- A metabolic poison, rotenone, inhibits the transfer of electrons from NADH dehydrogenase (Complex I) to ubiquinone in the electron transport chain. If isolated mitochondria are treated with rotenone and supplied with pyruvate, what would be the immediate effect?
Answer: The proton gradient across the inner mitochondrial membrane would decrease.
Inhibiting Complex I stops the pumping of protons by this complex and prevents subsequent complexes from receiving electrons from NADH. This leads to a reduction or collapse of the proton motive force (proton gradient). ATP synthesis would decrease, not increase. NADH would accumulate, causing NAD⁺ concentration to decrease. Oxygen consumption would decrease but not necessarily cease immediately if electrons from FADH₂ (which enters at Complex II) can still be processed, though the overall rate would plummet.
- From the complete aerobic respiration of one molecule of glucose, approximately what proportion of the total ATP yield is generated by substrate-level phosphorylation?
Answer: Approximately 12%, from reactions in glycolysis and the Krebs cycle.
Substrate-level phosphorylation involves the direct transfer of a phosphate group from a substrate to ADP. This occurs in glycolysis (net 2 ATP) and the Krebs cycle (2 ATP, via GTP). This gives a total of 4 ATP. The total yield from one glucose molecule is about 32-38 ATP. Therefore, 4 out of 32 (or 38) is approximately 12.5% (or 10.5%). Choice C is the closest estimate. The vast majority of ATP (~90%) is from oxidative phosphorylation.
- During strenuous exercise, the pH of muscle cell cytoplasm can decrease. What is the primary cause of this acidification?
Answer: The accumulation of lactic acid, which dissociates to form lactate and H⁺ ions.
During strenuous exercise, oxygen supply to muscles may be insufficient for aerobic respiration to meet all ATP demands. The muscle cells switch to lactate fermentation to supplement ATP production. Pyruvate is converted to lactic acid, which then dissociates into lactate and H⁺. The accumulation of these H⁺ ions is the main cause of the drop in pH (acidification) in the muscle cell cytoplasm, contributing to muscle fatigue.
- A genetic mutation deactivates the enzyme pyruvate decarboxylase, which is essential for the link reaction. In an affected individual's cells supplied with ample glucose and oxygen, where would a key substrate accumulate and which process would be directly halted?
Answer: Pyruvate would accumulate in the cytoplasm, halting the Krebs cycle.
Glycolysis produces pyruvate in the cytoplasm. Pyruvate is then transported into the mitochondrial matrix for the link reaction. If the link reaction is blocked, pyruvate cannot be converted to acetyl-CoA and will accumulate in the cytoplasm. Since acetyl-CoA is the entry point for the Krebs cycle, the Krebs cycle will be halted due to lack of substrate.
- The Respiratory Quotient (RQ) is the ratio of the volume of CO₂ produced to the volume of O₂ consumed. An animal is found to have an RQ of 0.7. What can be deduced about its metabolic state?
Answer: It is primarily metabolizing lipids, which require more oxygen for their complete oxidation.
Different macromolecules have different RQs. For carbohydrates (e.g., glucose), the RQ is 1.0 (6CO₂/6O₂). For lipids, the RQ is around 0.7 because they are more reduced and require proportionally more oxygen for their oxidation than the amount of CO₂ produced. For proteins, the RQ is around 0.8. An RQ of 0.7 strongly indicates that the animal's primary energy source is lipids, which might occur during fasting, hibernation, or prolonged exercise.
- How would the addition of DNP affect a culture of actively respiring animal cells?
Answer: Oxygen consumption would increase, but ATP synthesis via oxidative phosphorylation would decrease.
DNP dissipates the proton gradient, which is the energy source for ATP synthase. This uncouples electron transport from ATP synthesis. The cell attempts to re-establish the gradient by running the electron transport chain faster, which increases oxygen consumption. However, since protons bypass ATP synthase, ATP production via oxidative phosphorylation drops significantly. The cell would then upregulate glycolysis to compensate via substrate-level phosphorylation.
- Which statement correctly describes the role of electron carriers in the Krebs cycle?
