- A researcher analyzing a water sample from a stagnant, anoxic pond detects a distinct 'rotten egg' smell, which is identified as hydrogen sulfide (H₂S). The pond is known to have high concentrations of sulfate (SO₄²⁻) and organic matter. This observation is strong evidence for which of the following microbial processes?
Answer: Dissimilatory sulfate reduction, a form of anaerobic respiration using sulfate as the terminal electron acceptor.
The correct answer is B. Dissimilatory sulfate reduction is a type of anaerobic respiration where sulfate (SO₄²⁻) is used as the terminal electron acceptor, and it is reduced to hydrogen sulfide (H₂S). The presence of H₂S, coupled with the anoxic conditions and high sulfate and organic matter concentrations, strongly indicates that sulfate-reducing bacteria are active.
A is incorrect. Denitrification uses nitrate (NO₃⁻) or nitrite (NO₂⁻) as an electron acceptor, and its gaseous end products are typically N₂O or N₂. It does not produce H₂S.
C is incorrect. Sulfur-oxidizing bacteria consume H₂S as an electron donor; they do not produce it. This process typically occurs in environments where sulfide and an electron acceptor (like oxygen or nitrate) are both present, often at an oxic-anoxic interface.
D is incorrect. Fermentation pathways, such as homolactic fermentation, do not use external inorganic electron acceptors like sulfate. They produce organic end products like lactic acid, but not H₂S.
- A key distinction between anaerobic respiration and fermentation is the fate of electrons from NADH. Which of the following pairs correctly identifies a terminal electron acceptor for anaerobic respiration and a terminal electron acceptor for fermentation?
Answer: Anaerobic respiration: Sulfate (SO₄²⁻); Fermentation: Pyruvate
The correct answer is A. Anaerobic respiration is defined by the use of an external terminal electron acceptor other than oxygen that is fed into an electron transport chain. Sulfate (SO₄²⁻) is a classic example. Fermentation, in contrast, uses an internal, organic molecule generated during the catabolic pathway itself as the terminal electron acceptor to regenerate NAD⁺ from NADH. Pyruvate (or a derivative like acetaldehyde) is a common electron acceptor in fermentation, being reduced to products like lactate, ethanol, etc.
B is incorrect because oxygen is the terminal electron acceptor for aerobic, not anaerobic, respiration. Lactate is a product of fermentation, not an acceptor (pyruvate is the acceptor).
C is incorrect because nitrate is an external electron acceptor used in anaerobic respiration, not fermentation. Fumarate is a valid acceptor for anaerobic respiration.
D is incorrect because acetaldehyde is an internal electron acceptor in fermentation (specifically, alcohol fermentation), while carbon dioxide can be an external electron acceptor in a form of anaerobic respiration (methanogenesis). The pairing is reversed and mismatched.
- The processes of dissimilatory sulfate reduction and aerobic respiration are analogous in that both use an electron transport chain to conserve energy. However, they are fundamentally different. A bacterium carrying out dissimilatory sulfate reduction would differ from an obligate aerobe in which of the following key aspects?
Answer: The terminal electron acceptor for the sulfate reducer has a much lower standard reduction potential than O₂.
The correct answer is D. The defining difference that dictates the energy yield is the nature of the terminal electron acceptor. Sulfate (SO₄²⁻) has a standard reduction potential (E₀') of -0.22 V, whereas oxygen (O₂) has an E₀' of +0.82 V. This much lower potential for sulfate means that the total energy released from the oxidation of a given electron donor is significantly less, resulting in lower ATP yields and slower growth rates compared to aerobic respiration.
A is incorrect. Dissimilatory sulfate reduction is a form of anaerobic respiration, which by definition generates a proton (or sodium) motive force via an electron transport chain.
B is incorrect. Sulfate (SO₄²⁻) is an inorganic compound. Using an organic compound as a terminal electron acceptor is characteristic of fermentation.
C is incorrect. While some sulfate reducers are chemolithotrophs that use H₂, many are chemoorganotrophs that use organic compounds like lactate or acetate as electron donors, similar to many aerobes.
- A researcher is studying a novel bacterium isolated from an iron-rich, anoxic sediment. The bacterium's growth rate is measured and found to be directly proportional to the rate of ferrous iron (Fe²⁺) appearance in the medium, which is accompanied by a stoichiometric consumption of organic acids and ferric iron (Fe³⁺). This observation strongly suggests the bacterium is performing:
Answer: Anaerobic respiration using organic acids as the electron donor and Fe³⁺ as the terminal electron acceptor.
