- 2,4-dinitrophenol (DNP) is a chemical that uncouples the electron transport chain from ATP synthesis by making the inner mitochondrial membrane permeable to protons. In a person exposed to DNP, which metabolic effects would be observed?
Answer: An increased rate of electron transport and oxygen consumption, coupled with an increase in body temperature.
DNP allows protons to leak across the inner mitochondrial membrane, dissipating the proton gradient. The cell attempts to restore the gradient by running the electron transport chain (ETC) at a maximum rate, which increases oxygen consumption. However, since the energy of the gradient is not captured by ATP synthase, it is lost as heat, causing an increase in body temperature. The low ATP levels also signal the cell to increase its metabolic rate.
- A sprinter continues to breathe heavily for several minutes after completing a 100-meter race. What is the primary physiological reason for this sustained high rate of ventilation, often called 'oxygen debt'?
Answer: To provide the oxygen needed for the aerobic respiration required to metabolize the accumulated lactate.
During the sprint, muscles respired anaerobically, producing ATP rapidly but also accumulating lactate. After the race, the body needs to clear this lactate. A significant portion of the lactate is transported to the liver, where it is converted back to pyruvate and then to glucose (Cori cycle). This conversion is an energy-intensive process that requires a large amount of ATP, which is generated by aerobic respiration. The sustained heavy breathing provides the necessary oxygen for this aerobic metabolism.
- Chemiosmosis in mitochondria is dependent on the relative impermeability of the inner mitochondrial membrane to H⁺ ions. What would be the most immediate result if this membrane suddenly became freely permeable to H⁺ ions?
Answer: ATP synthesis via ATP synthase would cease.
The synthesis of ATP by ATP synthase is powered by the flow of H⁺ ions down their electrochemical gradient from the intermembrane space to the matrix. If the membrane became freely permeable to H⁺, this gradient would dissipate as H⁺ ions would flow back into the matrix through various points, not just through ATP synthase. Without the proton motive force, ATP synthase would have no energy source and ATP synthesis would stop immediately.
- A rare genetic disorder prevents the synthesis of the enzyme complex responsible for the link reaction. In a patient with this condition, which molecule would be expected to accumulate in their cells if they are fed glucose under aerobic conditions?
Answer: Lactate in the cytoplasm.
If the link reaction is blocked, pyruvate produced by glycolysis in the cytoplasm cannot be converted to acetyl-CoA and cannot enter the Krebs cycle. This creates a bottleneck. Even with oxygen present, the aerobic pathway is blocked. To regenerate the NAD⁺ needed for glycolysis to continue, the cell is forced to convert the accumulating pyruvate into lactate via anaerobic fermentation in the cytoplasm.
- Oligomycin is a poison that functions by directly inhibiting the ATP synthase enzyme in mitochondria. If a culture of actively respiring animal cells is treated with oligomycin, what are the most likely immediate consequences within the mitochondria?
Answer: The proton gradient across the inner membrane becomes steeper, and the rate of oxygen consumption decreases.
Inhibiting ATP synthase blocks the main channel for protons (H⁺) to flow back into the matrix. This causes the proton gradient to build up (become steeper). This steep gradient creates a large back-pressure that opposes further proton pumping by the electron transport chain (ETC). As the ETC slows down, its consumption of the final electron acceptor, oxygen, also decreases.
- A facultative anaerobe is an organism that can produce ATP by aerobic respiration if oxygen is present, but can switch to fermentation if oxygen is absent. If a population of facultative anaerobes is switched from an aerobic to an anaerobic environment, how would its rate of glucose consumption be expected to change to maintain the same output of ATP?
Answer: It would increase significantly, because fermentation yields far less ATP per molecule of glucose.
Aerobic respiration yields a large amount of ATP per glucose molecule (approx. 30-32 ATP). In contrast, fermentation (which includes glycolysis) yields only a net of 2 ATP per glucose. To produce the same total amount of ATP in an anaerobic environment, the cell must therefore metabolize glucose at a much higher rate. This phenomenon is known as the Pasteur effect.
- A molecule of pyruvate, with its carboxyl group (-COO⁻) containing a radioactive ¹⁴C atom, is supplied to a suspension of respiring mitochondria. In which molecular pool will this radioactivity first be detected?
