In electron transport system (ETS) which of the following cytochromes reacts with oxygen

Options

A. cyt b
B. `cyt c_1`
C. cyt a
D. `cyt a_3`

Answer

To solve the question regarding which cytochrome reacts with oxygen in the electron transport system (ETS), we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Electron Transport System (ETS)**:
- The ETS is a series of complexes located in the inner mitochondrial membrane that transfer electrons from electron donors to electron acceptors, ultimately leading to the production of ATP.

2. **Identify the Cytochromes**:
- The question provides four options: cytochrome b, cytochrome c1, cytochrome a, and cytochrome a3. We need to analyze each one to determine their roles in the ETS.

3. **Analyze Cytochrome b**:
- Cytochrome b is part of complex III in the ETS. It plays a role in transferring electrons but does not directly react with oxygen.

4. **Analyze Cytochrome c1**:
- Cytochrome c1 is a component of the cytochrome b-c1 complex (also part of complex III). It accepts electrons from the reduced form of cytochrome b but does not react with oxygen.

5. **Analyze Cytochrome a**:
- Cytochrome a is part of complex IV (cytochrome c oxidase) and plays a role in the electron transport chain, but it is not the final electron acceptor.

6. **Analyze Cytochrome a3**:
- Cytochrome a3 is also part of complex IV and is crucial in the final step of the electron transport chain. It directly transfers electrons to molecular oxygen, which is the final electron acceptor in the ETS.

7. **Conclusion**:
- Based on the analysis, the cytochrome that reacts with oxygen is **cytochrome a3**. Therefore, the correct answer is option D.

### Final Answer:
**The cytochrome that reacts with oxygen in the electron transport system is Cytochrome a3 (Option D).**