The correct answer is C. The data show that bacterial growth is linked to the conversion of Fe³⁺ to Fe²⁺ (a reduction) and the consumption of organic acids (an oxidation). This indicates that the bacteria are oxidizing the organic acids for energy and electrons, and passing those electrons to Fe³⁺, which serves as the terminal electron acceptor in an anaerobic respiratory chain. This is a classic example of dissimilatory iron reduction.
A is incorrect because the environment is anoxic, precluding aerobic respiration. While iron is a cofactor in cytochromes, the data show a bulk reduction of environmental iron linked to growth, not just its use as a cofactor.
B is incorrect because the tight coupling between growth and Fe²⁺ production indicates a direct biological process, not a secondary abiotic effect. Fermentation would not use an external acceptor like Fe³⁺.
D is incorrect because the organism is consuming organic acids, meaning it is a chemoorganotroph, not a chemolithoautotroph. Chemolithotrophs would oxidize an inorganic compound like Fe²⁺ for energy; this organism is producing Fe²⁺.
- The ratio of ATP synthesized per 2 electrons passed through the electron transport chain (the P/2e⁻ ratio) varies depending on the terminal electron acceptor. Which of the following sequences correctly ranks the terminal electron acceptors from the one that supports the HIGHEST P/2e⁻ ratio to the one that supports the LOWEST?
Answer: O₂ > NO₃⁻ > SO₄²⁻
The correct answer is A. The P/2e⁻ ratio (analogous to the P/O ratio in aerobes) is directly related to the magnitude of the proton motive force that can be generated, which in turn depends on the difference in reduction potential (ΔE₀') between the electron donor and the terminal acceptor. Oxygen (E₀' ≈ +0.82 V) has the most positive reduction potential, allowing for the highest energy yield and highest P/2e⁻ ratio. Nitrate (E₀' ≈ +0.42 V) is next, allowing for a moderate energy yield. Sulfate (E₀' ≈ -0.22 V) has a much lower reduction potential, resulting in a significantly smaller energy yield and the lowest P/2e⁻ ratio among the three.
B and C are incorrect as they do not place O₂ first, which is the most potent electron acceptor.
D is incorrect because it misplaces sulfate and nitrate. The reduction potential of nitrate is significantly more positive than that of sulfate, so respiration with nitrate yields more energy than respiration with sulfate.
- A microbiologist creates a mutant strain of Escherichia coli that lacks a functional cydB gene, which codes for a subunit of the cytochrome bd oxidase complex. This complex has a high affinity for oxygen and is expressed under microaerobic conditions. The mutant is then compared to the wild-type strain. In which of the following conditions would the mutant's growth be most significantly impaired relative to the wild type?
Answer: An aerobic environment with a very low, growth-limiting partial pressure of oxygen.
The correct answer is B. The cytochrome bd oxidase is specifically adapted for scavenging oxygen under microaerobic (low oxygen) conditions due to its high affinity for O₂. A mutant lacking this complex would be at a significant disadvantage compared to the wild type when oxygen is present but scarce, as it would be less efficient at performing aerobic respiration.
A is incorrect because this is an anaerobic condition. The cytochrome bd oxidase is a terminal oxidase for aerobic respiration and is not involved in anaerobic respiration with nitrate. Both strains would use nitrate reductase and grow similarly.
C is incorrect because fermentation does not involve the electron transport chain or terminal oxidases. Both the mutant and wild-type strains would rely on substrate-level phosphorylation and grow similarly under these conditions.
D is incorrect because under high oxygen conditions, E. coli primarily uses the cytochrome bo oxidase, which has a lower affinity for oxygen but is more efficient at pumping protons. While the cydAB operon might be expressed, the loss of this specific high-affinity oxidase would have a less significant impact when oxygen is not a limiting factor compared to microaerobic conditions.
- A bacterial culture is treated with rotenone, a potent inhibitor of Complex I (NADH dehydrogenase) in the electron transport chain. The bacteria are capable of both aerobic and anaerobic respiration. Under which condition would the addition of rotenone have the LEAST impact on ATP production via oxidative phosphorylation?
Answer: Aerobic conditions where the cells are forced to metabolize succinate as the sole carbon source.
The correct answer is C. Rotenone inhibits Complex I, which accepts electrons from NADH. Succinate, however, is oxidized by Complex II (succinate dehydrogenase), which donates electrons to the quinone pool, bypassing Complex I. Therefore, if succinate is the sole carbon source, the electron transport chain can continue to function from Complex II onwards to reduce oxygen and generate a proton motive force, minimizing the impact of the rotenone.
