- A bacterium capable of aerobic respiration is grown on two different sole carbon sources: glucose (a C₆ sugar) and acetate (a C₂ compound). Assuming complete oxidation to CO₂ and H₂O and equivalent transport costs, why does growth on glucose yield significantly more ATP per mole of substrate than growth on acetate?
Answer: Glucose oxidation generates more total reduced electron carriers (NADH, FADH₂) per mole than acetate oxidation does.
While substrate-level phosphorylation contributes (A), the vast majority of ATP comes from oxidative phosphorylation fueled by NADH and FADH₂. The complete catabolism of one glucose molecule involves glycolysis and the TCA cycle, yielding a total of 10 NADH and 2 FADH₂. One molecule of acetate is converted to acetyl-CoA and enters the TCA cycle, yielding only 3 NADH and 1 FADH₂ per molecule. Even on a per-carbon basis, glucose is more energy-rich because it is more reduced, and its breakdown through glycolysis generates significant reducing power before the TCA cycle, leading to a much higher total yield of reduced coenzymes.
- Escherichia coli is a facultative anaerobe. When an aerobic culture of E. coli growing on glucose exhausts its supply of O₂, it switches to mixed-acid fermentation. This metabolic shift is primarily driven by the immediate need to:
Answer: reoxidize NADH to NAD⁺ to allow glycolysis to continue producing ATP.
In the absence of O₂, the electron transport chain cannot function, and NADH produced during glycolysis accumulates. Glycolysis requires a constant supply of NAD⁺ to function (specifically for the glyceraldehyde-3-phosphate dehydrogenase step). The primary purpose of fermentation is to use organic molecules (pyruvate or its derivatives) as endogenous electron acceptors to reoxidize this excess NADH to NAD⁺, allowing glycolysis to continue and produce a small amount of ATP through substrate-level phosphorylation.
- An organism isolated from a deep-sea hydrothermal vent is cultured in a laboratory. It shows robust growth in a mineral-based medium containing H₂S, NH₄⁺, and phosphate, bubbled with a gas mixture of CO₂ and O₂. Growth ceases when H₂S is removed from the medium. Growth is also not observed when the medium is supplemented with glucose in the absence of H₂S. What is the metabolic classification of this organism?
Answer: Chemolithoautotroph
The organism uses an inorganic chemical (H₂S) as its energy source (chemo-), an inorganic electron donor (litho-), and an inorganic carbon source (CO₂) for growth (auto-). The lack of growth on glucose confirms it is not a heterotroph, and the inability to switch between inorganic and organic energy sources means it is not a mixotroph in this context. Lack of a light requirement rules out phototrophy. Therefore, it is a chemolithoautotroph.
- Clostridium acetobutylicum performs ABE (acetone-butanol-ethanol) fermentation on glucose. A key metabolic challenge in any fermentation is the regeneration of NAD⁺ from the NADH produced during glycolysis. How is this accomplished in ABE fermentation?
Answer: By reducing metabolic intermediates derived from acetyl-CoA to produce butanol and ethanol.
Fermentation pathways must reoxidize the NADH produced during glycolysis to NAD⁺ to maintain redox balance and allow glycolysis to continue. In ABE fermentation, intermediates derived from pyruvate and acetyl-CoA (like acetoacetyl-CoA and butyryl-CoA) serve as the electron acceptors. Their reduction to the final products of acetone, butanol, and ethanol consumes NADH, regenerating the required pool of NAD⁺.
- In an anoxic sediment, a co-culture of Syntrophomonas and a methanogen is observed. Syntrophomonas ferments butyrate to acetate and H₂, a reaction that is thermodynamically unfavorable (ΔG > 0) unless the products are efficiently removed. The methanogen consumes H₂ to reduce CO₂ and produce methane. Which statement best describes the carbon flow and metabolic roles in this syntrophic relationship?
Answer: Syntrophomonas acts as a chemoorganotroph converting butyrate to acetate, while the methanogen acts as a chemolithoautotroph using H₂ and CO₂.
