When electrons flow along the electron transport chains of mitochondria, which of the following changes occurs?

Options

A. pH of the matrix increases
B. NAD is oxidised
C. Cytochromes of the chain phosphorylate ADP to form АТР
D. ATP synthase pumps protons by active transport

Explanation

**Step-by-Step Solution:**

1. **Understanding the Electron Transport Chain (ETC):** The electron transport chain is a series of protein complexes located in the inner mitochondrial membrane. It plays a crucial role in cellular respiration by transferring electrons from electron donors like NADH and FADH2 to electron acceptors.

2. **Flow of Electrons:** As electrons flow through the ETC, they move from NADH and FADH2 to various cytochromes and other proteins in the chain. This process releases energy, which is used to pump protons (H⁺ ions) from the mitochondrial matrix into the intermembrane space.

3. **Effect on pH:** The pumping of protons into the intermembrane space increases the concentration of hydronium ions (H₃O⁺), leading to a decrease in pH in the intermembrane space. Consequently, the pH of the matrix (the space inside the inner membrane) will actually decrease, not increase.

4. **ATP Production:** The proton gradient created by the electron transport chain is utilized by ATP synthase, an enzyme located in the inner mitochondrial membrane. ATP synthase allows protons to flow back into the matrix through it, and this flow drives the phosphorylation of ADP to form ATP.

5. **Analyzing the Options:**
- **Option 1:** "pH of the matrix increases" - This is incorrect because the pH of the matrix actually decreases due to the increase of protons in the intermembrane space.
- **Option 2:** "NAD is oxidized" - This is also incorrect. NADH is oxidized to NAD⁺, but this is not a direct result of electron flow in the ETC.
- **Option 3:** "Cytochromes of the chain phosphorylate ADP to form ATP" - This is incorrect. Cytochromes do not directly phosphorylate ADP; ATP synthesis occurs through ATP synthase.
- **Option 4:** "ATP synthase pumps protons by active transport" - This is the correct statement. ATP synthase uses the proton gradient (created by the ETC) to synthesize ATP as protons flow back into the matrix.

6. **Conclusion:** The correct answer is that ATP synthase pumps protons by active transport, leading to ATP production.

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