Question

two nutrient solutions are maintained at the same ph. actively respiring mitochondria are isolated and placed into each of the two solutions. oxygen gas is bubbled into one solution. the other solution is depleted of available oxygen. which of the following best explains why atp production is greater in the tube with oxygen than in the tube without oxygen?
a the rate of proton pumping across the inner mitochondrial membrane is lower in the sample without oxygen.
b electron transport is reduced in the absence of a plasma membrane.
c in the absence of oxygen, oxidative phosphorylation produces more atp than does fermentation.
d in the presence of oxygen, glycolysis produces more atp than in the absence of oxygen.

Explanation:

• Option A: In the presence of oxygen, aerobic respiration occurs. The electron transport chain (ETC) in mitochondria uses oxygen as the final electron acceptor. Proton pumping across the inner mitochondrial membrane is a key part of the chemiosmotic mechanism that generates ATP. Without oxygen, the ETC slows down (since there is no final electron acceptor to keep the electron flow going smoothly), and thus the rate of proton pumping is lower. This leads to less ATP production via ATP - synthase (which uses the proton gradient).
• Option B: Mitochondria have their own inner and outer membranes. The electron - transport chain in mitochondria occurs on the inner mitochondrial membrane, not the plasma membrane. So, the absence of a plasma membrane (which is not relevant here as mitochondria are in a solution and not in a cell with a plasma - membrane - related electron - transport issue) does not explain the difference.
• Option C: In the absence of oxygen, oxidative phosphorylation (which requires oxygen) does not occur. Fermentation is an anaerobic process, but it produces very little ATP compared to oxidative phosphorylation. So, this statement is incorrect.
• Option D: Glycolysis is the first step of both aerobic and anaerobic respiration. It occurs in the cytoplasm and produces a small amount of ATP (2 ATP per glucose molecule). The presence or absence of oxygen does not significantly affect the amount of ATP produced by glycolysis itself. The big difference in ATP production (in favor of the oxygen - containing tube) comes from the subsequent steps (oxidative phosphorylation in the presence of oxygen).

Answer:

A. The rate of proton pumping across the inner mitochondrial membrane is lower in the sample without oxygen.