Q: Enzymes of phosphorus cycling: releasing orthophosphate Which enzymes catalyze the liberation of orthophosphate (Pi) from organic phosphorus compounds and from inorganic pyrophosphate in biological systems?

Introduction / Context: Phosphorus turnover in cells and ecosystems depends on enzymes that release inorganic phosphate from organic substrates. These activities are crucial in nutrient cycling, biotechnology assays, and diagnostic tests. Recognizing the correct enzyme class helps interpret lab results and environmental processes. Given Data / Assumptions: Target reaction: hydrolytic cleavage to liberate orthophosphate (Pi). Substrates include organic phosphate esters and inorganic pyrophosphate (PPi). Physiological pH may vary; alkaline variants are widely used/measured. Concept / Approach: Phosphatases hydrolyze phosphate esters to release Pi. Alkaline phosphatases are active at alkaline pH and can also act on pyrophosphate (with specific pyrophosphatases also contributing). In contrast, oxidoreductases change oxidation states; proteases hydrolyze peptide bonds; hydrogenases interconvert protons and hydrogen gas. Step-by-Step Solution: Identify reaction type: hydrolysis of phosphate esters or PPi to yield Pi.Match to enzyme class: phosphatases (not oxidoreductases or proteases).Select “Alkaline phosphatases”. Verification / Alternative check: Clinical chemistry assays for serum alkaline phosphatase activity use chromogenic phosphate esters, monitoring Pi release; microbial extracellular phosphatases mobilize phosphorus from organic matter. Why Other Options Are Wrong: Oxidoreductase/Dehydrogenase: Catalyze redox, not phosphate ester hydrolysis. Protease: Targets peptide bonds, not phosphate esters. Hydrogenase: Involves H2/H+ interconversion, unrelated to Pi release. Common Pitfalls: Confusing pyrophosphatase specificity with general phosphatase action; many alkaline phosphatases have PPi activity. Final Answer: Alkaline phosphatases

Q: Glyoxylate cycle identity check Which enzyme set is specific to the glyoxylate cycle (the bypass that enables growth on C2 compounds by conserving carbon)?

Introduction / Context: The glyoxylate cycle is a metabolic shunt used by many bacteria, fungi, and plants to convert acetyl-CoA into four-carbon dicarboxylic acids without loss of carbon as CO2. It is essential for growth on acetate, fatty acids, or other C2 substrates and is distinguished by two hallmark enzymes. Given Data / Assumptions: The cycle bypasses the CO2-producing steps of the TCA cycle. Two unique enzymes define this bypass. Objective: identify those specific enzymes. Concept / Approach: Isocitrate lyase cleaves isocitrate into succinate and glyoxylate. Malate synthase then condenses glyoxylate with acetyl-CoA to form malate. Together they enable net assimilation of acetyl units into cell material, avoiding the decarboxylating steps of TCA. Step-by-Step Solution: Recall the two signature enzymes: isocitrate lyase and malate synthase.Confirm that both are required for the glyoxylate shunt to operate.Choose “both (a) and (b)”. Verification / Alternative check: Mutants lacking either enzyme cannot grow on acetate as sole carbon source, underscoring their specificity to the glyoxylate pathway. Why Other Options Are Wrong: Isocitrate lyase or malate synthase alone: Incomplete pathway. Anaplerotic: Describes replenishing reactions (for example, PEP carboxylase), not the glyoxylate-specific enzymes. Pyruvate dehydrogenase: Part of central metabolism but not the glyoxylate-specific step. Common Pitfalls: Confusing “anaplerotic” with “glyoxylate”; while related to TCA balance, they are distinct concepts. Final Answer: both (a) and (b)

Question 1

In aerobic cellular respiration, which molecule serves as the terminal (final) electron acceptor in the electron transport chain?

Accepted Answer

Introduction / Context: Respiratory chains transfer electrons from reduced carriers to a terminal acceptor, generating a proton motive force to drive ATP synthesis. The identity of the terminal acceptor distinguishes aerobic respiration from various forms of anaerobic respiration. Given Data / Assumptions: The context is aerobic respiration. Standard mitochondrial or bacterial electron transport is implied. Common alternate acceptors (e.g., nitrate) are considered anaerobic. Concept / Approach: In aerobic respiration, oxygen (O2) is reduced to water at complex IV (cytochrome c oxidase). This high-potential acceptor allows maximal free energy capture, supporting efficient ATP production compared with most anaerobic acceptors. Step-by-Step Solution: Identify process: aerobic → must use O2.Recall ETC endpoint: electrons + O2 + protons → H2O.Select oxygen. Verification / Alternative check: Inhibitors like cyanide block cytochrome c oxidase, preventing reduction of O2 and halting aerobic respiration, demonstrating O2’s role as terminal acceptor. Why Other Options Are Wrong: Nitrogen/Nitrate: Nitrate (NO3−) can be a terminal acceptor in anaerobic respiration (denitrification), not aerobic. Hydrogen: Is an electron donor in many pathways, not the terminal acceptor here. Common Pitfalls: Confusing “presence of oxygen” with oxidative phosphorylation generally; the key is that O2 is the final acceptor, forming water. Final Answer: oxygen

Question 2

Photosynthesis basics: which combination lists all products generated directly by the light reactions (photochemical phase) of photosynthesis?

