Question 1

Rubisco (RuBP carboxylase–oxygenase), glyceraldehyde-3-phosphate (G3P), and NADPH all function in which process of photosynthesis?

Accepted Answer

Introduction / Context: Photosynthesis has two coordinated stages: the light reactions that capture light energy and the dark reactions (Calvin–Benson cycle) that fix CO2 into carbohydrates. This question asks you to identify where the enzyme Rubisco, the product G3P, and the reductant NADPH act together. Given Data / Assumptions: Rubisco fixes CO2 by carboxylating ribulose-1,5-bisphosphate (RuBP). NADPH and ATP are produced by the light reactions. G3P is the 3-carbon triose phosphate formed downstream of CO2 fixation and reduction steps. Concept / Approach: Rubisco initiates the Calvin cycle in the chloroplast stroma. ATP provides energy and NADPH provides reducing power to convert 3-phosphoglycerate into glyceraldehyde-3-phosphate (G3P). A portion of G3P exits the cycle for carbohydrate synthesis; the rest regenerates RuBP to continue CO2 fixation. Step-by-Step Solution: CO2 fixation: RuBP + CO2 → two 3-phosphoglycerate (Rubisco).Reduction: 3-phosphoglycerate → 1,3-bisphosphoglycerate (ATP used) → G3P (NADPH used).Regeneration: G3P rearrangements regenerate RuBP (ATP used).Output: G3P used to form sucrose/starch or other metabolites. Verification / Alternative check: Calvin cycle flux measurements and labeling experiments show NADPH consumption and G3P production in the stroma; Rubisco abundance in chloroplasts aligns with its central role. Why Other Options Are Wrong: Aerobic respiration/fermentation: mitochondrial or cytosolic processes, not chloroplast stroma reactions. Oxygen evolution: belongs to PSII light reactions, not Calvin cycle. Common Pitfalls: Equating “dark reactions” with nighttime; they are light-independent chemically but depend on light-generated ATP and NADPH. Final Answer: the dark reactions (Calvin–Benson cycle) of photosynthesis.

Question 2

In the light-dependent reactions of photosynthesis, which molecule is the terminal hydride/electron acceptor that becomes reduced to a key reductant for the Calvin cycle?

Accepted Answer

Introduction / Context: The light reactions produce two currencies: ATP and a powerful reductant for CO2 fixation. Identifying the correct electron acceptor at the end of the linear electron flow is essential for connecting light capture to carbon assimilation. Given Data / Assumptions: Linear electron flow moves electrons from H2O (via PSII) to PSI and ultimately to a soluble carrier. The Calvin cycle requires NADPH for reducing 3-phosphoglycerate to G3P. Distinct cofactors (NADP+ vs NAD+) serve different pathways. Concept / Approach: At the end of the light reactions, ferredoxin–NADP+ reductase transfers electrons from reduced ferredoxin to NADP+, forming NADPH. Thus, NADP+ is the final hydride/electron acceptor in the chloroplast light reactions, not NAD+ or FAD derivatives that are more typical of mitochondrial metabolism. Step-by-Step Solution: Water is oxidized at PSII → electrons flow through plastoquinone → cytochrome b6f → plastocyanin → PSI.PSI reduces ferredoxin (Fd).Ferredoxin–NADP+ reductase: Fd(red) + NADP+ + H+ → NADPH.NADPH supplies reducing power to the Calvin cycle. Verification / Alternative check: Biochemical assays reveal light-dependent accumulation of NADPH in isolated chloroplasts; inhibitors of ferredoxin–NADP+ reductase block NADPH formation and carbon fixation. Why Other Options Are Wrong: ADP: phosphate acceptor in ATP synthesis, not an electron acceptor. NAD+: primarily used in catabolic pathways (glycolysis/TCA) in the cytosol/mitochondria. FADH: nonstandard form; FAD/FADH2 participate in mitochondrial ETC, not chloroplasts. Ubiquinone: mitochondrial electron carrier; chloroplast uses plastoquinone instead. Common Pitfalls: Confusing NADP+ with NAD+ because of similar names; in chloroplasts, NADP+ is the principal terminal acceptor for the light reactions. Final Answer: NADP+.

Question 3

Across all of cellular metabolism, during which process is the greatest quantity of ATP manufactured per molecule of glucose under aerobic conditions?