A is incorrect. Under aerobic conditions with glucose, glycolysis and the Krebs cycle produce large amounts of NADH. The oxidation of this NADH is dependent on Complex I, so rotenone would severely inhibit ATP production.
B is incorrect. Anaerobic respiration with nitrate also relies on an electron transport chain that typically accepts electrons from NADH via an NADH dehydrogenase (a type of Complex I). Therefore, rotenone would also inhibit this process.
D is incorrect. Microaerobic conditions with high glucose catabolism would still produce a significant amount of NADH, making the cells sensitive to a Complex I inhibitor like rotenone.
- A deep-sea hydrothermal vent releases water rich in hydrogen sulfide (H₂S), methane (CH₄), and dissolved metals, but is nearly devoid of oxygen. Chemoautotrophic bacteria thrive in this environment, forming the base of the local food web. Given this chemical context, which metabolic strategy most likely predominates among these bacteria?
Answer: Anaerobic respiration using H₂S as the electron donor and sulfate (SO₄²⁻) from seawater as the terminal electron acceptor.
The correct answer is B. The environment is described as anoxic (nearly devoid of oxygen) but rich in potential electron donors like H₂S. Seawater contains sulfate (SO₄²⁻), which can serve as a terminal electron acceptor for anaerobic respiration. The use of an inorganic electron donor (H₂S) and an inorganic terminal electron acceptor other than oxygen (SO₄²⁻) is a classic example of anaerobic chemolithotrophy, a common strategy in such environments.
A is incorrect because the environment is described as nearly devoid of oxygen, making aerobic respiration an unlikely predominant strategy, even if some microaerobic respiration occurs at the fringes where vent water mixes with seawater.
C is incorrect because fermentation is a low-energy-yielding process that does not use an external electron acceptor. While some heterotrophic fermentation may occur, anaerobic respiration is a much more efficient way to capitalize on the available redox gradients and would likely be the dominant energy-generating strategy for primary producers.
D is incorrect because photosynthesis, by definition, uses light energy. While these organisms are chemoautotrophs (using chemical energy to fix carbon), the process is not photosynthesis, and geothermal heat cannot directly drive the photochemical reactions of photosynthesis.
- A mixed culture containing an obligate aerobe (Micrococcus luteus) and a facultative anaerobe (Escherichia coli) is inoculated into a sealed test tube containing a complex medium with glucose and a small, fixed amount of air in the headspace. Which of the following graphs best represents the population dynamics of the two organisms over time?
Answer: Both populations grow initially, but after the oxygen is consumed, the M. luteus population plateaus while the E. coli population continues to grow.
The correct answer is D. Initially, with oxygen present, both the obligate aerobe (M. luteus) and the facultative anaerobe (E. coli) will perform aerobic respiration and grow. Once the limited oxygen in the sealed tube is consumed, the obligate aerobe, M. luteus, can no longer perform respiration and will stop growing (enter stationary phase). However, E. coli, being a facultative anaerobe, will switch its metabolism to anaerobic respiration or fermentation. This will allow it to continue to grow, although likely at a slower rate than during the aerobic phase. Therefore, its population will continue to increase after the M. luteus population has plateaued.
A is incorrect because the depletion of oxygen, not glucose, will be the first limiting factor for the obligate aerobe.
B is incorrect because both organisms can grow simultaneously while oxygen is available.
C is incorrect because M. luteus would more likely enter a stationary phase rather than die off immediately, and the growth pattern described doesn't reflect the typical biphasic growth of facultative anaerobes.
- Both aerobic respiration and anaerobic respiration with nitrate as the electron acceptor generate a proton motive force (PMF) by pumping protons across a membrane. However, the magnitude of the PMF generated per pair of electrons from NADH is consistently lower when nitrate is the acceptor. Why is this the case?
Answer: The smaller difference in reduction potential between NADH and nitrate allows for fewer protons to be translocated.
The correct answer is C. The amount of energy available to pump protons is directly related to the difference in standard reduction potential (ΔE₀') between the electron donor (NADH) and the terminal acceptor. The ΔE₀' for the NADH/O₂ redox pair is much larger than for the NADH/NO₃⁻ redox pair. This smaller energy drop means that electrons traversing the anaerobic ETC release less energy, which is coupled to the translocation of fewer protons, resulting in a weaker PMF.
A is incorrect. While enzyme efficiencies vary, the primary reason for the lower PMF is thermodynamic, not a specific property like proton leakage of a single enzyme. The entire system is constrained by the lower energy yield.
