Question

Which statement best explains why NADH produces more ATP per molecule than a molecule of \[\;FAD{H_2}\] ?
A) There is more NADH than \[\;FAD{H_2}\] made for every glucose that enters cellular respiration.
B) The H+ gradient made from electron transport using NADH is located in a different part of the mitochondria than the H+ gradient made using \[\;FAD{H_2}\]
C) Fewer protons are pumped across the inner mitochondrial membrane when \[\;FAD{H_2}\] is the electron donor than when NADH is the electron donor.
D) It takes more energy to make ATP from ADP and Pi using \[\;FAD{H_2}\] than using NADH.

Answer 1

Hint: The \[\;NAD{H_2}\] produces more ATP per molecule than a molecule of \[\;FAD{H_2}\] as the hydrogen from the \[\;FAD{H_2}\] enters late in the electron transport chain from another route. It only goes through two ATP synthesis. Pumping hydrogen ions for the activation of ATP synthase is necessary for the production of ATP.

Step by step answer: The electron transport chain is the last step of aerobic respiration. It is the only part of glucose metabolism (anaerobic reactions) that uses oxygen for functioning. \[NA{D^ + }\] accepts the hydrogen and gets reduced to \[\;NAD{H_2}\]. This donates hydrogen to $FA{D^ + }$ gets oxidised to \[NA{D^ + }\]. This reduced form \[\;FAD{H_2}\] also gets oxidised to form $FA{D^ + }$. This oxidation and reduction produce energy for ATP synthesis. The hydrogen splits into two electrons and two protons. The two electrons are transferred through cytochromes namely b, c, ${c_1}$ and a, ${a_3}$ in sequence. This proton helps in the activation of ATP synthase (produces ATP from ADP and one inorganic phosphate group). \[\; NAD{H_2}\] gives its electron to complex I, which then passes to complex III and energy is given out to pump the electrons which creates a gradient. The electron passes to complex IV. \[\; FAD{H_2}\] joins this process with complex II and thus, it skips one ATP production.
Thus, the ATP produced by the \[\; NAD{H_2}\] produces more ATP per molecule than a molecule of \[\; FAD{H_2}\].
Hence, the correct answer is option C.

Note: The hydrogen ions in the matrix resulting from the splitting of the hydrogen remains in the matrix of the mitochondria. More the hydrogen ions, more is the splitting and more is the energy synthesized. Synthesis of ATP does not depend on the chemical nature of the electron donor or site of reaction.