloroplasts, and prokaryotic vs. eukaryotic compartmentalization, plus practice.

## Hardest Topics And Analytics

Snapshot: practice snapshot
This snapshot uses Fiveable practice activity to show where students tend to miss questions and which review moves are worth prioritizing first.
- **66% average MCQ accuracy** (Across 54k multiple-choice practice attempts for this unit.)
- **54k MCQ attempts** (Practice activity included in this snapshot.)
- **63% average FRQ score** (Across 96 scored free-response attempts for this unit.)
- **2.3: Plasma Membrane**: 40% MCQ miss rate across 6408 attempts. Review Plasma Membrane with attention to how the concept appears in AP-style source and evidence questions.
- **2.2: Cell Size**: 39% MCQ miss rate across 6257 attempts. Review Cell Size with attention to how the concept appears in AP-style source and evidence questions.
- **2.4: Membrane Permeability**: 35% MCQ miss rate across 5372 attempts. Review Membrane Permeability with attention to how the concept appears in AP-style source and evidence questions.
- **2.6: Facilitated Diffusion**: 34% MCQ miss rate across 3973 attempts. Review Facilitated Diffusion with attention to how the concept appears in AP-style source and evidence questions.

## Review Notes

### 2.1: Organelles and the endomembrane system

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.

- **Ribosome**: Non-membrane-bound structure of rRNA and protein that synthesizes proteins by translating mRNA; present in all cells.
- **Rough ER**: Membrane network studded with ribosomes; synthesizes, folds, and begins modifying proteins destined for secretion or membrane insertion.
- **Golgi complex**: Stack of membrane sacs that receives proteins from the ER, modifies them (including glycosylation), and packages them into vesicles for delivery.
- **Lysosome**: Membrane-bound organelle containing acid hydrolases that digest cellular waste, foreign particles, and damaged components.
- **Vesicle**: Small membrane-bound sac that transports cargo between organelles or to the plasma membrane via the secretory pathway.

**Checkpoint:** Trace a secretory protein from ribosome synthesis on the rough ER through the Golgi complex and out of the cell via exocytosis, naming each organelle and what happens at each step.

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

### 2.2: Cell size and surface area-to-volume ratio

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.

- **Surface area-to-volume ratio**: Ratio of membrane surface to internal volume; smaller cells have higher ratios and exchange materials more efficiently.
- **Membrane folding**: Structural adaptation such as microvilli or cristae that increases functional surface area without proportionally increasing volume.
- **Diffusion-limited cell size**: Principle that cells cannot grow indefinitely because diffusion becomes too slow to supply the interior as volume increases.

**Checkpoint:** A cube with side length 1 cm has SA:V = 6. A cube with side length 2 cm has SA:V = 3. Explain what this means for material exchange efficiency as cell size increases.

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

### 2.3: Plasma membrane structure and the fluid mosaic model

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.

- **Phospholipid bilayer**: Double layer of amphipathic phospholipids with hydrophilic heads outward and hydrophobic tails inward; forms the structural core of all cell membranes.
- **Fluid mosaic model**: Model describing the membrane as a dynamic bilayer in which proteins, cholesterol, glycoproteins, and glycolipids move laterally within the phospholipid framework.
- **Cholesterol**: Steroid embedded in animal cell membranes that stabilizes fluidity across temperature changes.
- **Glycoprotein**: Membrane protein with attached carbohydrate chains on the extracellular surface; involved in cell recognition and signaling.

**Checkpoint:** Explain why the hydrophobic interior of the phospholipid bilayer is essential for selective permeability, and describe how cholesterol affects membrane behavior at different temperatures.

### 2.4: Membrane permeability and cell walls

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.

- **Selective permeability**: Property of the plasma membrane that allows some molecules to cross freely while blocking others, based on size, polarity, and charge.
- **Simple diffusion**: Passive movement of small nonpolar molecules directly through the phospholipid bilayer from high to low concentration, requiring no energy or proteins.
- **Cell wall**: Rigid layer outside the plasma membrane in bacteria, archaea, fungi, and plants that provides structural support and protection from osmotic lysis.