Accepted Answer

Introduction / Context: The light reactions capture solar energy to produce high-energy compounds that power the Calvin–Benson cycle. Knowing exactly what the light reactions generate clarifies how the light-independent reactions are fueled. Given Data / Assumptions: Chloroplast thylakoid membranes host photosystems II and I. Water is split (photolysis) at photosystem II, releasing O2. ATP is made via photophosphorylation, and NADPH is produced via ferredoxin–NADP+ reductase. Concept / Approach: Light energy drives electron flow from water to NADP+, forming NADPH, while the proton gradient across the thylakoid membrane drives ATP synthase to produce ATP. Splitting water produces O2 as a by-product. These are the direct outputs of the photochemical phase. Step-by-Step Solution: Account for energy currency: ATP generated by photophosphorylation.Account for reducing power: NADPH formed at the end of the electron transport chain.Account for gas product: O2 released from water splitting at PSII. Verification / Alternative check: Isolated thylakoid experiments show ATP and NADPH formation upon illumination and oxygen evolution measured with oxygen electrodes. Why Other Options Are Wrong: Options listing only one or two of the three products are incomplete. The light reactions produce all three: ATP, NADPH, and O2. Common Pitfalls: Confusing products of light reactions with those of the Calvin cycle (e.g., carbohydrate synthesis). Sugars are produced in the dark reactions using ATP and NADPH from the light reactions. Final Answer: ATP, NADPH, and O2

Question 3

Metabolic pathways: the pentose phosphate pathway (also called the phosphogluconate pathway or hexose monophosphate shunt) occurs in which types of cells?

Accepted Answer

Introduction / Context: The pentose phosphate pathway (PPP) is a ubiquitous metabolic route that generates NADPH for reductive biosynthesis and ribose-5-phosphate for nucleotide synthesis. Knowing its distribution across life forms emphasizes its fundamental role. Given Data / Assumptions: PPP provides NADPH and pentose sugars in many organisms. It functions in parallel with glycolysis, with both oxidative and nonoxidative branches. Cell type options include prokaryotes and eukaryotes. Concept / Approach: The PPP is present in both prokaryotic and eukaryotic cells. In eukaryotes, it is prominent in the cytosol, especially in tissues requiring high NADPH (e.g., liver, adipose, adrenal cortex, and red blood cells). In bacteria, it supports biosynthesis and oxidative stress defense. Step-by-Step Solution: Identify the universal metabolic needs: NADPH and ribose-5-phosphate are required widely.Recognize PPP enzymes are conserved across domains of life.Select “prokaryotic and eukaryotic cells both.” Verification / Alternative check: Genomic and biochemical studies show PPP genes in bacteria and eukaryotes; clinical relevance includes G6PD deficiency affecting the oxidative branch in human RBCs. Why Other Options Are Wrong: Prokaryotic only / Eukaryotic only: Too narrow; PPP is widespread. None of these: False because PPP clearly exists. Common Pitfalls: Assuming PPP is “alternative” and rare; in fact, it is central for NADPH and nucleotide biosynthesis in many cell types. Final Answer: prokaryotic and eukaryotic cells both

Question 4

Cell transport physiology: passive diffusion across biological membranes In cellular physiology, solute molecules cross the lipid bilayer by passive diffusion primarily as a result of which driving force?

Accepted Answer

Introduction / Context: Passive diffusion is a fundamental transport process in cell biology. Without using metabolic energy, small nonpolar molecules (for example, O2, CO2) move across the phospholipid bilayer. Understanding what actually drives this movement clarifies how cells exchange gases and small solutes and why some substances require channels or carriers. Given Data / Assumptions: No transporter proteins or pumps are involved (that would be facilitated diffusion or active transport). The solute is moving directly through the lipid bilayer or an always-open pore without gating or coupling to ATP. The membrane is intact and at constant temperature and pressure for both sides of the bilayer in typical laboratory/physiological settings. Concept / Approach: Passive diffusion is driven by a gradient in chemical potential. For uncharged solutes, the chemical potential gradient is essentially the concentration difference across the membrane. Ions can be influenced by electrical potential as well, but simple passive permeation of ions through pure lipid is negligible, which is why cells require channels. Therefore, when we speak about passive diffusion across the bilayer, the operative driving force is the concentration difference of a permeant solute. Step-by-Step Solution: Identify the transport type: passive diffusion (no energy, no carrier).Recall Fick’s law: net flux is proportional to the concentration gradient across the membrane.Conclude that the primary driver is the concentration difference. Verification / Alternative check: Fick’s first law states: J = -D * (dC/dx). For a thin membrane, this scales with the difference in concentrations across the bilayer, consistent with observed oxygen and carbon dioxide fluxes in cells and tissues. Why Other Options Are Wrong: Pressure difference: Drives bulk flow/filtration through porous barriers, not molecular diffusion through lipid. Ionic difference: Electrical gradients matter for ions through channels; bare bilayers are effectively impermeable to most ions. All of these: Overbroad; only concentration difference is correct for passive diffusion across lipid bilayer. Temperature difference: Affects diffusion coefficient slightly but is not the directional driving force in typical physiology. Common Pitfalls: Confusing passive diffusion with osmosis/filtration, which involve water movement or pressure gradients. Assuming ions diffuse freely through lipid; in reality they require channels due to charge and hydration shell. Final Answer: concentration difference

Question 5

Photosynthetic pigments: absorption by chlorophyll in eukaryotes and cyanobacteria Chlorophyll molecules used by eukaryotic phototrophs and by cyanobacteria absorb radiant energy most strongly in which portion(s) of the visible spectrum?