Syntrophomonas catabolizes an organic compound (butyrate), making it a chemoorganotroph (specifically, a chemoorganoheterotroph). The hydrogenotrophic methanogen is a classic example of a chemolithoautotroph: it uses an inorganic energy/electron source (H₂) to generate energy and fixes an inorganic carbon source (CO₂) into methane and biomass. The removal of H₂ by the methanogen makes the butyrate fermentation thermodynamically favorable for Syntrophomonas.
- In a chemoorganoheterotroph growing aerobically, the tricarboxylic acid (TCA) cycle is a central metabolic hub that completely oxidizes acetyl-CoA for energy and provides precursors for biosynthesis. In a chemolithoautotroph that uses the Calvin cycle for carbon fixation, what is the primary role of the TCA cycle enzymes?
Answer: To serve as a source of precursor metabolites for biosynthesis, often as a branched, non-cyclic pathway.
An autotroph's primary goal is to build biomass from CO₂. It would be counterproductive to take the carbon fixed via the Calvin cycle and then immediately oxidize it completely in the TCA cycle. However, the cell still needs the intermediates of the TCA cycle (e.g., α-ketoglutarate, oxaloacetate) as starting materials for synthesizing amino acids and other essential molecules. Therefore, these organisms use the TCA cycle enzymes in a branched, biosynthetic fashion, pulling out intermediates as needed, rather than running it as a complete energy-generating cycle.
- A major evolutionary innovation that led to the global proliferation of cyanobacteria was the development of oxygenic photosynthesis. What is the key difference in the source of electrons for carbon fixation between anoxygenic purple sulfur bacteria and oxygenic cyanobacteria?
Answer: Purple sulfur bacteria use reduced sulfur compounds like H₂S, while cyanobacteria use water (H₂O).
The fundamental difference lies in the electron donor. Anoxygenic phototrophs like purple sulfur bacteria use reduced compounds such as H₂S, S⁰, or H₂ as their source of electrons. Cyanobacteria evolved the ability (via Photosystem II) to use H₂O as an electron donor. Since water is vastly more abundant than H₂S, this adaptation allowed cyanobacteria to colonize nearly every illuminated environment on Earth and led to the oxygenation of the atmosphere. While D is also a true difference, B describes the key difference in the source of electrons.
- A facultative anaerobe is grown in a glucose-rich medium. Condition 1 is anaerobic with nitrate (NO₃⁻) available. Condition 2 is anaerobic without any external electron acceptors. Which statement accurately contrasts the primary fate of glucose-derived carbon in these two conditions?
Answer: In Condition 1, carbon is mostly oxidized to CO₂ via the TCA cycle, whereas in Condition 2, it is converted to organic fermentation products.
Condition 1 describes anaerobic respiration, where nitrate serves as a terminal electron acceptor, allowing the TCA cycle and an electron transport chain to operate. This results in the relatively complete oxidation of glucose to CO₂. Condition 2 describes fermentation, where in the absence of an external electron acceptor, pyruvate is converted into various organic end products (e.g., lactate, acetate, ethanol) to regenerate NAD⁺ from NADH, resulting in incomplete oxidation of glucose.
- The reductive or reverse Krebs cycle is a carbon fixation pathway used by some anaerobic autotrophs. Which of the following is a key feature that distinguishes this pathway from the oxidative Krebs cycle found in chemoorganoheterotrophs?
Answer: The net consumption of ATP and reduced electron carriers (e.g., NADH, ferredoxin) to drive the cycle.
The oxidative Krebs cycle is catabolic, releasing energy and generating ATP and reduced electron carriers (NADH, FADH₂). The reverse Krebs cycle is anabolic; its purpose is to build organic molecules from CO₂. This process is energetically unfavorable and requires an input of energy (ATP) and reducing power (reduced electron carriers like NADH or ferredoxin) to drive the reactions in the reductive direction, ultimately producing acetyl-CoA from two molecules of CO₂.
