AAMC FL Practice Exam

During which step of cellular respiration is nicotinamide neither reduced nor oxidized?

1. Aerobic respiration

2. Glycolysis

3. Chemiosmosis

4. Fermentation

The correct answer is: Chemiosmosis

During the process of cellular respiration, nicotinamide adenine dinucleotide (NAD+) acts as a crucial electron carrier, being reduced to NADH during several steps of respiration. However, during chemiosmosis, nicotinamide is neither reduced nor oxidized. Chemiosmosis occurs in the mitochondria, specifically during the electron transport chain phase of aerobic respiration. Here, NADH produced in glycolysis, the citric acid cycle, and other processes donates electrons to the electron transport chain, resulting in the establishment of a proton gradient across the inner mitochondrial membrane. The energy generated from this gradient is utilized by ATP synthase to produce ATP. At this point, NAD+ has already been oxidized to NADH prior to chemiosmosis, and there are no further redox reactions involving NAD+ or NADH during this stage. Thus, chemiosmosis is characterized by the flow of protons and the synthesis of ATP rather than any direct interaction with nicotinamide in terms of oxidation or reduction, making it the correct context for this question.