Which of the following is true regarding the given electron transport chain?
`CoQrarrCyt c rarr Cyt aa_(3) rarr O_(2)`

Options

A. `CoQ -> Cyt cis` is `H^+` absorbing site
B. `aa_3 rarrO_2,H^+` yielding site
C. `CoQ -> Cytc` is `H^+` Yielding site and is `H^+` absorbing site
D. No `H^+` is absorbed or released

Answer

To solve the question regarding the electron transport chain (ETC) represented by the sequence `CoQ → Cyt c → Cyt aa3 → O2`, we will analyze the flow of electrons and the associated processes step by step.

### Step-by-Step Solution:

1. **Understanding the Components**:
- **CoQ (Coenzyme Q or Ubiquinone)**: This is a mobile electron carrier that accepts electrons from NADH and FADH2.
- **Cyt c (Cytochrome c)**: This is a small heme protein that transfers electrons from CoQ to the next complex.
- **Cyt aa3 (Cytochrome c oxidase)**: This is the terminal enzyme complex in the electron transport chain that transfers electrons to molecular oxygen (O2).

2. **Electron Flow**:
- Electrons flow from **CoQ** to **Cyt c**. This is a correct statement as CoQ donates electrons to Cyt c.
- Next, **Cyt c** transfers these electrons to **Cyt aa3**. This is also accurate as Cyt c serves as an electron carrier to the cytochrome oxidase complex.
- Finally, **Cyt aa3** transfers the electrons to **O2**, reducing it to water (H2O). This is the final step in the electron transport chain.

3. **Proton Pumping**:
- Complexes I (NADH dehydrogenase), III (cytochrome bc1 complex), and IV (cytochrome c oxidase) are involved in proton pumping, creating a proton gradient across the inner mitochondrial membrane. This gradient is essential for ATP synthesis.

4. **Conclusion**:
- The flow of electrons from CoQ to Cyt c to Cyt aa3 and finally to O2 is correct. The process involves the reduction of O2 to water and the pumping of protons by certain complexes.

### Correct Answer:
- The statement that is true regarding the electron transport chain is that the flow of electrons is from CoQ to Cyt c to Cyt aa3 and finally to O2.