USABO - USA Biology Olympiad Cellular Respiration and Metabolism Questions and Answers

Question 1

A scientist is studying a facultative anaerobe. Under which condition would the most ATP be produced per molecule of glucose?

Accepted Answer

In the presence of oxygen.

Answer

Facultative anaerobes can produce ATP through both aerobic respiration and fermentation. Aerobic respiration, which occurs in the presence of oxygen, yields significantly more ATP (approximately 30-32 ATP per glucose) than anaerobic respiration or fermentation (typically 2 ATP per glucose). An uncoupler would disrupt the proton gradient, severely reducing ATP synthesis. A high concentration of ATP would inhibit key enzymes in respiration through negative feedback.

Question 2

Which of the following molecules acts as an allosteric inhibitor of phosphofructokinase-1 (PFK-1), a key regulatory enzyme in glycolysis?

Accepted Answer

Citrate

Answer

Citrate is an intermediate of the citric acid cycle. A high concentration of citrate indicates that the citric acid cycle is saturated and does not require more acetyl-CoA from glycolysis. Citrate then allosterically inhibits PFK-1, slowing down the glycolytic pathway. AMP and ADP are allosteric activators, signaling a low energy state in the cell. Fructose-2,6-bisphosphate is a potent allosteric activator of PFK-1.

Question 3

During the complete oxidation of one molecule of glucose, how many molecules of CO2 are produced in the mitochondrial matrix?

Accepted Answer

6

Answer

The complete oxidation of a six-carbon glucose molecule (C6H12O6) produces six molecules of CO2. Two molecules of CO2 are released during the conversion of two pyruvate molecules to two acetyl-CoA molecules (pyruvate oxidation). Four more molecules of CO2 are released during two turns of the citric acid cycle in the mitochondrial matrix.

Question 4

A patient is diagnosed with a rare mitochondrial disease that specifically impairs the function of Complex I (NADH dehydrogenase) in the electron transport chain. Which of the following would be a direct consequence of this condition?

Accepted Answer

Reduced pumping of protons from the mitochondrial matrix and a decreased ATP yield.

Answer

Complex I is responsible for accepting electrons from NADH and pumping protons into the intermembrane space. Its impairment would lead to a reduced proton gradient, thereby decreasing the rate of ATP synthesis by ATP synthase. While electrons from FADH2 enter at Complex II and could still contribute to the proton gradient, the overall efficiency would be significantly reduced because the major entry point for electrons is blocked. This would lead to an accumulation of NADH, not NAD+, and while the citric acid cycle would slow down due to NADH accumulation, it would not necessarily halt completely.

Question 5

In the process of chemiosmosis, the energy used to synthesize ATP is directly derived from:

Accepted Answer

The flow of protons down their electrochemical gradient through ATP synthase.

Answer

Chemiosmosis is the process where the energy stored in a proton gradient across a membrane is used to drive cellular work, such as ATP synthesis. The electron transport chain establishes this proton (H+) gradient. The potential energy of this gradient is converted into chemical energy in ATP as protons flow back into the mitochondrial matrix through the enzyme ATP synthase. The oxidation of NADH/FADH2 and electron movement power the pumping of protons, but the direct energy source for the synthase enzyme is the proton flow itself.

USABO - USA Biology Olympiad Practice Test

USABO - USA Biology Olympiad Practice Test

USABO - USA Biology Olympiad Cellular Respiration and Metabolism Questions and Answers