B is incorrect. In many facultative anaerobes like E. coli, the Krebs cycle can operate under anaerobic conditions to completely oxidize substrates, providing electrons to the anaerobic respiratory chain. The cycle itself is not necessarily inhibited.
D is incorrect. Many bacterial anaerobic respiratory chains, including those for nitrate reduction, utilize a quinone pool (e.g., menaquinone) that plays a crucial role in electron and proton transport, similar to ubiquinone in aerobic respiration.
- A soil sample is taken from a waterlogged agricultural field that was recently treated with nitrogen-based fertilizer. Metagenomic analysis reveals a high abundance of genes encoding for the enzyme dissimilatory nitrate reductase. What is the primary metabolic function of this enzyme in the microorganisms within this specific environment?
Answer: To serve as the terminal enzyme in an electron transport chain, accepting electrons and conserving energy.
The correct answer is B. Dissimilatory nitrate reduction is the key process in anaerobic respiration using nitrate as a terminal electron acceptor (a process also known as denitrification). The enzyme dissimilatory nitrate reductase acts as the terminal reductase in the electron transport chain. It accepts electrons that have been passed down the chain, reducing nitrate (e.g., to nitrite) and in doing so contributes to the generation of a proton motive force for ATP synthesis.
A is incorrect. This describes assimilatory nitrate reduction. The purpose of assimilatory reduction is to acquire nitrogen for anabolic processes (building biomass), and it does not contribute to energy conservation via respiration.
C is incorrect. This describes nitrification, which is an aerobic process where ammonia is oxidized to nitrate. It is the opposite of denitrification and is carried out by different organisms (nitrifiers).
D is incorrect. While denitrification does convert nitrate to other forms like N₂, its primary purpose from the cell's perspective is not detoxification but energy conservation (respiration). Nitrate is not typically toxic to the organisms that use it for respiration.
- In facultative anaerobes like E. coli, the expression of respiratory enzymes is tightly regulated by environmental conditions. When an E. coli culture growing aerobically on glucose depletes the oxygen supply, a regulatory cascade is initiated. Which of the following represents the most critical transcriptional change for the cell to continue generating ATP via respiration?
Answer: Induction of genes encoding a terminal reductase for an alternative electron acceptor, such as nitrate reductase.
The correct answer is C. To switch from aerobic to anaerobic respiration, the cell must synthesize the appropriate terminal reductase for the available alternative electron acceptor (e.g., nitrate, fumarate). The transcription of genes like narGHIJ (for nitrate reductase) is induced under anaerobic conditions in the presence of nitrate. This allows the electron transport chain to be reconfigured and continue functioning.
A is incorrect. While fermentation genes might be upregulated if no external electron acceptors are available, the question asks how to continue ATP generation via respiration. Fermentation is not respiration.
B is incorrect. While the composition of the Krebs cycle may be altered (e.g., becoming a branched pathway), it is not necessarily repressed entirely, as it still provides precursor metabolites and can operate to provide electrons for anaerobic respiration. Downregulating NADH production would be counterproductive.
D is incorrect. While E. coli does have high-affinity oxidases for microaerobic conditions, this question describes a situation where oxygen is depleted, and the switch is to a completely anaerobic form of respiration. Scavenging trace oxygen is a different strategy from switching to a new acceptor.
- Methanogens are archaea that are obligate anaerobes. They conserve energy for growth by a process that uses hydrogen gas (H₂) as an electron donor and carbon dioxide (CO₂) as a terminal electron acceptor, producing methane (CH₄) as a byproduct. This metabolic strategy is best classified as:
Answer: Anaerobic respiration, because it utilizes an external, inorganic electron acceptor and an electron transport chain.
When you encounter questions about microbial metabolism, focus on identifying the electron donor, electron acceptor, and whether an electron transport chain is involved. These factors determine the metabolic classification.
Methanogens use a fascinating metabolic process called methanogenesis. They oxidize hydrogen gas (H₂) as their electron donor and reduce carbon dioxide (CO₂) as their terminal electron acceptor, transferring electrons through a specialized electron transport chain. This process generates a proton gradient that drives ATP synthesis, just like other forms of respiration. The methane (CH₄) produced is simply the reduced form of the CO₂ electron acceptor.
This process is anaerobic respiration because it uses an external, inorganic electron acceptor (CO₂) and involves electron transport for energy conservation, making answer D correct.
