B) In the inner membrane of mitochondria

The second process of aerobic respiration, also known as the electron transport chain (ETC), takes place in the inner membrane of mitochondria. This process involves a series of protein complexes and electron carriers that work together to generate ATP, the energy currency of the cell.

Here is a detailed explanation of this process:

1. Mitochondrial Structure:
Mitochondria have an outer membrane and an inner membrane. The inner membrane is highly folded, forming structures known as cristae, which increase the surface area available for the electron transport chain.

2. Electron Transport Chain:
The electron transport chain is a series of protein complexes embedded in the inner membrane of mitochondria. These complexes include NADH dehydrogenase, succinate dehydrogenase, cytochrome b-c1 complex, cytochrome c, and cytochrome oxidase.

3. Electron Carriers:
Electron carriers, such as NADH and FADH2, donate electrons to the electron transport chain. NADH is generated during glycolysis and the citric acid cycle, while FADH2 is produced in the citric acid cycle.

4. Electron Flow:
The electrons donated by NADH and FADH2 flow through the electron transport chain from one protein complex to another. As the electrons move, protons (H+) are pumped across the inner membrane from the matrix to the intermembrane space.

5. Proton Gradient:
The pumping of protons creates a concentration gradient across the inner membrane, with a higher concentration of protons in the intermembrane space compared to the matrix. This gradient is known as the proton motive force.

6. ATP Synthesis:
The proton motive force drives ATP synthesis through a process called chemiosmosis. ATP synthase, located in the inner membrane, uses the flow of protons back into the matrix to convert ADP and inorganic phosphate into ATP.

7. Oxygen as the Final Electron Acceptor:
At the end of the electron transport chain, oxygen (O2) acts as the final electron acceptor. It combines with electrons and protons to form water (H2O). This is why oxygen is essential for aerobic respiration.

In conclusion, the second process of aerobic respiration, the electron transport chain, takes place in the inner membrane of mitochondria. The highly folded inner membrane provides a large surface area for the protein complexes and electron carriers involved in this process.