**Checkpoint:** Rank the following by ease of crossing the plasma membrane without a transport protein: Na+ ion, O2, glucose, H2O. Justify each ranking using membrane structure.

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

### 2.5-2.6: Passive transport and facilitated diffusion

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.

- **Facilitated diffusion**: Passive transport of ions or large polar molecules down a concentration gradient through channel or carrier proteins, requiring no ATP.
- **Aquaporins**: Channel proteins that allow rapid, facilitated movement of water across membranes; critical for osmoregulation.
- **Gated ion channel**: Channel protein that opens in response to a specific stimulus (ligand or voltage), allowing ions such as Na+ or K+ to cross the membrane.
- **Endocytosis**: Bulk transport process in which the plasma membrane folds inward to form a vesicle that brings large molecules or particles into the cell, requiring energy.
- **Passive transport**: Net movement of molecules from high to low concentration across a membrane without direct ATP input.

**Checkpoint:** Compare how O2 and Na+ each cross the plasma membrane. Name the mechanism, whether energy is required, and what structural feature of the membrane or protein makes each crossing possible.

### 2.7: Tonicity, osmosis, and water potential

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.

- **Water potential**: Measure of the tendency of water to move; calculated as psi = psi-p + psi-s. Water moves from higher to lower water potential.
- **Osmosis**: Passive movement of water across a selectively permeable membrane from higher water potential to lower water potential.
- **Hypertonic**: External solution with higher solute concentration than the cell interior; water leaves the cell, causing it to shrink.
- **Turgor pressure**: Pressure exerted by water against the cell wall in plant cells; maintained by the central vacuole and essential for cell rigidity.
- **Osmoregulation**: Mechanisms cells and organisms use to control internal water and solute balance, such as contractile vacuoles in protists.

**Checkpoint:** A plant cell is placed in a hypertonic solution. Predict what happens to turgor pressure and cell volume, and explain using water potential.

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)

### 2.8: Active transport and the Na+/K+ pump

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.

- **Active transport**: Movement of molecules against a concentration gradient using ATP and membrane transport proteins.
- **Na+/K+ ATPase**: Pump that uses one ATP to move 3 Na+ out and 2 K+ into the cell, maintaining the electrochemical gradient and membrane potential.
- **Electrochemical gradient**: Combined concentration and electrical charge difference across a membrane that drives ion movement and powers secondary active transport.
- **Membrane potential**: Electrical potential difference across the plasma membrane, maintained largely by the Na+/K+ pump and ion gradients.

**Checkpoint:** Explain why the Na+/K+ pump requires ATP while facilitated diffusion of K+ through a channel protein does not. Reference gradient direction in your answer.

### 2.9-2.10: Cell compartmentalization and endosymbiosis

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.

- **Endosymbiosis**: Evolutionary process in which a host cell engulfed a free-living prokaryote, eventually giving rise to mitochondria (from alphaproteobacteria) and chloroplasts (from cyanobacteria).
- **Internal membranes**: Membrane systems within eukaryotic cells that create compartments for specific metabolic processes, increasing efficiency and surface area.
- **Prokaryotic cells**: Cells lacking membrane-bound organelles and a true nucleus; genetic material is in a nucleoid region; still have specialized internal functional regions.
- **Eukaryotic cells**: Cells with a nucleus and membrane-bound organelles that compartmentalize distinct metabolic processes.

**Checkpoint:** List three pieces of evidence that support endosymbiotic theory for the origin of mitochondria, and explain what each piece of evidence suggests about the ancestral relationship.