Answer: As gaseous ¹⁴CO₂ released from the mitochondrial matrix.
The link reaction, which converts pyruvate to acetyl-CoA, involves the oxidative decarboxylation of pyruvate. The carboxyl group is removed from pyruvate and released as a molecule of carbon dioxide. Therefore, the radioactive ¹⁴C in the carboxyl group will first appear as gaseous ¹⁴CO₂.
- Assuming aerobic conditions, what is the total number of decarboxylation events and substrate-level phosphorylation events that occur from the breakdown of one molecule of glucose to CO₂ and H₂O?
Answer: 6 decarboxylations and 4 substrate-level phosphorylations
One glucose (6C) yields two pyruvates (3C). Each pyruvate undergoes one decarboxylation in the link reaction (2 total). Each acetyl-CoA enters the Krebs cycle, which has two decarboxylation steps per turn. Since there are two turns per glucose, this is 4 more decarboxylations (2+4=6 total). Substrate-level phosphorylation occurs twice in glycolysis (net) and once per turn of the Krebs cycle (2 total). So, 2 (glycolysis) + 2 (Krebs) = 4 total substrate-level phosphorylations.
- Based on the setup described, what is being directly measured by the movement of fluid in the manometer?
Answer: The volume of oxygen consumed by the peas over time.
The potassium hydroxide (KOH) is a chemical that reacts with and absorbs carbon dioxide gas (CO₂ + 2KOH → K₂CO₃ + H₂O). By removing the CO₂ produced by respiration from the gas phase, any change in gas volume within the sealed chamber is solely due to the consumption of oxygen. Therefore, the movement of fluid in the manometer directly measures the rate of oxygen consumption.
- If the electron transport chain is inhibited by a poison such as cyanide, what is the most direct consequence for the Krebs cycle?
Answer: The Krebs cycle halts due to a lack of regenerated NAD⁺ and FAD from the electron transport chain.
The Krebs cycle requires a constant supply of the oxidized coenzymes NAD⁺ and FAD to accept electrons from its intermediates. The electron transport chain (ETC) is responsible for re-oxidizing NADH and FADH₂ back to NAD⁺ and FAD. If the ETC is inhibited, this regeneration stops. The available pool of NAD⁺ and FAD is quickly depleted, and as a result, the oxidation reactions of the Krebs cycle cannot proceed, causing the cycle to halt.
- During intense exercise, human muscle cells switch to anaerobic respiration. What is the primary reason that the accumulation of the product of this process leads to muscle fatigue?
Answer: Lactic acid dissociates, releasing H⁺ ions that lower the intracellular pH and inhibit key glycolytic enzymes.
During anaerobic respiration, pyruvate is converted to lactate, which can dissociate into lactate ions and H⁺. The accumulation of H⁺ ions lowers the pH (increases acidity) inside the muscle cell. This change in pH can denature or alter the shape of enzymes, particularly phosphofructokinase (a key regulatory enzyme in glycolysis), and can also interfere with calcium ion binding to troponin, thus inhibiting muscle contraction and causing fatigue.
- The complete oxidation of a 6-carbon fatty acid yields significantly more ATP than the complete oxidation of a 6-carbon glucose molecule. What is the primary biochemical reason for this difference?
Answer: The fatty acid is more reduced than glucose, yielding more NADH and FADH₂ per carbon atom during its oxidation.
Fatty acids are long hydrocarbon chains and are in a more reduced state (have a higher ratio of C-H bonds to C-O bonds) than carbohydrates like glucose. The process of beta-oxidation breaks the fatty acid down into acetyl-CoA molecules, generating a large amount of NADH and FADH₂ in the process. These reduced coenzymes then feed the electron transport chain. Because the fatty acid starts in a more reduced state, its complete oxidation yields more reduced coenzymes and therefore more ATP via oxidative phosphorylation compared to glucose.
- Cyanide is a potent poison because it binds to and inhibits cytochrome c oxidase, the final enzyme complex in the electron transport chain. How would cyanide poisoning affect the function of the sodium-potassium (Na⁺/K⁺) pump in a neuron?
Answer: The pump's activity would decrease and eventually stop due to the depletion of cellular ATP.