Accepted Answer

Introduction / Context: Understanding the absorption spectrum of chlorophyll explains why plants look green and how photosynthetic organisms harvest light. Chlorophyll a (and accessory chlorophylls) in algae, plants, and cyanobacteria have characteristic peaks that determine light-use efficiency in photosynthesis. Given Data / Assumptions: Organisms considered: eukaryotic phototrophs (plants/algae) and cyanobacteria. Main pigment: chlorophylls (especially chlorophyll a). We are focusing on the visible spectrum region where absorption is strongest. Concept / Approach: Chlorophyll strongly absorbs blue/violet (short wavelengths) and red (long wavelengths) light, while reflecting/transmitting green light, which is why many phototrophs appear green. Accessory pigments broaden capture, but the primary strong absorption bands remain in the blue and red regions for chlorophyll a-containing organisms, including cyanobacteria. Step-by-Step Solution: Recall chlorophyll absorption peaks near the blue (~430–453 nm) and red (~640–680 nm) ranges.Note weak absorption in green (~500–550 nm), leading to green appearance.Therefore, choose the option “red and blue”. Verification / Alternative check: Standard spectra show strong Soret band in blue and Qy band in red for chlorophyll a; cyanobacteria also contain phycobiliproteins that capture additional wavelengths but do not change the core chlorophyll peaks. Why Other Options Are Wrong: Red only: Ignores strong blue absorption. Green: Chlorophyll reflects green; absorption is minimal. Green and ultraviolet: Not aligned with chlorophyll’s main peaks. Yellow only: Not a primary chlorophyll absorption maximum. Common Pitfalls: Assuming the color seen equals the absorption peak; it is the opposite. Final Answer: red and blue

Question 6

Biological nitrogen fixation requirements Nitrogen fixation (conversion of N2 to ammonia by nitrogenase) requires which of the following conditions?

Accepted Answer

Introduction / Context: Biological nitrogen fixation is carried out by certain bacteria and archaea using the nitrogenase enzyme complex. The process is ecologically crucial but biochemically demanding. Knowing its requirements explains why only select microbes can fix nitrogen and why they often have special adaptations to manage oxygen exposure. Given Data / Assumptions: Nitrogenase reduces N2 to NH3. ATP consumption during electron transfer is substantial. Nitrogenase is irreversibly inactivated by molecular oxygen. Concept / Approach: Nitrogenase requires a large input of energy (ATP) and reducing power. Because oxygen damages key metal centers in nitrogenase, fixation typically proceeds under anaerobic or microaerobic conditions. Aerobic diazotrophs (for example, Azotobacter) use high respiration rates or protective proteins to keep nitrogenase protected, effectively creating a low O2 microenvironment. Step-by-Step Solution: Recognize ATP dependence: fixation is energy-intensive.Account for O2 sensitivity: enzyme requires anaerobic or protected conditions.Select “both (a) and (b)”. Verification / Alternative check: Typical stoichiometry shows high ATP cost per N2 reduced; legumes provide an O2-buffering protein (leghemoglobin) in root nodules to maintain an anaerobic environment around rhizobial nitrogenase. Why Other Options Are Wrong: Energy only or anaerobic only: Incomplete; both are required. Aerobic environment: Free O2 inactivates nitrogenase. Neither: Contradicts well-established biochemistry of nitrogenase. Common Pitfalls: Confusing organism-level aerobiosis with enzyme-level protection; even “aerobic” fixers protect nitrogenase from O2. Final Answer: both (a) and (b)

Question 7

Energy yield comparison: aerobic catabolism vs. fermentation Relative to glucose fermentation, how much ATP does complete aerobic catabolism of glucose synthesize in typical cells?

Accepted Answer

Introduction / Context: Cells can harvest energy from glucose via fermentation or aerobic respiration. Fermentation yields ATP only through substrate-level phosphorylation, whereas respiration uses both substrate-level and oxidative phosphorylation. Comparing yields is basic to microbial physiology and bioenergetics. Given Data / Assumptions: Fermentation of glucose typically yields 2 ATP per glucose. Aerobic respiration in bacteria can yield up to ~38 ATP per glucose (organism and conditions dependent); eukaryotes commonly yield ~30–32 ATP. We are asked for an order-of-magnitude comparison, not an exact number. Concept / Approach: Because aerobic respiration captures electrons into an electron transport chain and uses oxygen as the terminal electron acceptor, the proton motive force drives ATP synthase to generate far more ATP than fermentation alone. Even conservative yields far exceed ten times the fermentative yield. Step-by-Step Solution: Recall fermentation yield: ~2 ATP/glucose.Recall aerobic yield: often 30+ ATP/glucose.Compute ratio: 30 / 2 = 15, which is greater than 10.Select “More than 10 times as much”. Verification / Alternative check: Even with organism variability and P/O ratio differences, respiration’s yield is consistently an order of magnitude higher than fermentation. Why Other Options Are Wrong: Slightly less/About the same/Twice: All underestimate respiratory ATP gains. Exactly five times: Also too low; respiration greatly exceeds this ratio. Common Pitfalls: Treating the textbook 38 ATP as exact; actual yields vary, but still greatly surpass fermentation. Final Answer: More than 10 times as much

Question 8

Anoxygenic photosynthesis: primary electron donor In anoxygenic photosynthetic bacteria, which substance is typically oxidized as the electron donor instead of water?