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

## Study Guides

- [2.4 Membrane Permeability](/ap-bio/unit-2/plasma-membranes/study-guide/1aW0ZDGzS56ism3BJwTi)
- [2.3 Plasma Membrane](/ap-bio/unit-2/cell-size/study-guide/3oB8hJyGwvYACz8XlUmG)
- [2.6 Facilitated Diffusion](/ap-bio/unit-2/membrane-transport/study-guide/7yV52dtD7Vj7ZadTEd2P)
- [2.10 Origins of Cell Compartmentalization](/ap-bio/unit-2/cell-compartmentalization/study-guide/HRfoDYQgTXrvyzemUlwu)
- [2.2 Cell Size](/ap-bio/unit-2/cell-structure-function/study-guide/znjrRPCY6596o2nWt05n)
- [2.5 Membrane Transport](/ap-bio/unit-2/membrane-permeability/study-guide/1114cAD5d5VyivEBDKDJ)
- [2.8 Mechanisms of Transport](/ap-bio/unit-2/mechanisms-of-transport/study-guide/CCENUydXNVb7K4P7ikp7)
- [Origins of Cell Compartmentalization Review](/ap-bio/unit-2/origins-of-compartmentalization/study-guide/GC84zJJwIDYFWwSzFPNy)
- [2.7 Tonicity and Osmoregulation](/ap-bio/unit-2/tonicity-osmoregulation/study-guide/i3qUckt9PGfT4pQlHq5B)
- [2.1 Cell Structure and Function](/ap-bio/unit-2/cell-structure-subcellular-components/study-guide/oFM5gT3D8Pj5lZXmTNB9)
- [2.9 Cell Compartmentalization](/ap-bio/unit-2/mechanisms-transport/study-guide/sMlGCA9J4sGVE7S24xBM)

## Practice Preview

### Multiple-choice practice

- **Stimulus-based practice question**: Science Practice 2 - Visual Representations | Which of the following best explains how the data illustrate membrane potential principles?
- **Stimulus-based practice question**: Science Practice 5 - Statistical Tests and Data Analysis | Which decision about the null hypothesis is best supported by the 60-minute data?
- **Stimulus-based practice question**: Science Practice 5 - Statistical Tests and Data Analysis | Which conclusion about the null hypothesis is best supported?
- **Stimulus-based practice question**: Science Practice 5 - Statistical Tests and Data Analysis | What conclusion about the null hypothesis is supported at $\alpha = 0.05$?
- **Stimulus-based practice question**: Science Practice 4 - Representing and Describing Data | Which graph modification is needed to represent both variables accurately?
- **Stimulus-based practice question**: Science Practice 4 - Representing and Describing Data | Why does the graph give an inaccurate isotonic point?

### FRQ practice

- **CFTR protein localization and chloride transport dysfunction**: FRQ 6 – Analyze Data (Short) | CFTR protein localization and chloride transport dysfunction
- **CFTR protein hydrophobic amino acid interactions**: FRQ 2 – Interpreting and Evaluating Experimental Results with Graphing (Long) | CFTR protein hydrophobic amino acid interactions
- **Endomembrane system protein synthesis and secretion**: FRQ 5 – Analyze Model or Visual Representation (Short) | Endomembrane system protein synthesis and secretion

## Key Terms

- **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.

## Common Mistakes

- **Confusing the direction of osmosis with solute movement**: 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.
- **Treating facilitated diffusion as active transport**: 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.
- **Misidentifying which organelles belong to the endomembrane system**: 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.
- **Applying SA:V reasoning backwards**: 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.
- **Overstating endosymbiotic evidence**: 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.

## Exam Connections

- **Structure-to-function explanation tasks**: 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.
- **Quantitative water potential and SA:V problems**: 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.
- **Experimental design and evidence evaluation for endosymbiosis and transport**: 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.

