Which process utilizes NADH to create a proton gradient?

The correct choice is the process that utilizes NADH to create a proton gradient, which is the electron transport chain. In cellular respiration, NADH is generated from earlier metabolic processes such as glycolysis and the Krebs cycle. Once formed, NADH donates electrons to the electron transport chain, a series of protein complexes located in the inner mitochondrial membrane in eukaryotic cells or the plasma membrane in prokaryotes.

As electrons move through the chain, they lose energy, which is used to actively pump protons (H+) from the mitochondrial matrix into the intermembrane space. This pumping creates a proton gradient across the membrane, establishing a difference in proton concentration. The energy stored in this gradient is then harnessed by ATP synthase to synthesize ATP as protons flow back into the matrix.

Other metabolic processes mentioned, like glycolysis, Krebs cycle, and fermentation, do not create a proton gradient in this manner. Glycolysis and the Krebs cycle focus on breaking down glucose and generating electron carriers like NADH but do not involve proton pumping or the creation of a gradient. Fermentation acts to regenerate NAD+ in anaerobic conditions but similarly does not establish a proton gradient. Thus, the role of NADH in the electron transport