Accepted Answer

Introduction / Context: Not all phototrophs release oxygen. Anoxygenic photosynthetic bacteria (for example, purple sulfur bacteria, green sulfur bacteria) use alternative electron donors and thus do not split water or produce O2. Identifying their common donors is key to environmental microbiology and biogeochemical cycles. Given Data / Assumptions: Organisms: anoxygenic phototrophs. They lack photosystem II water-splitting oxygen-evolving complex. Common reduced substrates include hydrogen sulfide (H2S) or elemental sulfur; some can use H2 or organic acids. Concept / Approach: In oxygenic photosynthesis (plants, algae, cyanobacteria), water is oxidized to O2. In contrast, anoxygenic phototrophs often oxidize sulfide to sulfur or sulfate. Therefore, sulfide is the characteristic alternative electron donor in classic anoxygenic systems. Step-by-Step Solution: Identify process: anoxygenic photosynthesis (no O2 evolution).Recall typical donors: H2S > S0 > H2 in many taxa.Select “sulfide”. Verification / Alternative check: Ecophysiology of purple sulfur bacteria shows oxidation of H2S to S0 and SO4^2- as hallmark metabolism in illuminated anaerobic zones of stratified waters. Why Other Options Are Wrong: Water: Oxidized only in oxygenic phototrophs. Oxgyen (oxygen): Not an electron donor; it is an oxidant. Ammonia: Some chemolithotrophs oxidize ammonia, but that is not photosynthetic electron donation in these bacteria. Hydrogen peroxide: A reactive oxygen species, not the typical donor. Common Pitfalls: Equating all photosynthesis with oxygenic processes; many bacteria are anoxygenic. Final Answer: sulfide

Question 9

Bioenergetics: linking redox potential to standard free energy What is the correct relationship between the standard free energy change and the standard redox potential difference (n = moles of electrons, F = Faraday's constant, E° = standard redox …

Accepted Answer

Introduction / Context: In microbial metabolism, electron transport chains convert redox potential differences into usable energy. The quantitative link between a redox couple’s potential difference and the Gibbs free energy change allows prediction of directionality and ATP-generating capacity of respiratory pathways. Given Data / Assumptions: E° is the standard oxidation–reduction potential difference (volts). n is the number of electrons transferred. F is Faraday’s constant (approximately 96,485 C per mol e^-). Concept / Approach: The thermodynamic relation is ΔG° = -n F E°. A positive E° (favorable electron flow from donor to acceptor) yields a negative ΔG°, signifying a spontaneous process under standard conditions. This underpins how microbes harness energy by transferring electrons to acceptors with higher redox potentials. Step-by-Step Solution: Recall the electrochemical identity connecting electrical work to chemical free energy.Insert sign convention: favorable E° > 0 corresponds to ΔG° < 0.Choose the expression with the negative sign: ΔG° = -n F E°. Verification / Alternative check: Example: If E° = +0.2 V and n = 2, ΔG° = -2 * 96485 * 0.2 ≈ -38.6 kJ/mol, consistent with energy release usable for ATP synthesis. Why Other Options Are Wrong: ΔG° = n F E°: Wrong sign; would predict positive ΔG° for favorable E°. Expressions with lnE°: Dimensional inconsistency; E° is already in volts. ΔG° = -R T E°: Mixes gas constant relation improperly; the correct relation uses nF. Common Pitfalls: Forgetting the negative sign, which inverts spontaneity predictions. Final Answer: ΔG° = -nFE°

Question 10

Enzymes of phosphorus cycling: releasing orthophosphate Which enzymes catalyze the liberation of orthophosphate (Pi) from organic phosphorus compounds and from inorganic pyrophosphate in biological systems?