Accepted Answer

Introduction / Context: Energy yield from glucose comes from multiple processes: substrate-level phosphorylation in glycolysis and the TCA cycle, and oxidative phosphorylation driven by the electron transport chain. The question asks which process contributes the most ATP under aerobic conditions in respiring cells. Given Data / Assumptions: Glycolysis produces a small ATP yield directly. The TCA cycle generates GTP/ATP directly but mainly provides NADH/FADH2. Oxidative phosphorylation converts the energy in NADH/FADH2 into ATP using a proton gradient. Concept / Approach: Most ATP per glucose is made as electrons from NADH and FADH2 move through complexes I–IV of the electron transport chain, pumping protons that drive ATP synthase. Depending on P/O ratios assumed, the bulk—often >80%—of ATP comes from oxidative phosphorylation, far exceeding the small contributions from substrate-level steps. Step-by-Step Solution: Glycolysis: net 2 ATP per glucose by substrate-level phosphorylation.TCA: 2 GTP per glucose via substrate-level phosphorylation.NADH/FADH2 from glycolysis, PDH, and TCA feed the ETC.ATP synthase uses the proton motive force to synthesize the majority of ATP. Verification / Alternative check: Inhibitors (e.g., oligomycin) that block ATP synthase abolish the major ATP output; uncouplers increase O2 consumption but collapse ATP production, confirming where most ATP arises. Why Other Options Are Wrong: Fermentation/glycolysis: low ATP yield per glucose. Light reactions: pertain to photosynthetic organisms and generate ATP for the chloroplast, not general cellular ATP from glucose oxidation. Substrate-level steps alone can never match oxidative phosphorylation output. Common Pitfalls: Assuming glycolysis is the main ATP source because it happens first; in aerobic cells, oxidative phosphorylation dominates energy yield. Final Answer: oxidative phosphorylation (electron transport chain coupled to ATP synthase).

Question 4

Which substance is a reactant in photosynthesis and also a product of cellular respiration in the classic summary equations of these processes?

Accepted Answer

Introduction / Context: Cellular respiration and photosynthesis are complementary. Recognizing which molecules switch roles between these processes helps you understand ecosystem-level cycles of matter and energy. This item focuses on the classic balanced equations used in textbooks. Given Data / Assumptions: Photosynthesis (overall): 6 CO2 + 6 H2O + light → C6H12O6 + 6 O2. Respiration (overall): C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy (ATP + heat). We are interested in a molecule that is a reactant for photosynthesis and a product of respiration. Concept / Approach: From the canonical equations, CO2 is consumed (reactant) in photosynthesis and produced (product) in cellular respiration. Oxygen is the reverse: produced by photosynthesis and consumed in respiration. Water is more nuanced (reactant in photosynthesis, product in respiration), but among the given choices, CO2 most directly matches the prompt and the standard framing of these paired processes. Step-by-Step Solution: Write the two overall equations.Identify common species and their roles on each side.CO2 appears as a reactant in photosynthesis and a product in respiration.Therefore, select CO2. Verification / Alternative check: Isotopic labeling (e.g., 14C-CO2) demonstrates carbon incorporation into plant carbohydrates, while breath analysis shows CO2 increase during respiration in animals and microbes. Why Other Options Are Wrong: O2: product of photosynthesis and reactant of respiration (opposite of what is asked). Sunlight: an energy input to photosynthesis, not a product of respiration. ATP: produced in respiration and used within the Calvin cycle, but it is not the canonical cross-process molecule highlighted in the overall equations. H2O: also fits reactant/product roles, but the question’s standard single best answer in classical teaching is CO2, which is unambiguous across both equations. Common Pitfalls: Overthinking ATP’s role across pathways. The question refers to the classic equations most students memorize, for which CO2 is the clean match. Final Answer: CO2.

Question 5

A eukaryotic cell that can carry out only fermentation instead of complete aerobic respiration of glucose is most likely in which condition?

Accepted Answer

Introduction / Context: Fermentation is an anaerobic strategy to regenerate NAD+ so glycolysis can proceed when oxygen is unavailable or when the respiratory chain cannot be used. This question examines what situation forces a eukaryotic cell to rely only on fermentation rather than complete aerobic respiration of glucose. Given Data / Assumptions: Fermentation occurs in the cytosol and does not use mitochondria. Aerobic respiration requires O2 as the terminal electron acceptor in mitochondria. Some eukaryotes can select fermentation even with mitochondria present if O2 is absent. Concept / Approach: When O2 is not available, the electron transport chain cannot accept electrons from NADH/FADH2, preventing oxidative phosphorylation. To keep glycolysis running, cells reduce pyruvate to lactate (animals) or to ethanol and CO2 (yeast), regenerating NAD+ without using the mitochondrion. Therefore, oxygen lack is the key condition leading to “only fermentation.” Step-by-Step Solution: No O2 → ETC stalls → NADH accumulates.Cells regenerate NAD+ via fermentation pathways.ATP yield limited to substrate-level phosphorylation in glycolysis.Result: lower ATP output and, typically, less total CO2 than full oxidation. Verification / Alternative check: In yeast, air-lock fermentation releases modest CO2 (from pyruvate → acetaldehyde → ethanol) compared with the 6 CO2 per glucose in aerobic respiration. In muscle, lactate fermentation yields no CO2 at all, showing that total CO2 is indeed lower without the TCA cycle fully engaged. Why Other Options Are Wrong: Produces less CO2: often true, but this is a consequence, not the defining reason for using fermentation. Has mitochondria present and fully competent: mitochondria may be present, but “only fermentation” occurs because O2 is lacking; mitochondrial competence does not force fermentation. All of these: incorrect because one choice (mitochondria fully competent) does not explain the reliance on fermentation. Accelerated Krebs cycle: cannot proceed without O2; NADH reoxidation is blocked. Common Pitfalls: Confusing the presence of mitochondria with their ability to operate; without O2, the respiratory chain cannot run, regardless of organelle presence. Final Answer: is lacking in O2 (oxygen is unavailable).