- A photoheterotrophic bacterium, such as Rhodobacter sphaeroides, is growing anaerobically in the light on a medium with succinate as the sole carbon source. What is the primary role of light energy in this metabolic process?
Answer: To drive the synthesis of ATP via photophosphorylation, which powers cellular processes.
In photoheterotrophy, the organism uses two different sources for energy and carbon. Light energy is captured by photosynthetic pigments and used to generate a proton motive force, which drives the synthesis of ATP (photophosphorylation). The organic compound (succinate) is used as the carbon source for building biomass and may also serve as an electron source, but the primary energy for cellular work comes from light, not from the chemical oxidation of succinate.
- A methylotroph is an organism that can use reduced one-carbon compounds, such as methanol or methane, as its sole source of carbon and energy. Which statement correctly describes a key step in the assimilation of this carbon into biomass?
Answer: The C₁ compound is oxidized to formaldehyde, which is then assimilated into a larger sugar molecule.
In the major methylotrophic assimilation pathways (the serine cycle and the ribulose monophosphate cycle), the reduced C₁ substrate is first oxidized to the key intermediate formaldehyde (CH₂O). This formaldehyde is then incorporated into a larger acceptor molecule (glycine in the serine cycle, ribulose-5-phosphate in the RuMP cycle) to form a three-carbon or six-carbon compound, respectively, which can then enter central metabolism. While some methylotrophs are autotrophs that use the Calvin cycle (B), direct assimilation of formaldehyde is more common and characteristic.
- In a thermally stratified freshwater lake during summer, the epilimnion (upper layer) is oxygen-rich and well-lit, while the hypolimnion (bottom layer) is anoxic, dark, and rich in H₂S from decomposition. A layer of purple sulfur bacteria forms a dense plate at the chemocline, the interface between these two zones. Which combination of resources best explains the success of these bacteria in this specific niche?
Answer: Sunlight penetrating from the epilimnion and hydrogen sulfide (H₂S) diffusing up from the hypolimnion.
Purple sulfur bacteria are anoxygenic photoautotrophs. They require light for energy, which is available at the chemocline as it penetrates from the epilimnion. They are anaerobic and use a reduced inorganic compound, such as H₂S, as an electron donor for photosynthesis. H₂S is abundant in the anoxic hypolimnion and diffuses upwards. The chemocline is the optimal niche where both required resources, light and H₂S, are simultaneously available.
- A protist is classified as a mixotroph, capable of both photosynthesis and phagotrophy. Under which condition would this organism be most likely to rely primarily on phagocytosis of bacteria for its carbon and energy needs?
Answer: When incubated in a prolonged dark environment, even if dissolved inorganic nutrients are plentiful.
Mixotrophs use different metabolic strategies depending on environmental conditions. Photosynthesis is dependent on light as an energy source. In a dark environment, the organism cannot perform photosynthesis to generate ATP or fix carbon. Therefore, it must switch to a heterotrophic mode, such as phagocytosis (engulfing bacteria), to acquire pre-formed organic molecules that serve as both a carbon and energy source.
- Nitrosomonas europaea is a soil bacterium that plays a crucial role in the nitrogen cycle. It obtains energy by oxidizing ammonia (NH₃) to nitrite (NO₂⁻) and uses this energy to fix atmospheric CO₂ into biomass using the Calvin cycle. This organism is best described as a:
Answer: Chemolithoautotroph
The classification is based on energy, electron, and carbon sources. The energy source is a chemical reaction (oxidation of ammonia), so it is a 'chemotroph'. The electron donor is an inorganic molecule (ammonia), so it is a 'lithotroph'. The carbon source for biomass is inorganic (CO₂), so it is an 'autotroph'. Combining these terms gives chemolithoautotroph. It is not an organotroph because the electron donor is not organic.