Accepted Answer

Introduction / Context: Phosphorus turnover in cells and ecosystems depends on enzymes that release inorganic phosphate from organic substrates. These activities are crucial in nutrient cycling, biotechnology assays, and diagnostic tests. Recognizing the correct enzyme class helps interpret lab results and environmental processes. Given Data / Assumptions: Target reaction: hydrolytic cleavage to liberate orthophosphate (Pi). Substrates include organic phosphate esters and inorganic pyrophosphate (PPi). Physiological pH may vary; alkaline variants are widely used/measured. Concept / Approach: Phosphatases hydrolyze phosphate esters to release Pi. Alkaline phosphatases are active at alkaline pH and can also act on pyrophosphate (with specific pyrophosphatases also contributing). In contrast, oxidoreductases change oxidation states; proteases hydrolyze peptide bonds; hydrogenases interconvert protons and hydrogen gas. Step-by-Step Solution: Identify reaction type: hydrolysis of phosphate esters or PPi to yield Pi.Match to enzyme class: phosphatases (not oxidoreductases or proteases).Select “Alkaline phosphatases”. Verification / Alternative check: Clinical chemistry assays for serum alkaline phosphatase activity use chromogenic phosphate esters, monitoring Pi release; microbial extracellular phosphatases mobilize phosphorus from organic matter. Why Other Options Are Wrong: Oxidoreductase/Dehydrogenase: Catalyze redox, not phosphate ester hydrolysis. Protease: Targets peptide bonds, not phosphate esters. Hydrogenase: Involves H2/H+ interconversion, unrelated to Pi release. Common Pitfalls: Confusing pyrophosphatase specificity with general phosphatase action; many alkaline phosphatases have PPi activity. Final Answer: Alkaline phosphatases

Question 11

Glyoxylate cycle identity check Which enzyme set is specific to the glyoxylate cycle (the bypass that enables growth on C2 compounds by conserving carbon)?

Accepted Answer

Introduction / Context: The glyoxylate cycle is a metabolic shunt used by many bacteria, fungi, and plants to convert acetyl-CoA into four-carbon dicarboxylic acids without loss of carbon as CO2. It is essential for growth on acetate, fatty acids, or other C2 substrates and is distinguished by two hallmark enzymes. Given Data / Assumptions: The cycle bypasses the CO2-producing steps of the TCA cycle. Two unique enzymes define this bypass. Objective: identify those specific enzymes. Concept / Approach: Isocitrate lyase cleaves isocitrate into succinate and glyoxylate. Malate synthase then condenses glyoxylate with acetyl-CoA to form malate. Together they enable net assimilation of acetyl units into cell material, avoiding the decarboxylating steps of TCA. Step-by-Step Solution: Recall the two signature enzymes: isocitrate lyase and malate synthase.Confirm that both are required for the glyoxylate shunt to operate.Choose “both (a) and (b)”. Verification / Alternative check: Mutants lacking either enzyme cannot grow on acetate as sole carbon source, underscoring their specificity to the glyoxylate pathway. Why Other Options Are Wrong: Isocitrate lyase or malate synthase alone: Incomplete pathway. Anaplerotic: Describes replenishing reactions (for example, PEP carboxylase), not the glyoxylate-specific enzymes. Pyruvate dehydrogenase: Part of central metabolism but not the glyoxylate-specific step. Common Pitfalls: Confusing “anaplerotic” with “glyoxylate”; while related to TCA balance, they are distinct concepts. Final Answer: both (a) and (b)

Question 12

Metabolic definitions: reactions that capture energy In cellular metabolism, the set of reactions carried out to capture energy (for example, by oxidizing substrates and generating ATP) is called what?

Accepted Answer

Introduction / Context: Metabolism encompasses all biochemical reactions in a cell, but it is useful to divide it into energy-yielding processes (catabolism) and biosynthetic processes (anabolism). Recognizing this distinction helps in mapping pathways and predicting energy flow in microbes and higher organisms alike. Given Data / Assumptions: Energy capture typically involves substrate oxidation, electron transport, and ATP formation. Biosynthesis consumes ATP and reducing power. We are asked specifically for the name of energy-capturing reactions. Concept / Approach: Catabolism breaks down complex molecules into simpler ones while releasing energy that cells conserve as ATP or in ion gradients. Anabolism is the opposite: it builds complex molecules and requires energy input. “Metabolism” is the umbrella term for both. Step-by-Step Solution: Identify target: energy-yielding, degradative reactions.Match term: “catabolism”.Confirm that anabolism is the biosynthetic counterpart. Verification / Alternative check: Classic examples: glycolysis and TCA are catabolic; fatty acid synthesis and nucleotide synthesis are anabolic. Why Other Options Are Wrong: Metabolism: Too broad; includes both catabolism and anabolism. Anabolism: Energy-consuming synthesis, not energy capture. Activation energy: A thermodynamic barrier, not a set of reactions. Homeostasis: Physiological balance, not a biochemical pathway class. Common Pitfalls: Equating “metabolism” with “catabolism”; always specify the branch. Final Answer: catabolism

Question 13

Role of the TCA (citric acid) cycle The tricarboxylic acid (TCA) cycle in living cells primarily functions as which type of pathway?

Accepted Answer

Introduction / Context: The TCA cycle is central to metabolism. It oxidizes acetyl-CoA to CO2 while reducing NAD+ and FAD for ATP generation, yet it also provides key precursors for biosynthesis. The term that captures this dual role is important in biochemistry and microbiology curricula. Given Data / Assumptions: Catabolism produces energy; anabolism consumes energy to build biomolecules. Intermediates of the TCA cycle feed into amino acid, nucleotide, and heme biosynthesis. Cells use anaplerotic reactions to refill TCA intermediates when diverted for anabolism. Concept / Approach: Because the TCA cycle simultaneously supports energy production (catabolic) and supplies precursors for biosynthesis (anabolic), it is classified as amphibolic. Microbes dynamically balance withdrawal of intermediates with anaplerotic reactions (for example, PEP carboxylase) to maintain flux. Step-by-Step Solution: List catabolic roles: oxidation of acetyl-CoA, generation of NADH/FADH2 for oxidative phosphorylation.List anabolic roles: citrate to fatty acids; oxaloacetate to aspartate; alpha-ketoglutarate to glutamate, etc.Select “amphibolic reactions” to reflect both roles. Verification / Alternative check: Textbooks consistently label the TCA cycle amphibolic due to its central position linking energy and biosynthetic metabolism. Why Other Options Are Wrong: Catabolic only or anabolic only: Each captures part of the function but misses the duality. None of these/photolytic: Not applicable to the TCA cycle. Common Pitfalls: Ignoring biosynthetic withdrawals when thinking of the TCA cycle purely as an energy pathway. Final Answer: amphibolic reactions