Question 6

During the light-dependent reactions of photosynthesis, which of the following events does NOT occur?

Accepted Answer

Introduction / Context: Light reactions convert photon energy into chemical energy (ATP and NADPH) and release O2 from water splitting. CO2 fixation, by contrast, is part of the Calvin–Benson cycle (often called the dark reactions). This question distinguishes light-stage events from carbon fixation steps. Given Data / Assumptions: Photosystem II oxidizes water and evolves O2. Photosystem I reduces NADP+ to NADPH. The thylakoid membrane hosts electron carriers that generate a proton motive force. Concept / Approach: Light absorption by PSII and PSI drives electron transport, water splitting, proton pumping, ATP synthesis, and NADPH formation. CO2 fixation (carboxylation of RuBP by Rubisco) occurs in the stroma during the Calvin cycle and is not a light reaction event, even though it depends on ATP/NADPH from the light stage. Step-by-Step Solution: Identify light-stage hallmarks: photon capture, water splitting, electron transport.Recognize chemiosmosis: proton gradient → ATP via CF0CF1 ATP synthase.Separate carbon fixation: Rubisco acts in the stroma during Calvin cycle.Therefore, carbon fixation is not part of the light reactions. Verification / Alternative check: Isolated thylakoids in light synthesize ATP/NADPH and evolve O2 but will not fix CO2 without stromal enzymes and the Calvin cycle machinery, confirming the separation of stages. Why Other Options Are Wrong: Water splitting, O2 release, and light absorption are quintessential light reactions. Proton gradient formation is central to photophosphorylation. Common Pitfalls: Misinterpreting “dark reactions” as requiring darkness; they are light-independent chemically but rely on light-generated ATP and NADPH. Final Answer: Carbon dioxide fixation into organic molecules.

Question 7

In green plants, Photosystem I has a reaction center chlorophyll with a maximum absorption at approximately which wavelength?

Accepted Answer

Introduction / Context: Each photosystem is named after the approximate wavelength (in nanometers) at which its special pair of chlorophyll molecules absorbs maximally. Knowing these “P numbers” helps you map energy flow through PSI and PSII. Given Data / Assumptions: Photosystem I reaction center is termed P700. Photosystem II reaction center is termed P680. Absorption maxima are approximate and context-dependent but widely taught values. Concept / Approach: By definition, P700 indicates a chlorophyll special pair that absorbs best near 700 nm in the far-red region. Thus, PSI’s reaction center peaks around 700 nm, enabling it to accept electrons from plastocyanin and deliver high-energy electrons to ferredoxin. Step-by-Step Solution: Recall naming convention: P700 → ~700 nm.Place on spectrum: far-red region of visible/near-IR.Therefore, PSI maximum absorption ≈ 700 nm.Confirm: PSII is P680 (~680 nm), distinct from PSI. Verification / Alternative check: Spectral measurements of isolated PSI complexes show strong far-red absorption near 700 nm, consistent across plant species with minor shifts due to environment/protein context. Why Other Options Are Wrong: 550, 600, 650, 750 nm: do not match the well-known P700 designation for PSI in plants. Common Pitfalls: Swapping P680 and P700 between PSII and PSI; remember the order in the Z-scheme: PSII (P680) → PSI (P700). Final Answer: 700 nm.

Question 8

The rate of photorespiration in most C3 plants rises at higher temperatures. Some plants mitigate this by a metabolic strategy that concentrates CO2 around Rubisco. What is this pathway called?

