In cellular energy metabolism, which of the following components of the inner mitochondrial membrane directly phosphorylates ADP to produce ATP during oxidative phosphorylation?

Introduction / Context:
In aerobic respiration, most ATP is produced by oxidative phosphorylation, a process that takes place along the inner mitochondrial membrane. Several protein complexes form an electron transport chain and generate a proton gradient that drives ATP synthesis. This question asks which specific component directly phosphorylates ADP to form ATP.

Given Data / Assumptions:

• Electron transport chain complexes pump protons across the inner mitochondrial membrane.
• A proton gradient is established, creating a proton motive force.
• ADP must be phosphorylated to ATP within mitochondria.
• Normal eukaryotic cell metabolism is assumed.

Concept / Approach:
ATP synthase is a large enzyme complex embedded in the inner mitochondrial membrane. It allows protons to flow down their electrochemical gradient from the intermembrane space back into the matrix. The energy released by this proton flow is used to add a phosphate group to ADP, forming ATP. This process is sometimes called chemiosmotic phosphorylation. Other enzymes like DNA polymerase and RNA polymerase synthesize nucleic acids, not ATP, and ribosomes synthesize proteins. Therefore, ATP synthase is the correct answer.

Step-by-Step Solution:
Step 1: Recall that oxidative phosphorylation occurs on the inner mitochondrial membrane and produces most of the ATP during aerobic respiration. Step 2: Understand that electron transport chain complexes I to IV pump protons into the intermembrane space, creating a gradient. Step 3: Recognize that ATP synthase provides a channel for protons to flow back into the matrix. Step 4: During this flow, ATP synthase catalyzes the reaction ADP plus inorganic phosphate gives ATP. Step 5: Evaluate options and identify option A, ATP synthase in the inner mitochondrial membrane, as the enzyme that directly phosphorylates ADP to ATP.

Verification / Alternative check:
Biochemical experiments show that when the proton gradient is disrupted, ATP synthesis stops even if electron transport continues, indicating that ATP synthase is the key coupling factor. Structural studies of ATP synthase show a rotor like mechanism that turns as protons pass, leading to conformational changes that drive ATP formation. These findings confirm that ATP synthase is the direct enzyme responsible for phosphorylating ADP.

Why Other Options Are Wrong:
Option B, ribosomes on rough endoplasmic reticulum, are involved in protein synthesis, not direct ATP production. Option C, DNA polymerase, replicates DNA in the nucleus and does not phosphorylate ADP to ATP. Option D, RNA polymerase, synthesizes RNA from DNA templates and has no role in ATP synthesis. Option E, the sodium potassium pump, uses ATP to move ions across the plasma membrane, so it hydrolyzes ATP instead of producing it.

Common Pitfalls:
A frequent mistake is to think that any mitochondrial enzyme involved in respiration might produce ATP directly. In fact, only ATP synthase catalyzes the final step of ATP formation in oxidative phosphorylation. The electron transport chain complexes create the conditions needed, but they do not themselves directly add phosphate to ADP.

Final Answer:
The component that directly phosphorylates ADP to make ATP is the ATP synthase enzyme embedded in the inner mitochondrial membrane.