Unit 2: Cell Structure and Function is a cornerstone of the AP Biology curriculum, making up 10–13% of the AP Biology Exam. This unit bridges chemistry and biology, showing how cellular components work together to sustain life.
In this RevisionDojo Unit 2 Review, we’ll cover:
- Key concepts and organelles you must know
- The fluid mosaic model of membranes
- Types of membrane transport
- The relationship between surface area and cell efficiency
- Common AP exam question types for Unit 2
- Study tips and practice ideas for high scores
Unit 2 Overview
The College Board divides Unit 2 into four big ideas:
- Structure and function of organelles
- Plasma membrane structure and selective permeability
- Membrane transport mechanisms
- Cell size, surface area-to-volume ratio, and efficiency
1. Cell Organelles and Their Functions
Prokaryotic vs. Eukaryotic Cells
- Prokaryotes: No membrane-bound organelles, smaller, DNA in nucleoid region
- Eukaryotes: Membrane-bound organelles, larger, DNA in nucleus
Key Organelles:
- Nucleus: Stores DNA, site of transcription
- Ribosomes: Protein synthesis (free ribosomes → cytosolic proteins; bound ribosomes → proteins for secretion/membranes)
- Endoplasmic Reticulum (ER):
- Rough ER – protein folding and modification
- Smooth ER – lipid synthesis, detoxification
- Golgi Apparatus: Modifies, sorts, and ships proteins/lipids
- Mitochondria: Cellular respiration, ATP production
- Chloroplasts: Photosynthesis in plants/algae
- Lysosomes: Digestive enzymes, breakdown of macromolecules
- Vacuoles: Storage, water balance (large central vacuole in plants)
2. Plasma Membrane Structure
The fluid mosaic model describes membranes as:
- Phospholipid bilayer with hydrophilic heads and hydrophobic tails
- Embedded proteins for transport, signaling, and structural support
- Cholesterol for fluidity regulation
- Carbohydrate chains for cell recognition
3. Membrane Transport
Passive Transport (no energy required):
- Diffusion
- Osmosis (water movement)
- Facilitated diffusion (protein channels/carriers)
Active Transport (requires ATP):
- Sodium-potassium pump
- Proton pump
Bulk Transport:
- Endocytosis (phagocytosis, pinocytosis, receptor-mediated)
- Exocytosis
4. Surface Area-to-Volume Ratio
Smaller cells have a higher surface area-to-volume ratio, allowing more efficient exchange of materials.
AP Tip: Expect calculations comparing surface area-to-volume ratios in cubes or spheres.
5. Connections to Other Units
- Unit 3: Mitochondria and chloroplasts in energy transfer
- Unit 4: Membrane proteins in signal transduction
- Unit 5 & 6: Nucleus role in heredity and gene expression
Common AP Biology Exam Questions for Unit 2
Multiple Choice Examples:
- Identify which organelle is responsible for ATP production.
- Predict the effect of removing ribosomes from a cell.
FRQ Examples:
- Explain how the surface area-to-volume ratio affects cell size limits.
- Describe how membranes maintain homeostasis under environmental change.
Study Tips for Unit 2
- Draw and Label Organelles – Include function for each structure.
- Practice Osmosis & Diffusion Labs – Use graphs to interpret data.
- Memorize Transport Types – Group by passive vs. active.
- Solve SA:V Problems – Be ready for math-based questions.
Frequently Asked Questions
1. How many Unit 2 questions are on the AP Bio exam?
About 10–13% of the total exam.
2. Are prokaryotic structures tested?
Yes—know differences between prokaryotes and eukaryotes.
3. What’s the most tested Unit 2 concept?
Membrane transport and surface area-to-volume ratio.
4. How should I study organelles efficiently?
Use diagrams and actively quiz yourself on structure and function.
Call to Action
Unit 2 builds the bridge between biochemistry and cell biology, and mastering it is key to success on the AP Biology Exam.
👉 Access RevisionDojo’s complete AP Biology review series for unit guides, FRQ practice, and our AP Bio Score Calculator to keep your prep on track for a 5.
