IMAT - International Medical Admissions Bioenergetics and Metabolism Questions and Answers — Questions and Answers
Question 1: Which of the following correctly identifies the net products of glycolysis starting from one molecule of glucose?
- 2 Pyruvate, 2 NADH, 2 ATP (Correct answer)
- 1 Pyruvate, 1 NADH, 1 ATP
- 2 Acetyl-CoA, 2 CO2, 2 ATP
- 6 CO2, 6 H2O, ~32 ATP
Correct answer: 2 Pyruvate, 2 NADH, 2 ATP
Glycolysis is the metabolic pathway that converts one molecule of glucose into two molecules of pyruvate. This process, which occurs in the cytoplasm, has a net yield of two molecules of ATP (four are produced, but two are consumed) and two molecules of NADH. [14, 17, 24]
Question 2: A patient is exposed to a toxin that specifically inhibits Complex IV (cytochrome c oxidase) of the electron transport chain. Which of the following metabolic consequences would be most likely to occur?
- Increased ATP production via oxidative phosphorylation.
- Accumulation of NADH and FADH2. (Correct answer)
- Oxygen consumption would increase to compensate.
- The proton gradient across the inner mitochondrial membrane would become steeper.
Correct answer: Accumulation of NADH and FADH2.
Inhibiting Complex IV, the final complex in the electron transport chain, prevents the transfer of electrons to oxygen. This causes a 'backup' of electrons throughout the chain, leading to an accumulation of their reduced carriers, NADH and FADH2, as they cannot be re-oxidized. [11, 18, 20] This halt in electron flow stops the pumping of protons, collapsing the proton gradient and ceasing ATP synthesis. Oxygen consumption would decrease, not increase.
Question 3: The process of chemiosmosis, which is essential for ATP production during oxidative phosphorylation, is directly driven by:
- The direct transfer of a phosphate group from a substrate to ADP.
- The movement of electrons between protein complexes.
- The flow of protons down their electrochemical gradient through ATP synthase. (Correct answer)
- The reduction of NAD+ to NADH in the mitochondrial matrix.
Correct answer: The flow of protons down their electrochemical gradient through ATP synthase.
Chemiosmosis is the process where the energy stored in a proton (H+) gradient across a membrane is used to drive cellular work, such as the synthesis of ATP. [1, 3] In mitochondria, the electron transport chain pumps protons into the intermembrane space, creating this gradient. The protons then flow back into the matrix through the ATP synthase enzyme, and this flow provides the energy to convert ADP and inorganic phosphate into ATP. [5, 9, 16]
Question 4: What is the primary role of the Krebs cycle (citric acid cycle) in cellular respiration?
- To directly produce the majority of the cell's ATP.
- To generate reduced electron carriers (NADH and FADH2) for the electron transport chain. (Correct answer)
- To break down glucose into two molecules of pyruvate.
- To consume oxygen and produce water.
Correct answer: To generate reduced electron carriers (NADH and FADH2) for the electron transport chain.
While the Krebs cycle does produce a small amount of ATP (or GTP) directly per turn, its main purpose is to complete the oxidation of acetyl-CoA, derived from glucose. This oxidation process generates a large number of high-energy electrons, which are captured in the form of the reduced coenzymes NADH and FADH2. [2, 6, 12, 19] These molecules then donate their electrons to the electron transport chain, where the majority of ATP is produced.
Question 5: A key distinction between aerobic respiration and anaerobic respiration is the identity of the final electron acceptor. In aerobic respiration, the final electron acceptor is oxygen. In anaerobic respiration, the final electron acceptor is:
- Pyruvate or a derivative of pyruvate.
- Another organic molecule, such as ethanol.
- Water.
- An inorganic molecule other than oxygen, such as nitrate or sulfate. (Correct answer)
Correct answer: An inorganic molecule other than oxygen, such as nitrate or sulfate.
Anaerobic respiration is a form of cellular respiration that uses an electron transport chain, but the final electron acceptor is an inorganic molecule other than oxygen (O2). [7, 10, 22] Examples include nitrate (NO3-), sulfate (SO4^2-), and sulfur (S). [15, 23] The use of pyruvate or its derivatives as the final electron acceptor is characteristic of fermentation, not anaerobic respiration.
Question 6: How do enzymes catalyze metabolic reactions within a cell?
- By increasing the overall free energy change (ΔG) of the reaction.
- By decreasing the activation energy required for the reaction to proceed. (Correct answer)
- By providing the necessary reactant molecules for the reaction.
- By shifting the equilibrium of the reaction to favor the products.
Correct answer: By decreasing the activation energy required for the reaction to proceed.
Enzymes are biological catalysts that speed up the rate of chemical reactions without being consumed in the process. They achieve this by lowering the activation energy (Ea), which is the energy barrier that must be overcome for reactants to be converted into products. [4, 26, 27] Enzymes do not alter the overall free energy change (ΔG) of the reaction, nor do they change the reaction's equilibrium. [25, 28]
IMAT - International Medical Admissions Bioenergetics and Metabolism Questions and Answers
Which of the following correctly identifies the net products of glycolysis starting from one molecule of glucose?