The greatest contributor of electrons to the electron transport system is :

Options

A. oxygen
B. transition reaction
C. glycolysis
D. Krebs cycle

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the greatest contributor of electrons to the electron transport system (ETS) in cellular respiration.

2. **Identifying the Electron Transport System**: The electron transport system is a series of complexes located in the inner mitochondrial membrane where electrons are transferred from electron donors to electron acceptors through redox reactions. This process is crucial for ATP production.

3. **Role of Krebs Cycle**: The Krebs cycle, also known as the citric acid cycle or TCA cycle, plays a significant role in cellular respiration. It generates electron carriers (NADH and FADH2) that donate electrons to the ETS.

4. **Electron Carriers Produced**: In one turn of the Krebs cycle, 4 NADH and 2 FADH2 molecules are produced. These carriers are essential as they transport electrons to the electron transport chain.

5. **ATP Production from Electron Carriers**: Each NADH can produce approximately 3 ATP molecules, and each FADH2 can produce about 2 ATP molecules when they donate their electrons to the ETS. Therefore, from the 4 NADH produced, we can derive 12 ATP (4 NADH x 3 ATP/NADH) and from the 2 FADH2, we can derive 4 ATP (2 FADH2 x 2 ATP/FADH2).

6. **Conclusion**: The Krebs cycle is the greatest contributor of electrons to the electron transport system because it produces the highest number of electron carriers (NADH and FADH2) that ultimately lead to ATP synthesis.

7. **Final Answer**: The greatest contributor of electrons to the electron transport system is the Krebs cycle.

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