Answer: NAD⁺ and FAD are reduced as they accept electrons from the oxidation of intermediate carbon compounds.
The Krebs cycle involves a series of oxidation-reduction reactions. The intermediate carbon compounds (like isocitrate, α-ketoglutarate, succinate, malate) are oxidized, meaning they lose electrons. These electrons are accepted by the electron carriers NAD⁺ and FAD, which become reduced to NADH and FADH₂ respectively. Distractors describe the reverse process or misuse the terms oxidation and reduction.
- Why does the aerobic respiration of a 6-carbon fatty acid yield significantly more ATP than the aerobic respiration of a 6-carbon glucose molecule?
Answer: Fatty acids are more reduced molecules than glucose, containing more C-H bonds per carbon atom.
The amount of ATP produced is proportional to the number of electrons that can be donated to the electron transport chain. Fatty acids are highly reduced, meaning they have a higher proportion of C-H bonds compared to carbohydrates like glucose. The oxidation of these bonds releases more energy and yields more reduced electron carriers (NADH and FADH₂) per carbon atom, ultimately leading to more ATP synthesis via oxidative phosphorylation.
- ATP is an allosteric inhibitor of phosphofructokinase-1 (PFK-1), an enzyme that catalyzes an early, irreversible step in glycolysis. ADP acts as an allosteric activator of the same enzyme. How does this regulation contribute to homeostasis?
Answer: It slows down respiration when energy levels are high, conserving glucose for later use.
This is a classic example of negative feedback (end-product inhibition). When ATP levels are high, the cell has sufficient energy. The excess ATP binds to the allosteric site on PFK-1, inhibiting its activity and slowing down glycolysis. This prevents the cell from wasting glucose by producing ATP it does not need. When ATP is consumed and ADP levels rise, ADP activates PFK-1, stimulating glycolysis to replenish the ATP supply. This maintains a stable energy state (homeostasis).
- Mature mammalian red blood cells lack mitochondria. Which statement accurately describes the metabolic consequences of this characteristic?
Answer: They rely exclusively on glycolysis followed by lactate fermentation for their ATP supply.
Without mitochondria, red blood cells cannot perform the link reaction, Krebs cycle, or oxidative phosphorylation. Their sole source of ATP is substrate-level phosphorylation during glycolysis. To regenerate the NAD⁺ needed for glycolysis to continue, they must convert the resulting pyruvate into lactate via lactate fermentation. This entire process occurs in the cytoplasm.
- Brown adipose tissue contains mitochondria with a unique protein, thermogenin (UCP1), which acts as a proton channel. How does the activity of thermogenin affect metabolism in this tissue?
Answer: It allows the energy from the proton gradient to be released as heat instead of being used for ATP synthesis.
Thermogenin is an uncoupling protein. It provides an alternative pathway for protons to flow from the intermembrane space back to the matrix, bypassing ATP synthase. This uncouples the electron transport chain from ATP synthesis. The potential energy stored in the proton gradient is not captured in the chemical bonds of ATP but is instead dissipated as heat. This is a mechanism for non-shivering thermogenesis, important for maintaining body temperature.
- Malonate is a competitive inhibitor of succinate dehydrogenase, the enzyme that converts succinate to fumarate in the Krebs cycle. If malonate is added to mitochondria actively respiring on pyruvate, which of the following molecules would be expected to accumulate?
Answer: Succinate
A competitive inhibitor blocks the active site of an enzyme. In this case, malonate blocks succinate dehydrogenase. This means the substrate for this enzyme, succinate, cannot be converted to the product, fumarate. As the Krebs cycle continues to operate up to this point, the concentration of succinate will increase because its consumption is blocked while its production (from α-ketoglutarate) continues. Fumarate levels would decrease.
- A student correctly states that without oxygen, the electron transport chain and Krebs cycle stop. What is the most precise explanation for why the Krebs cycle ceases in the absence of oxygen?
Answer: The electron transport chain cannot unload electrons, causing a lack of regenerated NAD⁺ and FAD.