The sodium-potassium pump is an active transport mechanism that requires a large amount of energy in the form of ATP. Cyanide poisoning halts the electron transport chain, which in turn stops oxidative phosphorylation, the process that generates the vast majority of a cell's ATP. As ATP levels plummet, energy-dependent processes like the Na⁺/K⁺ pump will slow down and eventually cease to function.
- How does substrate-level phosphorylation differ from oxidative phosphorylation?
Answer: Substrate-level phosphorylation involves direct transfer of phosphate from substrate to ADP, while oxidative phosphorylation uses a proton gradient.
The key mechanistic difference is the source of energy for phosphorylation. In substrate-level phosphorylation, an enzyme directly transfers a phosphate group from a high-energy phosphorylated substrate to ADP. In oxidative phosphorylation, the energy comes from the flow of protons down their electrochemical gradient through ATP synthase (chemiosmosis), a process powered by the electron transport chain.
- Which statement best describes the primary role of NAD⁺ and FAD in aerobic respiration?
Answer: They are oxidizing agents that accept high-energy electrons from organic molecules during catabolism.
NAD⁺ and FAD are coenzymes that function as electron carriers. In their oxidized forms (NAD⁺ and FAD), they act as oxidizing agents, accepting electrons (and protons) from intermediates in glycolysis, the link reaction, and the Krebs cycle. In doing so, they become reduced to NADH and FADH₂. These reduced forms then carry the high-energy electrons to the electron transport chain.
- Phosphofructokinase, a key enzyme in glycolysis, is allosterically inhibited by high concentrations of ATP. What is the main metabolic advantage of this regulatory mechanism?
Answer: It prevents the cell from consuming glucose for energy when its energy reserves are already high.
This is a classic example of negative feedback inhibition. ATP is a final product of cellular respiration. When ATP levels are high, it signifies that the cell has sufficient energy. The ATP binds to an allosteric site on phosphofructokinase, inhibiting its activity and slowing down glycolysis. This prevents the cell from needlessly breaking down valuable glucose when it does not require more ATP, thus conserving resources.
- Which statement correctly identifies the net movement of protons and the location of ATP synthesis during chemiosmosis in a mitochondrion?
Answer: Protons move from the intermembrane space to the matrix, and ATP is synthesized in the matrix.
The electron transport chain actively pumps protons (H⁺) from the mitochondrial matrix into the intermembrane space, creating a high concentration there. During chemiosmosis, these protons flow down their concentration gradient back into the matrix. They pass through the enzyme ATP synthase, which is embedded in the inner mitochondrial membrane. The kinetic energy of this proton flow drives the synthesis of ATP on the matrix side of the membrane.
- In the Krebs cycle, the enzyme succinate dehydrogenase is competitively inhibited by malonate, a molecule structurally similar to succinate. If a high concentration of malonate is added to respiring mitochondria, which molecule would accumulate?
Answer: Succinate
Succinate dehydrogenase catalyzes the conversion of succinate to fumarate. A competitive inhibitor like malonate binds to the active site of this enzyme, preventing succinate from binding. This creates a bottleneck in the cycle. As the preceding reactions continue to produce succinate, its concentration will build up because its conversion to the next intermediate (fumarate) is blocked.
- Both ethanol fermentation in yeast and lactate fermentation in muscle cells achieve the same fundamental purpose required for glycolysis to continue under anaerobic conditions. What is this purpose?
Answer: To regenerate oxidized NAD⁺ from the NADH produced during glycolysis.
Glycolysis requires a supply of NAD⁺ to act as an oxidizing agent in the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate. Under anaerobic conditions, the electron transport chain is not available to regenerate NAD⁺ from NADH. Fermentation pathways serve to oxidize the NADH produced during glycolysis back to NAD⁺ by transferring its electrons to pyruvate (or a derivative), allowing glycolysis to continue producing a small amount of ATP.
- Which statement provides the most precise description of the role of molecular oxygen (O₂) in aerobic respiration?
Answer: It is the final electron acceptor in the electron transport chain, forming water.
Oxygen's role is highly specific. It is not involved in glycolysis or the Krebs cycle directly. Its function is to act as the terminal (final) electron acceptor at the end of the electron transport chain. By accepting electrons, it becomes reduced and combines with protons (H⁺) from the matrix to form water (H₂O). This removal of electrons is essential for the continued flow of electrons through the chain.
