Question

scientists were interested in testing the effects of rotenone, a broad - spectrum pesticide, on a cell culture. cell culture a was used as a control, while culture b was treated with rotenone. after a period of time, the scientists measured the concentration of several metabolites in the mitochondria of cells in both cultures. their results are shown in the table below.
based on the data in the table, which of the following best explains the effects of rotenone on cellular respiration?
a rotenone acts as an inhibitor of the enzymes in the krebs cycle.
b nadh produced during glycolysis, is not able to enter the mitochondria because transport proteins are blocked from entering
c treated cells are not able to break down nadh because certain enzymes of the electron transport chain are inhibited.
d rotenone acts as an allosteric inhibitor of glycolytic enzymes, thus inhibiting cellular respiration

Explanation:

• In cellular respiration, the electron transport chain (ETC) is crucial for ATP production. NADH donates electrons to the ETC. If certain enzymes in the ETC are inhibited (as rotenone does), NADH cannot be oxidized to NAD+. This leads to an accumulation of NADH (as seen in Culture B with a high NADH concentration of 550 μM compared to 55 μM in Culture A) and a decrease in NAD+ (5 μM in Culture B vs 55 μM in Culture A). Also, since the ETC is involved in ATP synthesis (oxidative phosphorylation), ATP levels drop (5 μM in Culture B vs 85 μM in Culture A) and ADP + Pi increase (100 μM in Culture B vs 55 μM in Culture A) as ATP is not being synthesized.
• Option A: If rotenone inhibited Krebs cycle enzymes, we would expect changes in Krebs - cycle - specific metabolites. But pyruvate (which is converted to acetyl - CoA for the Krebs cycle) has the same concentration in both cultures (25 μM), so this option is incorrect.
• Option B: NADH from glycolysis is not the main issue here. The data shows a problem with mitochondrial NADH (as FADH₂ and FAD levels are relatively unchanged compared to NADH and NAD+). Also, the problem is more about the ETC in the mitochondria rather than transport of glycolytic NADH.
• Option D: If rotenone inhibited glycolytic enzymes, pyruvate levels would change. But pyruvate levels are the same in both cultures (25 μM), so this option is incorrect.

Answer:

C. Treated cells are not able to break down NADH because certain enzymes of the electron transport chain are inhibited.