AP Biology Unit 2 Review – Cell Structure and Function (2025 Guide)

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

1. Draw and Label Organelles – Include function for each structure.
2. Practice Osmosis & Diffusion Labs – Use graphs to interpret data.
3. Memorize Transport Types – Group by passive vs. active.
4. 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.