AP Biology Unit 3 Review – Cellular Energetics (2025 Exam Prep)

Factors that influence enzyme activity:

• Temperature: Increases reaction rate up to an optimal point; high temps cause denaturation
• pH: Each enzyme has an optimal pH; deviation can reduce activity
• Substrate concentration: Rate increases until active sites are saturated
• Inhibitors:
  • Competitive inhibitors bind to the active site
  • Noncompetitive inhibitors bind to another site, changing enzyme shape

3. ATP and Energy Coupling

• ATP: Adenosine triphosphate – main energy currency of the cell
• Energy is stored in phosphate bonds; released when ATP → ADP + Pi
• Energy coupling: Using an exergonic reaction (e.g., cellular respiration) to drive an endergonic reaction (e.g., active transport)

4. Cellular Respiration

Stages:

1. Glycolysis: In cytoplasm; glucose → 2 pyruvate + 2 ATP + NADH
2. Pyruvate Oxidation: Pyruvate → Acetyl-CoA + CO₂ + NADH
3. Krebs Cycle: Produces NADH, FADH₂, ATP, and CO₂
4. Electron Transport Chain (ETC) & Oxidative Phosphorylation: Uses NADH/FADH₂ to power ATP synthase; O₂ is final electron acceptor

Net yield: ~36 ATP per glucose (in eukaryotes)

Fermentation: Occurs without oxygen; produces lactic acid or ethanol + CO₂; regenerates NAD⁺

5. Photosynthesis

Light-Dependent Reactions:

• Occur in thylakoid membranes
• Use light to produce ATP and NADPH; split water to release O₂

Calvin Cycle (Light-Independent Reactions):

• Occurs in stroma
• Uses ATP and NADPH to fix CO₂ into glucose
• Key enzyme: Rubisco

6. Connections Between Photosynthesis & Respiration

• Photosynthesis stores energy; respiration releases it
• Oxygen and glucose from photosynthesis fuel respiration
• CO₂ and water from respiration fuel photosynthesis

Common AP Biology Exam Questions for Unit 3

Multiple Choice Examples:

• Predict the effect of increased temperature on enzyme activity
• Identify where the Krebs cycle occurs in the cell

FRQ Examples:

• Design an experiment to test the effect of pH on catalase activity
• Explain how electron flow in the ETC results in ATP production

Study Tips for Unit 3

1. Know the Pathways: Draw glycolysis, Krebs cycle, and ETC flow charts.
2. Master Enzyme Graphs: Be able to read and explain temperature/pH vs. activity curves.
3. Connect the Cycles: Relate photosynthesis outputs to respiration inputs and vice versa.
4. Practice Experimental Design: Especially for enzyme activity labs.

Frequently Asked Questions

1. How much of the AP Bio exam is Unit 3?
Around 12–16% of the test.

2. Do I need to memorize every step of glycolysis?
Know the major inputs/outputs and where they occur, not every intermediate.

3. How important are enzymes in Unit 3?
Very—enzyme activity and regulation are tested every year.

4. What’s the hardest part of Unit 3?
Linking energy coupling concepts to real cellular processes.

Call to Action

Unit 3 is high-yield and high-impact—mastering enzymes, respiration, and photosynthesis can give you a major score boost.

Use RevisionDojo’s AP Bio practice tests and FRQ library to lock in your understanding and push your score toward a 5.