Answer A is wrong because fermentation doesn't use external electron acceptors - instead, organic molecules serve as both electron donors and acceptors. Answer B is incorrect because aerobic respiration specifically uses oxygen (O₂) as the terminal electron acceptor, not CO₂. Answer C misidentifies the process as photosynthesis; while CO₂ is being reduced, this isn't carbon fixation for biosynthesis, and there's no light energy involved - it's chemosynthesis using chemical energy from H₂ oxidation.
Remember this pattern: respiration always involves an external electron acceptor and electron transport. If it's oxygen, it's aerobic respiration. If it's anything else (nitrate, sulfate, CO₂, etc.), it's anaerobic respiration. This distinction is crucial for understanding microbial energy metabolism.
- An obligate anaerobe is found to perform respiration, but its electron transport chain completely lacks cytochromes. Electrons are passed from a dehydrogenase to a quinone, then to a series of iron-sulfur proteins, and finally to a terminal reductase. The absence of cytochromes in this respiratory chain most likely implies that:
Answer: the terminal electron acceptor has a very low standard reduction potential.
When you encounter questions about electron transport chains, focus on the relationship between the components present and the thermodynamic requirements of the system. Electron transport must follow thermodynamic rules - electrons flow from donors with lower reduction potentials to acceptors with higher potentials, releasing energy to pump protons.
In this cytochrome-free respiratory chain, electrons flow from dehydrogenase → quinone → iron-sulfur proteins → terminal reductase. Cytochromes typically have intermediate reduction potentials and help bridge the gap between electron donors and acceptors. Their absence suggests the terminal electron acceptor must have a very low (more negative) standard reduction potential, making the overall electron flow thermodynamically favorable without needing cytochromes as intermediates.
Answer A is correct because a terminal acceptor with very low reduction potential would make cytochromes unnecessary - the energy gap could be bridged by quinones and iron-sulfur proteins alone.
Answer B is wrong because organisms can still generate proton motive force without cytochromes. The dehydrogenase and terminal reductase can pump protons, and quinone cycling can contribute to the gradient.
Answer C is incorrect because both organic and inorganic compounds can serve as terminal acceptors with low reduction potentials. The acceptor type isn't determined by cytochrome absence.
Answer D is wrong because both archaea and bacteria can have cytochrome-free respiratory chains. This feature alone doesn't distinguish between domains.
Remember: cytochrome presence or absence reflects the thermodynamic requirements of the specific electron transport pathway, not the organism's domain or basic respiratory capacity.
- A researcher constructs artificial membrane vesicles (liposomes) containing two purified components from Wolinella succinogenes: a formate dehydrogenase complex and a fumarate reductase complex. The liposomes are suspended in a buffer containing formate. When fumarate is added to the buffer, the medium outside the liposomes becomes more alkaline. This experiment provides direct evidence that:
Answer: the oxidation of formate and reduction of fumarate can be coupled to generate a proton motive force.
This question tests your understanding of chemiosmotic coupling and electron transport chains in bacterial respiration. When you see experiments involving membrane vesicles and pH changes, think about proton gradients and energy conservation.
The key observation is that the medium outside becomes more alkaline (higher pH, fewer H⁺ ions) when fumarate is added. This means protons are being consumed from or pumped out of the external medium. In this reconstituted system, formate dehydrogenase oxidizes formate (releasing electrons), and fumarate reductase reduces fumarate using those electrons. The pH change indicates that this electron flow is coupled to proton translocation across the membrane, creating a proton motive force. This is exactly what answer A describes - the coupling of these redox reactions to generate electrochemical energy.
Answer B is wrong because the alkalinity increase is due to proton movement, not hydroxyl ion pumping, and there's no evidence fumarate reductase itself pumps ions. Answer C is incorrect because the experiment only shows the two complexes can function together in the same membrane - they don't need to be physically associated in a supercomplex. Answer D misunderstands the metabolism entirely: this is anaerobic respiration (using fumarate as an electron acceptor), not fermentation, and formate is inorganic, not organic.
Remember: when you see pH changes in membrane experiments, immediately think about proton gradients and energy conservation mechanisms. The direction of pH change tells you which way protons are moving.
- A facultative anaerobe, such as Paracoccus denitrificans, is cultured in a bioreactor with a continuous supply of glucose. The culture is initially grown under vigorous aeration. A sensor indicates that dissolved oxygen is completely depleted at hour 12, at which point a sterile nitrate solution is infused into the medium. Which of the following outcomes is most likely to be observed immediately following the nitrate infusion?
Answer: The electron transport chain will resume activity using a different terminal reductase, restoring a proton motive force.
