Unit 3: Cellular Energetics is one of the most heavily tested units on the AP Biology Exam, making up 12–16% of the test. It focuses on how cells capture, store, and use energy to sustain life.
In this RevisionDojo Unit 3 Review, we’ll cover:
- Enzyme structure and function
- Environmental effects on enzyme activity
- ATP and energy coupling
- Cellular respiration and fermentation
- Photosynthesis
- Common AP exam question types
- Study tips to maximize points in this unit
Unit 3 Overview
The College Board divides Unit 3 into five key topics:
- Enzyme structure and catalysis
- Environmental impacts on enzyme function
- Coupling of energy-releasing and energy-requiring reactions
- Cellular respiration and fermentation
- Photosynthesis
1. Enzymes: Structure and Function
- Biological catalysts that speed up reactions without being consumed
- Made of proteins (and sometimes RNA—ribozymes)
- Active site binds to the substrate (specificity like a lock and key or induced fit)
- Lower the activation energy of reactions
AP Tip: Be ready to explain enzyme-substrate specificity and why enzyme shape is crucial.
2. Environmental Impacts on Enzyme Activity
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:
- Glycolysis: In cytoplasm; glucose → 2 pyruvate + 2 ATP + NADH
- Pyruvate Oxidation: Pyruvate → Acetyl-CoA + CO₂ + NADH
- Krebs Cycle: Produces NADH, FADH₂, ATP, and CO₂
- 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
- Know the Pathways: Draw glycolysis, Krebs cycle, and ETC flow charts.
- Master Enzyme Graphs: Be able to read and explain temperature/pH vs. activity curves.
- Connect the Cycles: Relate photosynthesis outputs to respiration inputs and vice versa.
- 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.