Question 14

Bioenergetics and equilibrium: If the Gibbs free energy change (ΔG) of a chemical reaction is positive and the equilibrium constant (Keq) is less than 1, what does this imply about the reaction's direction under standard biological conditions?

Accepted Answer

Introduction / Context: In biochemistry and microbiology, predicting whether a reaction will proceed forward or backward is fundamental for understanding metabolism, enzyme regulation, and pathway flux. Two key indicators are the Gibbs free energy change (ΔG) and the equilibrium constant (Keq). Their relationship determines spontaneity and the favored direction of a reaction under defined conditions. Given Data / Assumptions: ΔG > 0 (positive) for the reaction as written. Keq < 1 for the reaction as written. Standard biochemical conditions are implied (activities near 1 unless otherwise stated). Concept / Approach: The fundamental relationship is ΔG°′ = −R * T * ln(Keq). If Keq < 1, then ln(Keq) is negative, making −R * T * ln(Keq) positive; thus ΔG°′ is positive. A positive ΔG means the forward direction is non-spontaneous under standard conditions, while the reverse reaction would be spontaneous (negative ΔG when written in reverse). Step-by-Step Solution: Start with ΔG°′ = −R * T * ln(Keq).Since Keq < 1, ln(Keq) < 0; multiplying by −R * T yields a positive ΔG°′.Positive ΔG°′ indicates the forward reaction is not thermodynamically favored.Therefore, the reverse reaction is favored (it would have ΔG°′ of equal magnitude but negative sign). Verification / Alternative check: Consider Le Châtelier’s principle at equilibrium: when Keq < 1, the equilibrium mixture contains more reactants than products, reinforcing that the forward formation of products is disfavored unless cellular conditions (mass action) are altered. Why Other Options Are Wrong: “proceeed in forward direction”: contradicts ΔG > 0 and Keq < 1. “not take place in any of the direction”: reactions are reversible; direction depends on ΔG and concentrations. “none of these”: incorrect because the reverse direction is clearly favored. Common Pitfalls: Confusing ΔG (actual conditions) with ΔG°′ (standard). Also, ignoring concentration effects: cells can drive endergonic steps by coupling or by changing reactant/product ratios. Final Answer: proceed in reverse direction

Question 15

Photosynthetic pigments in anoxygenic bacteria: Bacteriochlorophylls used by anoxygenic phototrophs show absorbance maxima predominantly in which portion of the electromagnetic spectrum?

Accepted Answer

Introduction / Context: Anoxygenic phototrophic bacteria use bacteriochlorophyll pigments to harvest light energy without evolving oxygen. The specific wavelengths absorbed determine ecological niches (e.g., light available in sediments or microbial mats) and dictate how these microbes compete or coexist with oxygenic phototrophs such as cyanobacteria and plants. Given Data / Assumptions: Bacteriochlorophylls differ from plant chlorophylls in structure and spectral properties. Question asks for the portion of the spectrum where absorbance maxima typically occur. Focus is on predominant, characteristic maxima, not minor bands. Concept / Approach: Bacteriochlorophyll variants (e.g., BChl a, b, c, d, e, g) commonly exhibit strong absorbance in the near-infrared region (approximately 800–1020 nm depending on type and protein environment). This enables these organisms to use wavelengths that penetrate deeper into biofilms or sediments and are not efficiently captured by oxygenic phototrophs that primarily absorb in blue and red regions. Step-by-Step Solution: Recognize that plant chlorophylls absorb blue (~430–450 nm) and red (~660–680 nm).Bacteriochlorophylls are tuned toward near-infrared bands (e.g., 800–900+ nm).Among choices, “infrared” best matches the characteristic maxima.Therefore, select “infrared.” Verification / Alternative check: Spectral data from photochemical reaction centers of purple and green sulfur bacteria show strong Qy bands in near-infrared, confirming the ecological partitioning of light across microbial communities. Why Other Options Are Wrong: Green/blue: major bands for oxygenic chlorophylls, not the signature of bacteriochlorophyll maxima. Ultraviolet: damaging to biomolecules and not a primary absorption band for these pigments. Common Pitfalls: Assuming all “chlorophylls” absorb identically; confusing accessory carotenoids (which can absorb blue/green) with the core bacteriochlorophyll maxima. Final Answer: infrared

Question 16

Aromatic pathway entry step: DAHP synthetase catalyzes the condensation of which substrates to form 3-deoxy-D-arabino-heptulosonate-7-phosphate (DAHP) in the shikimate pathway?

