Cells need energy to function, and they get it through cellular respiration. This process converts glucose into ATP, the molecule that powers everything from muscle movement to brain activity. In this guide, we’ll break down cellular respiration into simple, easy-to-understand steps so you can master this important AP Biology topic.
What Will You Learn in This Guide?
By the end of this blog, you’ll understand:
- What cellular respiration is and why it’s important
- The three main stages: Glycolysis, Krebs Cycle, and Electron Transport Chain
- Key terms and their definitions
- How ATP is made step by step
- The difference between aerobic and anaerobic respiration
- Helpful study resources to strengthen your understanding
Every living thing relies on cellular respiration to stay alive. Let’s take a closer look at how it works!
What Is Cellular Respiration?
Cellular respiration is the process where cells convert glucose into energy (ATP) using oxygen. This energy is used for essential life functions like muscle contractions, nerve signals, and even cell repair.
Equation for Cellular Respiration
What this means:
- Glucose (C₆H₁₂O₆) and oxygen (O₂) are used as fuel.
- The process produces carbon dioxide (CO₂), water (H₂O), and ATP (energy).
Key Terms and Definitions
| Term | Definition |
|---|---|
| ATP (Adenosine Triphosphate) | The main energy carrier in cells. |
| Glucose | A simple sugar that serves as fuel for cellular respiration. |
| Glycolysis | The first stage of respiration, where glucose is split into pyruvate. |
| Krebs Cycle (Citric Acid Cycle) | A series of reactions that generate high-energy molecules (NADH, FADH₂). |
| Electron Transport Chain (ETC) | The final stage where most ATP is produced. |
| NADH and FADH₂ | Electron carriers that help generate ATP. |
| Aerobic Respiration | Respiration that requires oxygen. |
| Anaerobic Respiration | Respiration that happens without oxygen, producing less ATP. |
| Mitochondria | The “powerhouse” of the cell where most of cellular respiration takes place. |
| ATP Synthase | The enzyme that produces ATP in the ETC. |
Stages of Cellular Respiration
There are three major stages in cellular respiration:
| Stage | Location | Main Function | ATP Produced |
|---|---|---|---|
| Glycolysis | Cytoplasm | Splits glucose into pyruvate | 2 ATP |
| Krebs Cycle (Citric Acid Cycle) | Mitochondrial Matrix | Produces high-energy electron carriers | 2 ATP |
| Electron Transport Chain (ETC) | Inner Mitochondrial Membrane | Uses electrons to generate ATP | 32-34 ATP |
Now, let’s break each stage down further
Stage 1: Glycolysis – Breaking Down Glucose
✅ Where does it happen? Cytoplasm
✅ What happens?
- One glucose molecule (C₆H₁₂O₆) is split into two pyruvate molecules.
- 2 ATP molecules are invested to start the process.
- 4 ATP molecules are made (net gain: 2 ATP).
- 2 NADH molecules store electrons for later use.
Steps of Glycolysis
| Step | What Happens? | Products |
|---|---|---|
| Glucose Breakdown | Glucose is split | 2 Pyruvate |
| ATP Investment | 2 ATP used | Activation Energy |
| ATP Production | 4 ATP made | Net 2 ATP |
| Electron Carrier | NADH stores electrons | 2 NADH |
✅ Oxygen needed? ❌ No, glycolysis is anaerobic (does not require oxygen).
Stage 2: Krebs Cycle (Citric Acid Cycle) – Generating Energy Carriers
✅ Where does it happen? Mitochondrial Matrix
✅ What happens?
- Each pyruvate is converted into Acetyl-CoA.
- Acetyl-CoA enters the Krebs Cycle, producing:
- NADH and FADH₂ (electron carriers).
- 2 ATP per glucose molecule.
- CO₂ as a waste product.
Steps of the Krebs Cycle
| Step | What Happens? | Products |
|---|---|---|
| Pyruvate Conversion | Pyruvate becomes Acetyl-CoA | 2 Acetyl-CoA, 2 CO₂ |
| Electron Carrier Formation | NADH and FADH₂ store energy | 6 NADH, 2 FADH₂ |
| ATP Production | Energy is used to make ATP | 2 ATP |
✅ Oxygen needed? ✅ Yes, indirectly.
Stage 3: Electron Transport Chain (ETC) – ATP Factory
✅ Where does it happen? Inner Mitochondrial Membrane
✅ What happens?
- NADH and FADH₂ drop off electrons.
- Electrons move through proteins, pumping protons (H⁺).
- Protons flow back through ATP synthase, generating 32-34 ATP.
- Oxygen (O₂) acts as the final electron acceptor, forming water (H₂O).
For more details, check Nature’s ATP Synthesis Guide.
Final ATP Summary
| Stage | ATP Produced |
|---|---|
| Glycolysis | 2 ATP |
| Krebs Cycle | 2 ATP |
| Electron Transport Chain | 32-34 ATP |
| Total ATP from One Glucose | 36-38 ATP |
Practice for AP Biology
Want to improve your understanding of cellular respiration? Check out our AP Biology course:
➡️ Schedule a Free Class
➡️ AP Biology Full Course
For additional practice questions, try College Board’s AP Biology Free-Response Questions.
Understanding cellular respiration is key for AP Biology. Keep practicing, and you’ll be ready for your next exam!
