In an electron transport chain , the cytochrome which donates electrons to `O_2` is

Options

A. Cytochrome b
B. Cytochrome c
C. Cytochrome `a_3`
D. Cytochrome a

Answer

To solve the question regarding which cytochrome donates electrons to oxygen in the electron transport chain, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Electron Transport Chain (ETC)**:
- The ETC is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions. This process is crucial for ATP production in cellular respiration.

2. **Identify the Role of Cytochromes**:
- Cytochromes are heme-containing proteins that play a key role in the electron transport chain. They facilitate the transfer of electrons through the chain.

3. **Locate the Final Electron Acceptor**:
- In aerobic respiration, the final electron acceptor in the electron transport chain is molecular oxygen (O₂).

4. **Determine Which Cytochrome Donates Electrons to Oxygen**:
- Cytochrome a3 is specifically known for its role in transferring electrons to oxygen. It accepts electrons from cytochrome a and then donates them to oxygen, forming water.

5. **Conclude the Answer**:
- Therefore, the cytochrome that donates electrons to O₂ is **cytochrome a3**.

### Final Answer:
The cytochrome which donates electrons to O₂ in the electron transport chain is **cytochrome a3**.

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