Accepted Answer

Introduction / Context: The shikimate pathway is central to the biosynthesis of aromatic amino acids (phenylalanine, tyrosine, tryptophan) in bacteria, fungi, and plants. The first committed step is catalyzed by DAHP synthetase, making it a classic enzyme target in antimicrobial and herbicidal strategies. Given Data / Assumptions: The product is DAHP (3-deoxy-D-arabino-heptulosonate-7-phosphate). We must identify the two substrates condensed by DAHP synthetase. Phenylalanine is an end product/feedback effector, not a substrate. Concept / Approach: DAHP synthetase condenses phosphoenolpyruvate (PEP) with erythrose-4-phosphate (E4P). This C3 + C4 condensation yields the seven-carbon DAHP, committing carbon flow into the shikimate pathway toward chorismate and downstream aromatic compounds. Step-by-Step Solution: Identify central carbon substrates feeding shikimate: PEP (glycolysis) and E4P (pentose phosphate pathway).DAHP synthetase catalyzes their aldol-like condensation.Confirm that phenylalanine is not a substrate, but often a feedback inhibitor of certain DAHP isoenzymes.Select “both (a) and (b).” Verification / Alternative check: Pathway schematics consistently show PEP + E4P → DAHP; later steps proceed via shikimate, EPSP, and chorismate to the aromatic amino acids. Why Other Options Are Wrong: E4P only or PEP only: incomplete; both are required. Phenylalanine: an end-product and regulator, not a substrate at this step. Common Pitfalls: Confusing substrates with allosteric regulators; mixing up shikimate pathway entry (DAHP) with downstream branch points. Final Answer: both (a) and (b)

Question 17

Bacteriochlorophyll versus chlorophyll: In what key ways do bacteriochlorophylls differ from the chlorophylls of oxygenic phototrophs?

Accepted Answer

Introduction / Context: Bacteriochlorophylls power anoxygenic photosynthesis in diverse bacteria. While structurally related to plant chlorophylls, they are tuned for distinct ecological niches and light regimes, which is reflected in both chemical substituents and spectral properties. Given Data / Assumptions: Bacteriochlorophylls and chlorophylls are porphyrin-like (or chlorin/bacteriochlorin) macrocycles with a central metal. We compare metal center, side-chain chemistry, and absorption spectra. The question seeks the combination that is correct. Concept / Approach: Both chlorophylls and bacteriochlorophylls typically coordinate magnesium (Mg) at the center of the macrocycle. However, bacteriochlorophylls differ in hydrogenation of rings and in R-group substituents (e.g., presence of different side chains), which shift electronic transitions. Consequently, bacteriochlorophylls display absorption maxima shifted toward the near-infrared relative to plant chlorophylls. Therefore, differences lie in side-group chemistry and spectra, not in the chelated metal. Step-by-Step Solution: Evaluate option (a): central metal remains Mg → statement is false.Confirm chemical differences in side chains between pigment families → true.Confirm distinct absorption spectra, often near-infrared for bacteriochlorophylls → true.Select the combined true statements, “Both (b) and (c).” Verification / Alternative check: Spectroscopic data from purple and green bacteria show Qy bands in the near-infrared; structural analyses document side-chain variations (e.g., bacteriochlorin vs chlorin cores). Why Other Options Are Wrong: (a) Metal identity is not the distinguishing feature; both commonly use Mg. Single-statement options omit part of the correct picture; both chemistry and spectra differ. Common Pitfalls: Assuming “bacterio-” implies a different central metal; ignoring how subtle side-chain changes produce major spectral shifts. Final Answer: Both (b) and (c)

Question 18

Isotopic tracing in lipid biosynthesis: If bacterial cells synthesizing fatty acids are fed radioactive bicarbonate (HCO3−), where would you expect the bulk of the radioactivity to be found after synthesis?

Accepted Answer

Introduction / Context: Radioisotopic tracers help map carbon flow through metabolic pathways. In fatty acid biosynthesis, bicarbonate (as CO2) participates in a key carboxylation step, but whether its carbon remains in the final lipid depends on the detailed chemistry of the elongation cycle. Given Data / Assumptions: Bacteria are actively synthesizing fatty acids de novo. Radioactive bicarbonate (e.g., 14C-HCO3−) is supplied. Standard malonyl-CoA pathway via acetyl-CoA carboxylase is operative. Concept / Approach: Acetyl-CoA carboxylase catalyzes the biotin-dependent carboxylation of acetyl-CoA to form malonyl-CoA, using bicarbonate as the source of the carboxyl carbon. In the subsequent Claisen condensation during chain elongation, the carboxyl group of malonyl (derived from bicarbonate) is released as CO2. Therefore, although bicarbonate is essential for the chemistry, its carbon does not become part of the long-chain fatty acid; it is lost each cycle as CO2. Step-by-Step Solution: HCO3− + acetyl-CoA → malonyl-CoA (carboxylation).Malonyl-ACP condenses with the growing acyl chain; decarboxylation releases CO2.The isotope from HCO3− is largely lost as CO2 rather than incorporated into the fatty acid chain.Thus, radioactivity remains in cellular inorganic carbon pools (bicarbonate/CO2), not in the lipids or membranes. Verification / Alternative check: Classic labeling experiments show poor incorporation of 14C from bicarbonate into fatty acid carbon skeletons compared with labeled acetate or acetyl units; the decarboxylation step accounts for isotope loss. Why Other Options Are Wrong: “the fatty acids” or “the cytoplasmic membrane”: the labeled carbon is not retained in the acyl chain carbons; membrane lipids thus show little incorporation from bicarbonate. “nucleic acids”: bicarbonate is not a primary carbon donor for nucleic acid synthesis; labeled incorporation would be minimal relative to cellular inorganic pools. Common Pitfalls: Assuming all carbon sources fed to cells end up in macromolecules; overlooking decarboxylation steps that release the label. Final Answer: cellular bicarbonate

