Electron transport chains pump which of the following across membranes?

• A. Oxygen molecules (O\(_2\))
• B. Electrons (e\(^-\))
• C. Glucose molecules (C\(_6\)H\(_{12}\)O\(_6\))
• D. Protons (H\(^+\))

Answer: (D)

Verified step by step guidance

1. Understand the concept of the electron transport chain (ETC): The ETC is a series of protein complexes located in the inner mitochondrial membrane (in eukaryotes) or the plasma membrane (in prokaryotes). It plays a key role in cellular respiration by transferring electrons and creating a proton gradient.
2. Recognize the role of electrons (e\(^-\)): Electrons are transferred through the protein complexes of the ETC, originating from electron carriers like NADH and FADH\(_2\). These electrons drive the pumping of protons across the membrane.
3. Identify the role of oxygen (O\(_2\)): Oxygen acts as the final electron acceptor in the ETC. It combines with electrons and protons to form water (H\(_2\)O), but it is not pumped across the membrane.
4. Clarify the role of glucose (C\(_6\)H\(_{12}\)O\(_6\)): Glucose is the initial substrate for cellular respiration, broken down during glycolysis and subsequent processes to produce electron carriers. It is not directly involved in the ETC or pumped across membranes.
5. Explain the correct answer: Protons (H\(^+\)) are actively pumped across the membrane by the protein complexes of the ETC. This creates a proton gradient, which is essential for ATP synthesis via chemiosmosis in the enzyme ATP synthase.