Within the mitochondrion, the proton gradient develops across the :

Options

A. outer membrane
B. inner membrane
C. intermembrane space
D. matrix

Explanation

**Step-by-Step Solution:**

1. **Understanding the Structure of Mitochondria:**
- Mitochondria have a double membrane structure consisting of an outer membrane and an inner membrane. Between these two membranes is the intermembrane space, and inside the inner membrane is the mitochondrial matrix.

2. **Role of the Electron Transport Chain (ETC):**
- The electron transport chain is a series of protein complexes located in the inner mitochondrial membrane. It is responsible for transferring electrons from NADH and FADH2 to oxygen.

3. **Formation of the Proton Gradient:**
- As electrons are transferred through the complexes of the ETC, protons (H+) are pumped from the mitochondrial matrix into the intermembrane space. This process creates a proton gradient (higher concentration of protons in the intermembrane space compared to the matrix).

4. **Function of ATP Synthase:**
- The proton gradient generated across the inner membrane drives protons back into the matrix through ATP synthase, an enzyme located in the inner mitochondrial membrane. This flow of protons through ATP synthase is what generates ATP from ADP and inorganic phosphate.

5. **Conclusion:**
- Therefore, the proton gradient develops across the inner membrane of the mitochondrion.

**Final Answer:**
The proton gradient develops across the **inner membrane** of the mitochondrion.

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