Krebs Cycle Step-by-Step
Step through the 8 stages of the Citric Acid Cycle, visualizing the breakdown of Acetyl-CoA and the tracking of Carbon, NADH, FADH₂, and ATP yields.
THE KREBS CYCLE (CITRIC ACID CYCLE)
The **Krebs Cycle** occurs in the mitochondrial matrix and is the central metabolic hub of the cell. It completes the breakdown of glucose by oxidizing derivatives of pyruvate into . While the cycle only produces a small amount of ATP directly, its most important role is generating high-energy electron carriers (NADH and ) for the electron transport chain.
KEY INPUTS AND OUTPUTS
The cycle begins when Acetyl-CoA combines with a four-carbon molecule (oxaloacetate) to form **Citric Acid**. For every two turns of the cycle (one for each original glucose molecule), the outputs are: 1. **4 **: Released as waste. 2. **6 NADH and 2 **: Sent to the ETC. 3. **2 ATP (or GTP)**: Produced via substrate-level phosphorylation.
HOW TO USE THIS VISUALIZATION
1. **Track the Carbon atoms**: Watch the 6-carbon Citric Acid lose carbons as and return to 4-carbon oxaloacetate. 2. **Monitor Electron Loading**: Observe and molecules docking to the cycle and leaving as high-energy NADH and . 3. **Identify Substrate-Level Phosphorylation**: Locate the step where the single ATP is formed. **Try This**: Count how many molecules are produced for every one molecule of Acetyl-CoA that enters. How does this relate to the total breakdown of the original 6-carbon glucose?
AP EXAM CONNECTION
Unit: Unit 3: Cellular Energetics (Topic 3.6)
Learning Objective: ENE-1.K
COMMON MISCONCEPTIONS
- Thinking the Krebs cycle requires Oxygen directly (it is aerobic because it depends on the ETC, which needs ).
- Believing the cycle makes all the cell's ATP.
- Confusing the Krebs cycle with the Calvin cycle (which is for photosynthesis).
KEY TAKEAWAYS
- Occurs in the mitochondrial matrix.
- Oxidizes Acetyl-CoA to CO2.
- Loads NADH and FADH2 with electrons.
- Produces 2 ATP per glucose molecule.
PRACTICE QUESTIONS
Q1 (CONCEPTUAL): Which molecule is 'recycled' at the end of the Krebs cycle to allow it to begin again?
Show Answer & Explanation
Answer: Oxaloacetate.
Explanation: Oxaloacetate is the four-carbon starting material that combines with Acetyl-CoA to form Citric Acid.
Q2 (CONCEPTUAL): Most of the energy removed from glucose during the Krebs cycle is currently contained in which molecules?
Show Answer & Explanation
Answer: NADH and FADH2.
Explanation: The cycle is primarily an electron-harvesting process, with most energy stored as reduced coenzymes.