Accepted Answer

Introduction / Context: Photorespiration increases when temperature rises and when internal CO2 levels drop, causing Rubisco to act more as an oxygenase than a carboxylase. Some plants evolved anatomical and biochemical innovations to suppress photorespiration by elevating CO2 near Rubisco. This question asks you to recognize that adaptation. Given Data / Assumptions: Rubisco can react with O2, causing wasteful photorespiration. High temperature and low CO2 favor oxygenation reactions. Plants have evolved CO2-concentrating mechanisms. Concept / Approach: The C4 pathway uses mesophyll phosphoenolpyruvate carboxylase (PEPC) to fix HCO3- into a 4-carbon acid (e.g., malate), which is transported to bundle sheath cells. There, CO2 is released near Rubisco, raising the CO2:O2 ratio and suppressing photorespiration. This spatial separation (Kranz anatomy) helps maintain high photosynthetic efficiency in hot, bright environments. Step-by-Step Solution: PEPC in mesophyll fixes HCO3- → oxaloacetate → malate/aspartate.4-C acids move to bundle sheath cells and decarboxylate, releasing CO2.Localized CO2 saturates Rubisco, favoring carboxylation over oxygenation.Calvin cycle proceeds with reduced photorespiration losses. Verification / Alternative check: Gas exchange and isotope studies show lower photorespiratory flux in C4 plants (e.g., maize, sugarcane) compared with C3 plants at high temperatures; anatomical Kranz pattern correlates with function. Why Other Options Are Wrong: ETS and Photosystem II: components of light reactions; they do not constitute a CO2-concentrating mechanism. Calvin cycle: the standard carbon fixation pathway common to C3, C4, and CAM; it does not by itself concentrate CO2. CAM: another CO2-concentrating mechanism but temporally separated (night/day), not what the question highlights; the widely taught answer here is the C4 pathway responding to heat-induced photorespiration. Common Pitfalls: Confusing C4 (spatial separation) with CAM (temporal separation). Both concentrate CO2, but the classic response to high-temperature photorespiration in many grasses is the C4 pathway. Final Answer: C4 pathway.

Question 9

Photosynthetic bioenergetics — A “cyclic” electron transport process (producing ATP without net NADPH formation or water splitting) is characteristic of which metabolic context?

Accepted Answer

Introduction / Context: Cyclic electron flow is a special mode of photosynthetic electron transport in which excited electrons from Photosystem I return through the cytochrome b6f complex back to the special chlorophyll, generating a proton gradient and ATP but no net NADPH or O2. This question checks whether you can associate “cyclic electron transport” with the correct biological process. Given Data / Assumptions: Cyclic electron flow involves only Photosystem I and cytochrome b6f. Non-cyclic flow produces both ATP and NADPH and evolves O2 from water. Respiratory and chemolithotrophic chains are typically non-cyclic linear chains to terminal acceptors. Concept / Approach: Identify the process in which electrons excited by light re-enter the same reaction center, driving additional proton pumping without reducing NADP+. That hallmark belongs to photosynthesis, not to methane or sulfide oxidation or methanogenesis. Step-by-Step Solution: Recognize “cyclic” refers to PSI electrons cycling back via ferredoxin and cytochrome b6f.Proton pumping across thylakoid membrane increases ATP output.No net electrons reach NADP+, and water is not split; hence no O2 is produced. Verification / Alternative check: Plant and algal chloroplasts switch between cyclic and non-cyclic flow to balance the ATP/NADPH ratio required by the Calvin–Benson cycle. Why Other Options Are Wrong: b,c) Chemolithotroph electron transport is typically linear to an external acceptor.d) Methanogenesis reduces CO2 to CH4 using unique cofactors, not cyclic PSI flow.e) Mitochondrial respiration passes electrons to O2; the chain is not cyclic. Common Pitfalls: Confusing “cyclic” with the circular TCA cycle; here it refers to electron path around PSI. Final Answer: Photosynthesis (cyclic photophosphorylation around Photosystem I).

Question 10

Carbohydrate metabolism — The stepwise breakdown of glucose to pyruvate in the cytosol is known as what process (independent of whether O2 is present later)?

