In mitochondrial electron transport system

Options

A. Number of ATP molecules synthesised does not depend on nature of electron donor
B. Ubiquinone 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

### Step-by-Step Solution:

1. **Understanding the Electron Transport System (ETS)**:
- The electron transport system, also known as the electron transport chain (ETC), is a series of protein complexes located in the inner mitochondrial membrane. It is responsible for the transfer of electrons derived from reduced coenzymes (NADH and FADH2) to molecular oxygen.

2. **Components of the Electron Transport Chain**:
- The ETS consists of four main protein complexes (Complex I, II, III, and IV) and mobile electron carriers like ubiquinone (coenzyme Q) and cytochrome c.
- Complex I (NADH dehydrogenase) oxidizes NADH to NAD+ and transfers electrons to ubiquinone.
- Complex II (succinate dehydrogenase) oxidizes FADH2 and also transfers electrons to ubiquinone.

3. **Electron Flow**:
- Electrons from NADH and FADH2 flow through the complexes in a specific order:
- From Complex I to ubiquinone, then to Complex III, and finally to Complex IV.
- Ubiquinone can also receive electrons from Complex II.

4. **Role of Cytochrome c**:
- Cytochrome c is a small protein that acts as a mobile electron carrier between Complex III and Complex IV. It facilitates the transfer of electrons to Complex IV.

5. **ATP Production**:
- The number of ATP molecules synthesized during oxidative phosphorylation depends on the nature of the electron donor:
- NADH can produce approximately 2.5 ATP molecules.
- FADH2 can produce approximately 1.5 ATP molecules.
- This difference is due to the number of protons (H+) pumped across the inner mitochondrial membrane, which is used to generate ATP via ATP synthase.

6. **Evaluating Statements**:
- The question presents several statements regarding the ETS:
- **Statement 1**: "The number of ATP molecules synthesized does not depend on the nature of the electron donor." - This is incorrect.
- **Statement 2**: "Ubiquinone receives reducing equivalents via FADH2." - This is correct.
- **Statement 3**: "Cytochrome c is a large protein." - This is incorrect; it is a small protein.
- **Statement 4**: "Complex IV has cytochrome A, cytochrome A3, but no copper centers." - This is incorrect; Complex IV contains two copper centers.

7. **Conclusion**:
- The correct answer is that the second statement is true, while the others are false. Thus, the statement that "the number of ATP molecules synthesized does not depend on the nature of the electron donor" is the incorrect statement.