Question 19

Calvin cycle reduction phase: During the reduction phase of the Calvin–Benson cycle, 3-phosphoglyceric acid (3-PGA) is reduced to which compound, and which reductant supplies the reducing power?

Accepted Answer

Introduction / Context: The Calvin–Benson cycle fixes CO2 into organic molecules. After carboxylation by RuBisCO, the reduction phase converts 3-phosphoglyceric acid (3-PGA) into triose phosphates that serve as precursors for carbohydrates and other biomolecules. Recognizing the product and reductant is essential for linking photosynthetic light reactions to carbon assimilation. Given Data / Assumptions: Starting intermediate: 3-PGA. Photosynthetic organisms use NADPH generated by light reactions. Product commonly termed glyceraldehyde-3-phosphate (G3P) or phosphoglyceraldehyde (PGAL). Concept / Approach: 3-PGA is first phosphorylated by ATP to 1,3-bisphosphoglycerate and then reduced by NADPH + H+ to glyceraldehyde-3-phosphate (phosphoglyceraldehyde). NADPH provides the high-energy electrons derived from photosystem I, directly coupling light energy to CO2 reduction. Step-by-Step Solution: 3-PGA + ATP → 1,3-bisphosphoglycerate + ADP.1,3-bisphosphoglycerate + NADPH + H+ → glyceraldehyde-3-phosphate + Pi + NADP+.Therefore, the reduced product is phosphoglyceraldehyde (G3P).The reductant is NADPH + H+, not NADH + H+. Verification / Alternative check: Textbook stoichiometry for one CO2 fixed requires ATP and NADPH; NADPH specifically supplies the reducing equivalents in chloroplasts and many photoautotrophic bacteria. Why Other Options Are Wrong: NADH + H+: typically used in catabolic pathways; Calvin cycle reduction uses NADPH. Ribulose-1,5-bisphosphate is the CO2 acceptor regenerated later, not the reduction product of 3-PGA. Pyruvic acid is not a Calvin cycle intermediate. Common Pitfalls: Confusing G3P (PGAL) with 3-PGA; mixing up NADH and NADPH roles; forgetting the ATP-dependent phosphorylation preceding the reduction step. Final Answer: phosphoglyceraldehyde; NADPH+H+

Question 20

Glyoxylate cycle and carbon sources: Some microorganisms employ the glyoxylate shunt when which compound is the sole source of carbon for growth?

Accepted Answer

Introduction / Context: The glyoxylate cycle is a metabolic bypass that enables cells to grow on C2 compounds by conserving carbon skeletons that would otherwise be lost as CO2 in the tricarboxylic acid (TCA) cycle. Understanding when microbes switch on this pathway explains growth on acetate, fatty acids, or ethanol. Given Data / Assumptions: Organism faces a sole carbon source with two carbons per molecule. Objective is net synthesis of C4 and C6 intermediates for biosynthesis. Key enzymes: isocitrate lyase and malate synthase. Concept / Approach: When acetate is the only carbon source, acetyl-CoA is the entry currency. The conventional TCA cycle decarboxylates carbon at isocitrate dehydrogenase and α-ketoglutarate dehydrogenase steps, preventing net carbon gain. The glyoxylate shunt bypasses these CO2-losing steps: isocitrate → succinate + glyoxylate (via isocitrate lyase), and glyoxylate + acetyl-CoA → malate (via malate synthase). This preserves carbon and allows gluconeogenesis and biosynthetic precursor formation. Step-by-Step Solution: Acetate → acetyl-CoA.Two acetyl-CoA + glyoxylate shunt → succinate + malate without CO2 loss.Succinate/malate → oxaloacetate → gluconeogenesis and anabolism.Therefore, the glyoxylate cycle is specifically used with acetate as sole carbon source. Verification / Alternative check: Mutants lacking isocitrate lyase cannot grow on acetate but grow normally on glucose, confirming the pathway’s role in C2 metabolism. Why Other Options Are Wrong: Nitrate: a nitrogen source/electron acceptor, not a carbon source. Carbon dioxide: chemoautotrophs fix CO2 via Calvin cycle or other CO2-fixing routes, not glyoxylate shunt. “all of these”: incorrect because only acetate fits the context. Common Pitfalls: Confusing glyoxylate cycle with Calvin cycle; assuming any poor carbon source requires glyoxylate shunt. Final Answer: acetate

Microbial Metabolism Questions