USABO - USA Biology Olympiad Cellular Respiration and Metabolism Questions and Answers — 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?
- In the presence of oxygen and an uncoupler of oxidative phosphorylation.
- In the absence of oxygen.
- In the presence of oxygen. (Correct answer)
- In the presence of a high concentration of ATP.
Correct answer: In the presence of oxygen.
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?
- AMP
- Fructose-2,6-bisphosphate
- Citrate (Correct answer)
- ADP
Correct answer: Citrate
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?
- 2
- 4
- 6 (Correct answer)
- 0
Correct answer: 6
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?
- Increased ATP production from FADH2.
- A complete halt of the citric acid cycle.
- Reduced pumping of protons from the mitochondrial matrix and a decreased ATP yield. (Correct answer)
- An accumulation of oxidized NAD+ in the mitochondrial matrix.
Correct answer: Reduced pumping of protons from the mitochondrial matrix and a decreased ATP yield.
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:
- The oxidation of NADH and FADH2.
- The movement of electrons between protein complexes.
- The phosphorylation of glucose in the cytoplasm.
- The flow of protons down their electrochemical gradient through ATP synthase. (Correct answer)
Correct answer: The flow of protons down their electrochemical gradient through ATP synthase.
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.
Question 6: Which metabolic pathway is common to both aerobic cellular respiration and fermentation?
- The citric acid cycle
- Oxidative phosphorylation
- Glycolysis (Correct answer)
- Pyruvate oxidation
Correct answer: Glycolysis
Glycolysis is the initial pathway for glucose breakdown and occurs in the cytoplasm. It does not require oxygen and is therefore a universal step in both aerobic respiration and anaerobic fermentation. It splits glucose into two pyruvate molecules, producing a net of 2 ATP and 2 NADH.
USABO - USA Biology Olympiad Cellular Respiration and Metabolism Questions and Answers
A scientist is studying a facultative anaerobe.
Under which condition would the most ATP be produced per molecule of glucose?