Question

6. Glucose is completely oxidized to carbon dioxide with the concomitant production of several reduced electron carrier molecules. Oxidative phosphorylation ensures the regeneration of these electron carrier molecules.

A. Cell Y is able to carry out glycolysis, citric acid cycle, and oxidative phosphorylation. However, a certain mutation in ATP synthase allows the passage of H+ ions but does not produce ATP. When grown in aerobic conditions, would you expect Cell Y to generate a little less OR a little more OR the same amount of ATP as a cell that was fermenting. Explain your choice. (2.5 points)

B. Cell Z is able to carry out glycolysis, citric acid cycle and the first few steps of the oxidative phosphorylation pathway. An experiment showed that addition of Inhibitor X prevented the transfer of electrons to the last protein in the electron transfer chain and glycolysis stopped immediately, even before all the ATP was depleted. Explain this observation. (2.5 points)

Answer 1

Ans. #6A. Correct option. Less ATP.

The process of “leaking” of H⁺ from intermembrane space (IMS) to mitochondrial matrix (MM) reduces the electrochemical gradient across inner membrane.

The electrochemical gradient across inner membrane is established through transport of protons from MM to IMS during ETC. The potential energy of electrochemical gradient is harvested in form of ATP by ATP synthase.

Since proton leak reduces the electrochemical gradient, cell Y will produce less ATP because it has lesser potential energy in form of electrochemical gradient.

#6B. #1. Electron transport chain consists of four complexes that mediate the sequential transfer of electrons from NADH and FADH2 oxidation to complex IV. Inhibition of any complex stops the “chain of electron transfer” which leads the whole ETC to shut down. So, the inhibitor stops the whole ETC. Note the ETC does not require ATP input, so it has nothing to do with depletion of ATP.

#2. Glycolysis required input of ATP, so glycolysis shall stop when ATP is depleted.

However, glycolysis stops before ATP is depleted because of unavailability of NAD⁺.

Cytoplasm has limited pool of NAD⁺/NADH. During glycolysis NAD⁺ is reduced to NADH. So, [NAD⁺] gradually decreases or reduces as glycolysis continues. If not regenerated, the unavailability of NAD⁺ will cause glycolysis to shut down.

Similarly, mitochondria has its own limited pool of NAD⁺/NADH. NADH is oxidized into NAD⁺ during ETC.

The cytoplasm and Mitochondrial pools circulate between the two to replenish the molecules as needed.

Inhibition of any complex in ETC by the inhibitor stops the regeneration of NAD⁺ in mitochondria. Inhibition of NAD⁺ regeneration in mitochondria further inhibits the regeneration/ replenishment of NAD⁺ in cytoplasm. Therefore, cytoplasm faces the unavailability of NAD⁺.

Therefore, glycolysis stops before depletion of ATP.