- The bacterium Acidithiobacillus ferrooxidans thrives in acidic environments (pH ~2) and obtains energy by oxidizing ferrous iron (Fe²⁺) to ferric iron (Fe³⁺). The standard redox potential for the Fe³⁺/Fe²⁺ couple is high (+0.77 V), close to that of the O₂/H₂O couple (+0.82 V), resulting in a very small free energy yield. How does this organism generate sufficient ATP to grow?
Answer: By oxidizing enormous quantities of Fe²⁺ to generate a proton motive force across its membrane.
The small potential difference between the electron donor (Fe²⁺) and the typical acceptor (O₂) means that the energy yield per electron transferred is very low. To compensate, these chemolithotrophs must process a very large amount of the substrate (Fe²⁺). They have a highly active electron transport chain that oxidizes vast quantities of iron, allowing them to pump enough protons to generate a sufficient proton motive force for ATP synthesis and reverse electron flow to make NADH.
- A groundwater site is contaminated with tetrachloroethylene (PCE), a chlorinated solvent. A bioremediation strategy involves stimulating the activity of Dehalococcoides species. These bacteria use PCE as a terminal electron acceptor in a process called dehalorespiration, reducing it to ethene. To be effective, this strategy must also include the addition of which two types of compounds to the groundwater?
Answer: An electron donor (like H₂ or lactate) and an organic carbon source (like acetate).
Dehalococcoides are typically chemoorganoheterotrophs that perform a type of anaerobic respiration. Like all respiring organisms, they need an electron donor to provide electrons for their electron transport chain (in this case, H₂ or simple organic acids are common donors). They also require a source of organic carbon for building cellular materials (acetate is a common requirement). PCE serves only as the terminal electron acceptor, not as a carbon or energy source.
- A chemoorganoheterotroph is growing aerobically on acetate as its sole carbon source. To sustain the TCA cycle for both energy production and biosynthesis of precursors (e.g., α-ketoglutarate, oxaloacetate), the cell must have a mechanism to replenish cycle intermediates that are withdrawn for anabolism. Which pathway is essential for this organism under these conditions?
Answer: The glyoxylate shunt
When growing on two-carbon compounds like acetate, the cell cannot afford to lose carbon as CO₂ in the TCA cycle if it also needs to draw off four-carbon intermediates for biosynthesis. The glyoxylate shunt is an anaplerotic pathway that bypasses the two decarboxylation steps of the TCA cycle (isocitrate dehydrogenase and α-ketoglutarate dehydrogenase). This allows for the net synthesis of a four-carbon compound (succinate or malate) from two molecules of acetyl-CoA, thus replenishing the cycle.
- In the metabolism of Thiobacillus denitrificans, elemental sulfur (S⁰) is oxidized to sulfate (SO₄²⁻), a process coupled to the reduction of nitrate (NO₃⁻) to nitrogen gas (N₂). The organism fixes CO₂ for biomass. How are sulfur and CO₂ functioning in its metabolism?
Answer: Sulfur is the energy and electron source, and CO₂ is the carbon source.
This organism is a chemolithoautotroph. The oxidation of an inorganic compound, sulfur (S⁰), provides the energy (chemo-) and electrons (litho-) for the cell. CO₂ is used as the sole source of carbon to build cellular components (autotroph). Nitrate, not CO₂, serves as the terminal electron acceptor in this example of anaerobic respiration.
- A microbiologist cultures a purple non-sulfur bacterium in a medium containing malate as the sole carbon and electron source. The culture is incubated in the light under anaerobic conditions. If the malate provided is labeled with ¹⁴C, where would the label be most prominently detected after one generation of growth?
Answer: Primarily in newly synthesized cellular proteins, lipids, and nucleic acids.
Purple non-sulfur bacteria growing under these conditions are photoheterotrophs. They use light for energy (photophosphorylation) but utilize organic compounds (malate) as their carbon source for building biomass. Therefore, the ¹⁴C from malate will be incorporated into all major classes of macromolecules (proteins, lipids, nucleic acids, etc.) as the cells grow and divide. They do not primarily fix CO₂ under these conditions, nor do they completely oxidize the organic carbon to CO₂ as a chemoheterotroph would.