Accepted Answer

Introduction / Context: Glucose catabolism begins with a universal, cytosolic pathway that yields ATP and NADH and ends in pyruvate. That pathway is glycolysis, and it operates in virtually all cells. This question asks you to name that process, regardless of whether cells later use oxygen or ferment. Given Data / Assumptions: Location: cytosol. Products: pyruvate, ATP (net 2), and NADH. Downstream fates: aerobic oxidation in mitochondria or anaerobic fermentation. Concept / Approach: Differentiate upstream glycolysis from downstream branches. Fermentation is a post-glycolytic process that reoxidizes NADH to NAD+ in the absence of oxygen; anaerobic respiration uses an electron transport chain with non-O2 acceptors; the TCA cycle is mitochondrial and follows pyruvate oxidation. Step-by-Step Solution: Identify the initial pathway: glucose → glucose-6-phosphate → … → pyruvate.Recognize ATP/NADH production steps (substrate-level phosphorylation, glyceraldehyde-3-phosphate dehydrogenase).Select “Glycolysis.” Verification / Alternative check: The Embden–Meyerhof–Parnas pathway name is synonymous with glycolysis in most textbooks. Why Other Options Are Wrong: b) Fermentation is after glycolysis and does not break down glucose to pyruvate.c) Anaerobic respiration uses an ETC with non-oxygen acceptors.d) The TCA cycle oxidizes acetyl-CoA, not glucose directly.e) Gluconeogenesis synthesizes glucose, the reverse direction. Common Pitfalls: Equating fermentation with the entire process of glucose utilization; fermentation is only one possible fate of pyruvate. Final Answer: Glycolysis.

Question 11

Photosynthesis — In the very first photochemical event, sunlight excites an electron pair primarily associated with which component?

Accepted Answer

Introduction / Context: Light reactions begin when photons are absorbed by pigments. The crucial event is excitation of chlorophyll special pairs (P680 in Photosystem II, P700 in Photosystem I), which triggers charge separation and electron transfer down the photosynthetic electron transport chain. Given Data / Assumptions: Reaction centers house specialized chlorophyll molecules. Water is the electron donor in oxygenic photosynthesis but is oxidized by the oxygen-evolving complex after charge separation. CO2 is fixed later in the Calvin–Benson cycle. Concept / Approach: Identify the molecule that first interacts with light to move electrons to an acceptor. Chlorophyll absorbs light; the excited electron is transferred to a primary acceptor, leaving an oxidized chlorophyll that is re-reduced by electrons ultimately from water. Step-by-Step Solution: Photon absorbed → chlorophyll special pair excited.Charge separation → electron passed to primary acceptor.Oxidized chlorophyll is reduced by electrons extracted from water (via OEC in PSII). Verification / Alternative check: Fluorescence and ultrafast spectroscopy experiments demonstrate the rapid photoexcitation of chlorophyll and subsequent electron transfer events. Why Other Options Are Wrong: a) ATP is a product of photophosphorylation, not a light absorber.c) Water does not absorb the initiating photon; it is the substrate for oxidation.d) CO2 participates later in carbon fixation, not photochemistry.e) RuBP is the CO2 acceptor in the Calvin cycle. Common Pitfalls: Assuming the donor (water) is the first to interact with light; pigments mediate the primary event. Final Answer: Chlorophyll pigments within the reaction center.

Question 12

Cell types with full photosynthetic machinery — Antenna complexes (light-harvesting), photosynthetic electron transport chains, and carbon fixation (Calvin–Benson cycle) are all present together in which cells?

Accepted Answer

Introduction / Context: Complete oxygenic photosynthesis requires three coordinated components: light capture (antenna), photochemical electron transfer (ETC), and carbon fixation. This combination is a hallmark of chloroplast-bearing plant cells and many algae. Given Data / Assumptions: Plants possess chloroplasts with thylakoid membranes housing antenna and ETC complexes. Carbon fixation (Calvin–Benson cycle) occurs in the chloroplast stroma. Not all bacteria are photosynthetic; many lack these systems entirely. Concept / Approach: Choose the cell type that generally and consistently contains all three systems. While some bacteria (for example, cyanobacteria) do have analogous components, the option “bacterial cells” as a blanket statement is overly broad and thus misleading. Animal cells do not photosynthesize, and the citric acid cycle is unrelated to the triad listed. Step-by-Step Solution: Identify universal presence of antenna + ETC + Calvin cycle → chloroplast-bearing plant cells.Exclude animals (no photosynthesis) and general bacteria category (not universally true).Select plant cells as the best, consistently correct answer. Verification / Alternative check: Chloroplast ultrastructure shows grana/thylakoids with photosystems and stroma enzymes for carbon fixation. Why Other Options Are Wrong: a) Animals lack photosystems and RuBisCO.b) Statement is not true for all bacteria; many are heterotrophs lacking photosystems and Calvin cycle.d) The citric acid cycle is a respiratory pathway, not photosynthetic.e) Methanogens are anaerobic archaea without oxygenic photosynthesis. Common Pitfalls: Generalizing from cyanobacteria to “all bacteria.” Final Answer: Plant cells (chloroplast-containing).

Question 13

Energy metabolism — Identify the single false statement (others are true with proper wording and scope).

