Which of the following is NOT a primary energy-carrying molecule involved in cellular respiration?

Glucose serves as a primary energy source for cells, but it is not technically classified as an energy-carrying molecule in the same sense as ATP, NAD, or FAD. Instead, glucose is a substrate that gets metabolized during cellular respiration to produce energy.

As a carbohydrate, glucose undergoes glycolysis and subsequent processes that ultimately convert its chemical energy into ATP. ATP is the direct energy currency of the cell, while NAD and FAD are crucial cofactors that carry electrons during the redox reactions of cellular respiration. These molecules are integral to the process of energy extraction from the breakdown of glucose and other substrates, but glucose itself is not a carrier; it is a reactant in those energy-releasing pathways.

Therefore, the distinction lies in the role of glucose as a fuel source rather than as a carrier of energy, which is why it is the correct answer in identifying which option does not fit with the others.