The “fall” of electrons during respiration is stepwise, via NAD + and an electron transport chain. Cellular respiration does not oxidize glucose in a single step that transfers all the hydrogen in the fuel to oxygen at one time. Rather, glucose and other fuels are broken down in a series of steps, each catalyzed by a specific enzyme. At key steps, electrons are stripped from the glucose. In many oxidation reactions, the electron is transferred with a proton, as a hydrogen atom. The hydrogen atoms are not transferred directly to oxygen but are passed first to a coenzyme called NAD + (nicotinamide adenine dinucleotide). How does NAD + trap electrons from glucose? Dehydrogenase enzymes strip two hydrogen atoms from the fuel (e.g., glucose), oxidizing it. The enzyme passes two electrons and one proton to NAD + . The other proton is released as H
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