In the context of cellular respiration, which molecule is the source of the protons that are used to establish the proton gradient in the mitochondria?

• A. $\mathrm{Glucose}$
• B. $\mathrm{Oxygen}$
• C. $\mathrm{ATP}$
• D. $\mathrm{NADH}$

Answer: (D)

Verified step by step guidance

1. Understand that cellular respiration involves several stages, including glycolysis, the citric acid cycle, and oxidative phosphorylation.
2. Recognize that during oxidative phosphorylation, a proton gradient is established across the inner mitochondrial membrane, which is crucial for ATP synthesis.
3. Identify that NADH and FADH2 are electron carriers that donate electrons to the electron transport chain (ETC) in the mitochondria.
4. Realize that as electrons are transferred through the ETC, energy is released, which is used to pump protons (H⁺ ions) from the mitochondrial matrix to the intermembrane space, creating a proton gradient.
5. Conclude that NADH is the molecule that donates electrons to the ETC, leading to the pumping of protons and the establishment of the proton gradient necessary for ATP production.