Accepted Answer

Introduction / Context: This concept question integrates ecology and physiology. It asks you to distinguish accurate generalizations about energy flow and respiration from a deliberately incorrect statement about the gases used by animals during cellular respiration. Given Data / Assumptions: Energy for nearly all ecosystems is solar in origin, captured by photosynthesis. Animals are heterotrophs; their energy comes from organic matter originating from primary producers (plants, algae, some bacteria). Aerobic respiration uses O2 as the terminal electron acceptor and produces CO2 and water as products. Concept / Approach: Evaluate each statement using core principles. The false statement should attribute the wrong gases to animal respiration requirements. Animals require O2, not CO2, to carry out aerobic respiration; they release CO2. The other statements are broadly true with proper scope qualifiers (for example, “most” or “ultimately”). Step-by-Step Solution: Check (a): Primary energy source for Earth’s biosphere is the sun → broadly true.Check (b): Food chains originate with primary producers → true as a general rule.Check (c): Claims animals need CO2 for respiration → false; animals need O2 and produce CO2.Check (d): Aerobic respiration in eukaryotes occurs in mitochondria → true when O2 is present.Check (e): Consistent with thermodynamics in ecosystems → true. Verification / Alternative check: Basic physiology confirms that hemoglobin transports O2 to tissues; mitochondria consume O2 and generate CO2 via the TCA cycle and pyruvate oxidation. Why Other Options Are Wrong: a) Correct—photosynthesis captures solar energy that fuels ecosystems.b) Correct—animals depend on producer biomass directly or indirectly.d) Correct—many eukaryotes respire aerobically when oxygen is available.e) Correct—decomposers recycle matter; energy is dissipated as heat. Common Pitfalls: Confusing animal respiration with plant gas exchange; both use O2 in mitochondria, but plants also produce O2 in photosynthesis. Final Answer: Plants do not provide CO2 for animal respiration; animals require O2 and produce CO2, so option (c) is false.

Question 14

Redox cofactors — What do NAD+, NADP+, and FAD (oxidized forms) all have in common in metabolism?

Accepted Answer

Introduction / Context: NAD+, NADP+, and FAD are ubiquitous redox cofactors. In their oxidized forms, they accept electrons from substrates, becoming NADH, NADPH, and FADH2. Recognizing their shared role clarifies many pathways from glycolysis to fatty acid synthesis. Given Data / Assumptions: NAD+ and FAD commonly accept electrons in catabolic reactions (e.g., dehydrogenases). NADP+ is often used in anabolic pathways, reduced to NADPH as a reductant. “+” denotes oxidized form; “H/H2” denotes reduced form. Concept / Approach: Identify the property common to all: in the listed forms they are oxidized and ready to accept electrons. They are not yet “full” of electrons; that would be NADH/NADPH/FADH2. They are not direct carbon-fixing reagents in photosynthesis. Step-by-Step Solution: NAD+ + 2 e− + H+ → NADH.NADP+ + 2 e− + H+ → NADPH.FAD + 2 e− + 2 H+ → FADH2. Verification / Alternative check: Enzyme names such as “dehydrogenase” indicate oxidation of a substrate with concomitant reduction of these cofactors. Why Other Options Are Wrong: a,b) Describe the reduced state, not the listed oxidized forms.d) Carbon fixation uses RuBP and CO2 with RuBisCO; NADPH provides reducing power but is not the carbon-accepting substrate.e) These cofactors function broadly across metabolism, not only in glycolysis. Common Pitfalls: Confusing the oxidized (NAD+, NADP+, FAD) and reduced (NADH, NADPH, FADH2) forms. Final Answer: They are oxidized electron acceptors that become reduced during metabolism.

Question 15

Oxygenic photosynthesis — Identify the primary electron donor that replaces the photo-excited electrons and ultimately releases O2 as a by-product.

Accepted Answer

Introduction / Context: In plants, algae, and cyanobacteria, the light reactions remove electrons from water via the oxygen-evolving complex of Photosystem II. This process generates O2 and feeds electrons into the photosynthetic electron transport chain. Given Data / Assumptions: PSII extracts electrons from H2O, releasing O2 and protons into the thylakoid lumen. Electrons travel through plastoquinone → cytochrome b6f → plastocyanin → PSI → NADP+. NADPH is produced at the end of non-cyclic flow, not used as the initial donor. Concept / Approach: Identify the substrate of the oxygen-evolving complex. Only water can account for both electron donation and O2 evolution in oxygenic photosynthesis. Step-by-Step Solution: Light excites P680 → strong oxidant forms.OEC oxidizes H2O → O2 + 4 H+ + 4 e−.Electrons proceed through the chain to reduce NADP+ to NADPH. Verification / Alternative check: Isotopic tracer experiments with H2^18O demonstrate that the oxygen evolved comes from water. Why Other Options Are Wrong: b) O2 is a product, not an electron donor.c,d) NADH/NADPH are reduced carriers; they are electron donors elsewhere but not the initial donor in oxygenic photosynthesis.e) CO2 is the carbon source in the Calvin cycle, not the electron donor. Common Pitfalls: Confusing the direction of electron flow; in photosynthesis, electrons originate from water and end in NADPH. Final Answer: Water (H2O).

