Question

The P/O ratio takes into account the fact that, in the electron transport chain, there are ________ protons pumped across the membrane per NADH and ________ protons pumped across the membrane per FADH2. A. 3 protons per NADH, 2 protons per FADH2.

B. 2 protons per NADH, 3 protons per FADH2.
C. 4 protons per NADH, 2 protons per FADH2.
D. 3 protons per NADH, 4 protons per FADH2.

Answer 1

The P/O ratio reflects protons pumped by NADH and FADH2 during ATP synthesis. The answer is C: 4 protons per NADH and 2 per FADH2, indicating that NADH contributes more to ATP production in the electron transport chain than FADH2.

Step-by-step explanation:

The P/O ratio is a measure of the efficiency of ATP synthesis related to the electron transport chain (ETC) in mitochondria. In the context of this question, the correct answer is C. 4 protons per NADH, and 2 protons per FADH2. Electrons from NADH enter the ETC at Complex I, which pumps 4 H+ into the intermembrane space, while electrons from FADH2 enter at Complex II and proceed to Complexes III and IV, pumping a total of 2 protons. This generates the proton gradient that drives ATP synthesis through oxidative phosphorylation.

The chemi-osmotic hypothesis explains how the transfer of electrons through the ETC is coupled with the pumping of protons across the inner mitochondrial membrane. A difference in proton concentration and electrical charge (the proton motive force) across the membrane is established, which powers the synthesis of ATP by the FF_1 ATPase. The higher the P/O ratio, the more efficient the process is for generating ATP, with NADH contributing more to the proton gradient and thus to ATP synthesis compared to FADH2.