## Final Review Checklist

- **Trace the secretory pathway**: Starting from a ribosome on the rough ER, follow a secretory protein through the Golgi complex, into a vesicle, and out of the cell via exocytosis. Name what happens at each organelle.
- **Explain SA:V and its consequences**: Calculate or compare SA:V for cells of different sizes. Explain why smaller cells exchange materials more efficiently and describe two structural adaptations that compensate for low SA:V.
- **Connect membrane structure to permeability**: For any given molecule (O2, glucose, Na+, H2O), explain whether it crosses the membrane freely, by facilitated diffusion, or by active transport, and link the answer to the molecule's size, polarity, and charge.
- **Apply the water potential equation**: Use psi = psi-p + psi-s and psi-s = -iCRT to predict the direction of water movement between two solutions or compartments. Practice with both plant cell and open-solution scenarios.
- **Distinguish passive from active transport**: For each transport type (simple diffusion, facilitated diffusion, active transport, endocytosis, exocytosis), state whether ATP is required and whether movement is with or against the concentration gradient.
- **Explain the Na+/K+ pump mechanism**: Describe the 3 Na+ out / 2 K+ in stoichiometry, the role of ATP hydrolysis, and why this pump is essential for maintaining membrane potential and enabling secondary active transport.
- **Evaluate endosymbiotic evidence**: List at least three structural or molecular features of mitochondria and chloroplasts that support endosymbiotic theory, and explain what each feature implies about their prokaryotic ancestry.

## Study Plan

- **Step 1: Organelles and the endomembrane system (Topic 2.1)**: Read the topic guide for 2.1 and draw the secretory pathway from ribosome to exocytosis. Label each organelle and write one sentence about what it does. Use the key terms list to check that you can define endomembrane system, rough ER, Golgi complex, lysosome, and vesicle without looking.
- **Step 2: Cell size and membrane structure (Topics 2.2-2.3)**: Work through the SA:V concept by calculating ratios for cubes of different sizes. Then review the fluid mosaic model: sketch the bilayer and label phospholipid heads and tails, cholesterol, integral proteins, and glycoproteins. Connect each component to its function.
- **Step 3: Permeability and transport mechanisms (Topics 2.4-2.6)**: Build a comparison of all transport types: simple diffusion, facilitated diffusion (channels and carriers), active transport, endocytosis, and exocytosis. For each, note the direction relative to the gradient, whether ATP is needed, and what type of molecule uses it. Use practice questions to test your ability to classify transport scenarios.
- **Step 4: Tonicity, water potential, and active transport (Topics 2.7-2.8)**: Practice the water potential equation with numerical problems: calculate psi-s using psi-s = -iCRT, then determine water movement direction. Review the Na+/K+ pump stoichiometry and explain how it maintains the electrochemical gradient. Use the AP score calculator to estimate how these quantitative topics affect your overall score.
- **Step 5: Compartmentalization and endosymbiosis (Topics 2.9-2.10)**: Review the benefits of eukaryotic compartmentalization and list the evidence for endosymbiotic theory. Practice explaining the prokaryote-to-eukaryote comparison using the comparison table. Then do a timed review of all unit key terms to consolidate vocabulary before attempting FRQ practice.

## More Ways To Review

- [Topic study guides](/ap-bio/unit-2#topics)
- [FRQ practice](/ap-bio/frq-practice)
- [Cram archive videos](/cram-archives?subject=ap-biology&unit=unit-2)
- [Cheatsheets](/ap-bio/cheatsheets/unit-2)
- [Key terms](/ap-bio/key-terms)

## FAQs

### What topics are covered in AP Bio Unit 2?

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](/ap-bio/unit-2) for matched study resources.

### How much of the AP Bio exam is Unit 2?

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.

### What's on the AP Bio Unit 2 progress check (MCQ and FRQ)?

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).

### How do I practice AP Bio Unit 2 FRQs?

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](/ap-bio/unit-2).

### Where can I find AP Bio Unit 2 practice questions?

The best place to find AP Bio Unit 2 practice questions, including multiple-choice and practice test sets, is [AP Bio Unit 2](/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.

### How should I study AP Bio Unit 2?

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](/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.

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