Question 16

Mitochondrial ETC — The cytochrome c oxidase complex (Complex IV) performs which set of functions during oxidative phosphorylation?

Accepted Answer

Introduction / Context: Complex IV, cytochrome c oxidase, is the terminal enzyme of the mitochondrial electron transport chain. It couples the reduction of oxygen to water with proton translocation, strengthening the proton-motive force used by ATP synthase. Given Data / Assumptions: Electron donor to Complex IV is reduced cytochrome c. O2 is the terminal electron acceptor in aerobic respiration. Proton pumping by Complex IV is part of chemiosmotic coupling. Concept / Approach: Evaluate each role: electron acceptance, oxygen reduction, and proton pumping. Complex IV accomplishes all three to complete the chain and maintain the gradient. Step-by-Step Solution: cyt c (reduced) → Complex IV → O2 reduction to H2O (four electrons overall).Conformational changes drive proton pumping across the inner membrane.This contributes to membrane potential and pH gradient for ATP synthesis. Verification / Alternative check: Inhibitors like cyanide and carbon monoxide bind Complex IV and block both oxygen consumption and ATP production, demonstrating its roles. Why Other Options Are Wrong: e) Contrary to well-established function; Complex IV both reduces O2 and pumps protons. Common Pitfalls: Underestimating Complex IV’s proton pumping; remembering only Complexes I and III pump protons, but IV does as well. Final Answer: All of these.

Question 17

Citric acid cycle connections — What do coenzyme A, carbon dioxide (CO2), oxaloacetate (OAA), and FADH2 have in common in core metabolism?

Accepted Answer

Introduction / Context: The citric acid cycle is a central metabolic hub. CoA forms acetyl-CoA to enter the cycle, CO2 is released during decarboxylation steps, oxaloacetate is both the starting and ending four-carbon acceptor, and FADH2 is a reduced cofactor produced at succinate dehydrogenase (Complex II). Given Data / Assumptions: Acetyl-CoA condenses with OAA to form citrate. CO2 is produced at isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase steps. FAD is reduced to FADH2 when succinate is oxidized to fumarate. Concept / Approach: Relate each item to a specific TCA step to confirm their commonality with the Krebs cycle. Step-by-Step Solution: CoA → part of acetyl-CoA entry to the cycle.CO2 → product of oxidative decarboxylations.OAA → regenerated acceptor closing the cycle.FADH2 → produced at succinate dehydrogenase, feeding electrons to the ETC. Verification / Alternative check: Standard biochemical maps show these roles explicitly in the TCA cycle scheme. Why Other Options Are Wrong: b) Dark reactions use CO2 and NADPH; CoA and FADH2 are not Calvin cycle components.c) Lactic fermentation mainly involves pyruvate ↔ lactate and NADH ↔ NAD+.d) FADH2 participates in oxidative phosphorylation after being generated in the TCA cycle.e) The TCA cycle operates in both prokaryotes and eukaryotes. Common Pitfalls: Separating TCA from oxidative phosphorylation; they are linked via NADH/FADH2 supply. Final Answer: They are all components or products of the citric acid cycle.

Question 18

Phycology — In algae (eukaryotic phytoplankton and macroalgae), photosynthesis occurs in which organelles?

Accepted Answer

Introduction / Context: Eukaryotic algae are photosynthetic organisms whose light reactions and carbon fixation are compartmentalized within chloroplasts (plastids), similar to those in higher plants, though plastid structure and pigments can vary widely among algal lineages. Given Data / Assumptions: Chloroplasts contain thylakoid membranes with Photosystems I and II and ATP synthase. Calvin–Benson cycle enzymes reside in the stroma. Mitochondria carry out respiration, not photosynthesis; the plasma membrane is not the site of eukaryotic photosystems. Concept / Approach: Select the organelle that houses both the light and dark reactions in algal cells: the chloroplast. Step-by-Step Solution: Identify photosynthetic machinery → thylakoids (chloroplast).Identify carbon fixation location → stroma (chloroplast).Choose “Chloroplasts.” Verification / Alternative check: Microscopy and fractionation studies localize chlorophyll, photosystems, and RuBisCO within algal plastids. Why Other Options Are Wrong: b) The plasma membrane does not host chloroplast thylakoids in eukaryotic algae.c) Mitochondria perform oxidative phosphorylation using O2, not photosynthesis.d,e) Contradict the well-known plastid localization of photosynthesis. Common Pitfalls: Confusing cyanobacteria (prokaryotes that use thylakoid-like membranes in the cytoplasm) with eukaryotic algae (which use chloroplasts). Final Answer: Chloroplasts.