Oxygen is the final electron acceptor in the ETC. Without it, NADH and FADH₂ cannot be re-oxidized to NAD⁺ and FAD. The Krebs cycle requires a constant supply of NAD⁺ and FAD to accept electrons from the oxidation of its intermediates. When these carriers are not regenerated, the Krebs cycle reactions halt due to a lack of reactants (oxidized coenzymes).
- What is the primary distinction between the fate of pyruvate in alcoholic fermentation by yeast and lactate fermentation in muscle cells?
Answer: Only alcoholic fermentation involves a decarboxylation step, releasing CO₂.
In alcoholic fermentation, the 3-carbon pyruvate is first decarboxylated to form a 2-carbon compound (acetaldehyde), releasing a molecule of CO₂. The acetaldehyde is then reduced by NADH to ethanol. In lactate fermentation, the 3-carbon pyruvate is directly reduced by NADH to form the 3-carbon lactate, with no loss of a carbon atom as CO₂. Both processes regenerate NAD⁺ and occur in the cytoplasm.
- What is the net production of reduced electron carriers (NADH and FADH₂) from one molecule of acetyl-CoA entering the Krebs cycle?
Answer: 3 NADH and 1 FADH₂
One turn of the Krebs cycle processes one molecule of acetyl-CoA. During this cycle, NAD⁺ is reduced to NADH at three points, and FAD is reduced to FADH₂ at one point. This results in a net production of 3 NADH and 1 FADH₂ per acetyl-CoA. Choice D represents the yield from a whole glucose molecule (which produces two acetyl-CoA molecules). The other choices are incorrect counts.
- A researcher creates artificial vesicles containing mitochondrial ATP synthase and a light-driven proton pump. The vesicles are placed in a solution with a pH of 7, containing ADP and inorganic phosphate. The interior of the vesicles also has a pH of 7. What must happen for ATP synthesis to occur?
Answer: Light must be shone on the vesicles to pump protons into the vesicles.
ATP synthase uses the flow of protons from a high concentration area to a low concentration area. In mitochondria, protons flow from the intermembrane space (high H⁺) into the matrix (low H⁺). For the artificial vesicle, a gradient must be established. The light-driven pump must pump protons into the vesicle, creating a high concentration inside and a low concentration outside. Protons will then flow out through the ATP synthase, driving ATP synthesis in the external solution. Pumping protons out (B) would create a gradient in the wrong direction for external ATP synthesis.
- In the context of chemiosmosis, what is the most direct and immediate source of energy used by ATP synthase to convert ADP and Pi to ATP?
Answer: The flow of protons down their electrochemical gradient through the enzyme.
Chemiosmosis describes the coupling of ion movement to energy storage. The electron transport chain uses the energy from electron transfers to pump protons, establishing an electrochemical gradient (the proton-motive force). The potential energy of this gradient is converted to kinetic energy as protons flow back into the matrix through the channel in ATP synthase. This flow of protons drives the rotation of the enzyme's components, which in turn catalyzes the synthesis of ATP. The other options are indirect sources of energy for the overall process, not the direct energy for ATP synthase.
- To generate the same total amount of ATP per minute, how must a yeast cell adjust its rate of glucose consumption when it is transferred from an oxygen-rich environment to a completely anaerobic one?
Answer: The rate must increase by a factor of 16, as fermentation is less efficient.
To produce the same amount of ATP, the cell must compensate for the lower efficiency of fermentation. The ratio of ATP yield (aerobic:anaerobic) is 32:2, which simplifies to 16:1. Therefore, to produce the same amount of ATP, the cell must consume glucose 16 times faster in the anaerobic environment.
- If the student forgets to add the potassium hydroxide (KOH) solution, which is the CO₂ absorbent, what change in gas volume would be observed in the pipette?
Answer: Little to no net change in volume, as O₂ consumption is balanced by CO₂ production.
The overall equation for aerobic respiration of glucose is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. For every mole of oxygen consumed, one mole of carbon dioxide is produced. According to Avogadro's law, equal moles of gases occupy equal volumes at the same temperature and pressure. Therefore, the volume of O₂ consumed is approximately equal to the volume of CO₂ produced, resulting in little to no net change in gas volume if the CO₂ is not absorbed.
