Which of the following is correct , for the mitochondrial ETS ?

Options

A. Number of ATP molecules synthesised does not depend on nature of electron donor
B. Ubiquinon receives reducing equivalents via `FADH_(2)` also
C. Cytochrome c is a large protein, attached to outer surface of inner mitochondrial membrane.
D. Complex IV has cyt a and cyt `a_(3)`, but no copper centres.

Answer

To solve the question regarding the mitochondrial electron transport system (ETS), we will analyze the provided options step by step.

### Step-by-Step Solution:

1. **Understanding the Mitochondrial Structure**:
- The mitochondria have a double membrane structure: an outer membrane and an inner membrane. The space between these membranes is called the intermembrane space, and the space inside the inner membrane is known as the matrix.

2. **Location of the Electron Transport System**:
- The electron transport system occurs at the inner mitochondrial membrane. This is where various complexes and carriers are located that facilitate the transfer of electrons.

3. **Components of the Electron Transport Chain**:
- The ETS consists of several complexes (Complex I, II, III, and IV) and mobile carriers (like ubiquinone and cytochrome c).
- Complex I (NADH dehydrogenase), Complex II (succinate dehydrogenase), Complex III (cytochrome bc1 complex), and Complex IV (cytochrome c oxidase) are involved in the transfer of electrons.

4. **Role of Ubiquinone**:
- Ubiquinone (also known as coenzyme Q) is a mobile hydrogen carrier that accepts electrons and protons, thus playing a crucial role in the electron transport process.

5. **ATP Synthesis and Electron Donors**:
- The number of ATP molecules synthesized does depend on the nature of the electron donor. For example, NADH contributes to the pumping of more protons compared to FADH2, leading to the synthesis of more ATP from NADH than from FADH2.

6. **Evaluating the Options**:
- **Option 1**: "Number of ATP molecules synthesized does not depend on the nature of electron donor." - This statement is incorrect.
- **Option 2**: "Ubiquinone receives reducing equivalents via FADH2." - This statement is correct.
- **Option 3**: "Cytochrome C is a large protein." - This statement is incorrect; cytochrome c is a small protein.
- **Option 4**: "Complex IV has cytochrome A and A3 but no copper centers." - This statement is incorrect; Complex IV does contain copper centers.

7. **Conclusion**:
- The correct statement regarding the mitochondrial electron transport system is **Option 2**, which states that ubiquinone receives reducing equivalents via FADH2.