Question 19

Photosynthesis—Tracing Oxygen Atoms from Labeled CO2 If the oxygen atom is labeled in carbon dioxide (CO2) and this labeled CO2 is supplied to a plant, after photosynthesis where would you expect to find that labeled oxygen atom?

Accepted Answer

Introduction / Context: This question tests understanding of the fate of oxygen atoms during photosynthesis by using an isotopic tracer. It probes whether the oxygen released as O2 comes from water (H2O) or from carbon dioxide (CO2), and where the oxygen from CO2 ends up in the final products of photosynthesis. Given Data / Assumptions: Labeled oxygen is placed in CO2 (for example, 18O in CO2). Plant performs oxygenic photosynthesis with a light-dependent stage and the Calvin cycle. We track the labeled oxygen after the process proceeds. Concept / Approach: Classical isotope-labeling experiments showed that the O2 evolved in photosynthesis derives from water, not from CO2. In water-splitting at photosystem II, H2O is oxidized to O2, and those oxygen atoms are released as gas. The CO2 carbon is reduced and incorporated into carbohydrates; its oxygen atoms ultimately reside in sugar carbonyl/hydroxyl groups or in downstream metabolites derived from these sugars. Step-by-Step Solution: Water is split at photosystem II: 2 H2O → O2 + 4 H+ + 4 e−. The O2 evolved here comes from water.Electrons pass through the electron transport chain to NADP+, forming NADPH; these carriers do not incorporate oxygen from CO2.The Calvin cycle fixes CO2 into 3-phosphoglycerate and ultimately into triose phosphates and glucose.The oxygen atom(s) bound to carbon in CO2 remain associated with the carbon skeleton as it becomes carbohydrate hydroxyl/carbonyl oxygens. Verification / Alternative check: Historical tracer studies using 18O-labeled water vs 18O-labeled CO2 demonstrate that labeled O2 appears only when water is labeled, confirming that O2 comes from H2O and CO2 oxygen is retained within organic products. Why Other Options Are Wrong: In the water used: that is the source of evolved O2, not the sink for CO2 oxygen. NADPH: carries electrons and a proton; it does not sequester the CO2 oxygen. In the oxygen given off: incorrect—released O2 originates from water. ATP phosphate groups: ATP synthesis does not incorporate CO2 oxygen into its phosphates. Common Pitfalls: Assuming that because O2 is a product, its oxygen must come from CO2; experiments show it comes from H2O. Also confusing electron flow (to NADPH) with oxygen atom tracing. Final Answer: In the carbohydrate (sugars) synthesized during the Calvin cycle

Question 20

Light Reactions—Identify What Is NOT a Direct Product Which of the following is NOT a direct product of the light-dependent reactions of photosynthesis in chloroplasts?

Accepted Answer

Introduction / Context: Photosynthesis comprises light-dependent reactions (capturing light energy) and the Calvin cycle (light-independent reactions) that fix carbon. Distinguishing the immediate outputs of the light reactions from downstream carbohydrate synthesis is a core learning objective in plant physiology and biochemistry. Given Data / Assumptions: Light reactions occur in thylakoid membranes. Primary outputs are energy carriers and oxidized/ reduced species enabling carbon fixation. Glucose formation requires carbon fixation steps beyond light reactions. Concept / Approach: Light reactions use photosystems II and I to split water, release O2, generate a proton motive force, and produce ATP and NADPH. These energy carriers fuel the Calvin cycle in the stroma, which synthesizes triose phosphates that can be assembled into glucose or starch. Therefore, glucose is not a direct product of the light reactions. Step-by-Step Solution: Water-splitting at PSII produces O2, electrons, and protons.Electron transport and photophosphorylation yield ATP and reduce NADP+ to NADPH.The Calvin cycle uses ATP and NADPH to fix CO2 into carbohydrates; glucose arises later from triose phosphate pools.Thus, among the options, glucose is not produced directly by the light reactions. Verification / Alternative check: Isolated thylakoids illuminated in vitro evolve O2 and make ATP and NADPH but do not synthesize glucose without the stromal enzymes of carbon fixation. Why Other Options Are Wrong: ATP/NADPH: canonical outputs of light reactions. O2: released from water splitting at PSII. Proton gradient: immediate intermediate driving ATP synthase in the light reactions. Common Pitfalls: Equating “photosynthesis” generically with “making sugar” and forgetting the two-stage nature of the process. Final Answer: Glucose

Photosynthesis and Respiration Questions