The correct answer is C. When oxygen (the terminal electron acceptor for aerobic respiration) is depleted, the electron transport chain (ETC) ceases to function. For a facultative anaerobe capable of anaerobic respiration, the addition of an alternative electron acceptor like nitrate allows the cell to synthesize and use a different terminal reductase (nitrate reductase). This allows the ETC to resume, re-establishing the proton motive force (PMF) and allowing for continued ATP synthesis via oxidative phosphorylation, albeit at a lower yield than with oxygen.
A is incorrect because the standard reduction potential of the NO₃⁻/NO₂⁻ couple is lower than that of the O₂/H₂O couple. This results in a smaller proton motive force and thus a lower ATP yield per glucose molecule compared to aerobic respiration.
B is incorrect because the presence of an external electron acceptor (nitrate) allows the organism to perform anaerobic respiration, which is much more energy-efficient than fermentation. Fermentation would only be the primary strategy if no suitable external electron acceptors were available.
D is incorrect because the ATP yield from anaerobic respiration is much higher than from fermentation but lower than from aerobic respiration. To compensate for the lower ATP yield compared to the aerobic phase, the rate of glycolysis (and thus glucose consumption) would likely need to increase, not decrease, to maintain a similar overall rate of ATP production for cellular maintenance and growth.
- The theoretical maximum ATP yield from aerobic respiration is substantially greater than from any form of anaerobic respiration. The fundamental reason for this difference lies in the properties of the terminal electron acceptors. Which statement provides the most accurate and fundamental explanation?
Answer: The reduction of O₂ to H₂O has a highly positive standard reduction potential, maximizing the potential energy drop of the ETC.
The correct answer is B. The energy yield of an electron transport chain (ETC) is determined by the difference in electrochemical potential (ΔE) between the primary electron donor (like NADH) and the terminal electron acceptor. Oxygen has a very high standard reduction potential (E₀' = +0.82 V) compared to other acceptors like nitrate (E₀' = +0.42 V) or sulfate (E₀' = -0.22 V). This large potential difference allows for the maximum release of free energy as electrons traverse the ETC, which in turn allows for the maximum number of protons to be pumped, generating the strongest proton motive force and the highest ATP yield.
A is incorrect. Anaerobic respiration can also be coupled with the complete oxidation of glucose to CO₂ via the Krebs cycle. The final acceptor, not the completeness of glucose oxidation, is the primary determinant of the difference in energy yield.
C is incorrect. Anaerobic electron transport chains in many bacteria, such as E. coli, do utilize quinones (e.g., menaquinone instead of ubiquinone) to transfer electrons. The composition of the chain may differ, but quinones are not absent.
D is incorrect. Substrate-level phosphorylation occurs during glycolysis and the Krebs cycle, which can proceed under both aerobic and anaerobic conditions. It is independent of the terminal electron acceptor and oxygen.
- A facultative anaerobe is grown in a medium containing glucose, oxygen, and nitrate. The organism is observed to consume oxygen preferentially, only beginning to use nitrate as a terminal electron acceptor after the oxygen is fully depleted. What is the primary reason for this metabolic preference?
Answer: Aerobic respiration provides a significantly higher ATP yield, making it the more energetically favorable process.
The correct answer is C. Cells regulate their metabolic pathways to maximize energy efficiency. Aerobic respiration, using O₂ as the terminal electron acceptor, has a much larger free energy change (ΔG) and thus a much higher ATP yield per mole of glucose than anaerobic respiration with nitrate. Therefore, organisms will preferentially use the pathway that provides the most energy for growth and maintenance. This preference is enforced by complex regulatory networks (like ArcAB and FNR in E. coli) that repress the expression of anaerobic respiratory enzymes in the presence of oxygen.
A is incorrect. While this is a plausible mechanism, the primary level of control is transcriptional regulation. The expression of the nar operon (encoding nitrate reductase) is repressed by oxygen, so the enzyme isn't even present in significant amounts until oxygen is depleted.
B is incorrect. This describes catabolite repression by glucose, which controls the use of alternative carbon sources (like lactose). The choice of electron acceptor is controlled by regulators that sense oxygen or anaerobiosis, not glucose concentration in this context.
D is incorrect. While new enzymes (like nitrate reductase) must be synthesized, many components of the electron transport chain (like NADH dehydrogenase and the quinone pool) are shared. It is not an entirely new chain, and the primary reason for the preference is the superior energy yield of the aerobic pathway, not the lag time to synthesize components.
