Hydrogen joins oxygen at the end of
Glycolysis
Electron transport system
Krebs cycle
All of these

Options

A. Glycolysis
B. Electron transport system
C. Krebs cycle
D. All of these

Answer

To solve the question "Hydrogen joins oxygen at the end of," we need to analyze each of the provided options: glycolysis, electron transport system, Krebs cycle, and all of these.

### Step-by-Step Solution:

1. **Understanding Glycolysis**:
- Glycolysis is the first step of cellular respiration, where glucose is broken down into pyruvate.
- In this process, hydrogen does not directly combine with oxygen. Instead, it is involved in the formation of NADH and ATP.

2. **Understanding the Krebs Cycle**:
- The Krebs cycle, also known as the citric acid cycle or TCA cycle, occurs after glycolysis and involves the conversion of acetyl-CoA into carbon dioxide and energy carriers (NADH and FADH2).
- Similar to glycolysis, hydrogen does not directly join with oxygen in this cycle; it is released in the form of NADH and FADH2.

3. **Understanding the Electron Transport System (ETS)**:
- The electron transport system (or chain) is the final stage of cellular respiration.
- In this process, electrons from NADH and FADH2 are transferred through a series of proteins in the inner mitochondrial membrane.
- At the end of this chain, hydrogen ions (protons) combine with oxygen to form water (H2O). This is the key reaction where hydrogen joins with oxygen.

4. **Conclusion**:
- Based on the analysis, the only pathway where hydrogen directly joins with oxygen to form water is the electron transport system.
- Therefore, the correct answer to the question is **Electron Transport System**.

### Final Answer:
**Electron Transport System**