Question 1

Overall Output of Photosynthesis—Select the Correct Product As a result of oxygenic photosynthesis in plants, which of the following is produced as a net product of the process?

Accepted Answer

Introduction / Context: In the overall photosynthetic reaction, plants use carbon dioxide and water to form carbohydrates and release oxygen gas. Understanding which molecules are consumed and which are produced helps clarify the role of photosynthesis in global biogeochemical cycles and ecosystem respiration balance. Given Data / Assumptions: Oxygenic photosynthesis splits water at photosystem II. CO2 is fixed during the Calvin cycle into carbohydrates. We are considering net products, not transient intermediates. Concept / Approach: The classic summary is: 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2 (simplified stoichiometry). Water is consumed, not produced, in the light reactions; additional water may appear in various metabolic reactions, but the canonical net effect is oxygen generation and carbohydrate synthesis. Carbon dioxide is a reactant, not a product. Therefore, the best answer among the options is oxygen alone. Step-by-Step Solution: Identify reactants: CO2 and H2O.Recognize that splitting water releases O2 gas.Calvin cycle fixes CO2 into carbohydrate; CO2 is not released as a product in this process.Conclude that oxygen is the explicit gaseous product listed. Verification / Alternative check: Isotopic labeling with 18O-labeled water leads to 18O in the O2 evolved, confirming water as the oxygen source and O2 as a product. Why Other Options Are Wrong: Water: primarily a reactant consumed during light reactions. Carbon dioxide: a substrate for fixation, not a product. Both oxygen and water: incorrect because net water production is not the hallmark output. Nitrogen gas: unrelated to photosynthetic outputs. Common Pitfalls: Confusing cellular respiration (which consumes O2 and produces CO2) with photosynthesis. Final Answer: Oxygen

Question 2

Electron Carriers—Identify the Reduced Form Which of the following is the reduced form of a temporary electron carrier molecule used in cellular redox reactions?

Accepted Answer

Introduction / Context: Biological redox reactions transfer electrons using carrier molecules that cycle between oxidized and reduced states. Recognizing the reduced versus oxidized forms is essential for tracking electron flow through metabolic pathways such as the TCA cycle and oxidative phosphorylation, and in photosynthesis and respiration. Given Data / Assumptions: Flavin adenine dinucleotide (FAD) ⇄ FADH2. Nicotinamide adenine dinucleotide phosphate (NADP+) ⇄ NADPH. ATP is a high-energy phosphate carrier, not primarily an electron carrier. Concept / Approach: The reduced form carries additional electrons and (often) protons. For FAD, the reduced state is FADH2; for the NADP system it is NADPH (not listed among options). NADP+ and NAD+ are oxidized forms. CO2 is fully oxidized carbon and not an electron carrier, and ATP stores energy in phosphoanhydride bonds rather than as reducing power. Step-by-Step Solution: Identify which candidates function as electron carriers.Recall oxidation states: FADH2 is reduced; FAD is oxidized. NADP+ is oxidized; NADPH is reduced.Select FADH2 as the correct reduced carrier among the choices. Verification / Alternative check: In oxidative phosphorylation, FADH2 donates electrons to Complex II (succinate dehydrogenase), confirming its reduced-carrier status. Why Other Options Are Wrong: ATP: energy currency, not a redox carrier. NADP+ and NAD+: oxidized forms lacking the hydride. CO2: terminal oxidized product of carbon catabolism. Common Pitfalls: Mixing up ATP's role with that of electron carriers; conflating NADP+ with NADPH. Final Answer: FADH2

Question 3

Photosynthetic Electron Flow—Final Electron Acceptor of Water-Splitting Electrons During photosynthesis, electrons released by the photolysis (splitting) of water ultimately reduce which molecule?

Accepted Answer

Introduction / Context: In oxygenic photosynthesis, water is split at photosystem II to provide electrons, protons, and O2. The path of these electrons through the photosynthetic electron transport chain determines which molecules become reduced and how energy is stored for carbon fixation. Given Data / Assumptions: Photolysis occurs at PSII's oxygen-evolving complex. Electrons travel from PSII → plastoquinone → cytochrome b6f → plastocyanin → PSI → ferredoxin. Ferredoxin-NADP+ reductase catalyzes the terminal reduction of NADP+ to NADPH. Concept / Approach: The ultimate purpose of linear electron flow is to generate NADPH (reducing power) and ATP (energy). Electrons from water end up reducing NADP+ to NADPH at the stromal side of PSI via ferredoxin. ATP is produced by chemiosmosis but does not accept electrons. O2 is produced from water and released; it is not reduced. Rubisco catalyzes CO2 fixation and is not an electron sink. Plastoquinone is an intermediate carrier, not the final destination. Step-by-Step Solution: Water splitting: 2 H2O → O2 + 4 H+ + 4 e− (at PSII).Electron transport: PSII → PQ → b6f → PC → PSI → Fd.Terminal step: ferredoxin-NADP+ reductase uses electrons to reduce NADP+ → NADPH.Therefore, electrons from water ultimately reside in NADPH. Verification / Alternative check: Measuring stromal NADPH accumulation upon illumination confirms that NADP+ is the terminal electron acceptor in linear flow. Why Other Options Are Wrong: ATP: synthesized via ATP synthase; does not accept electrons. O2: generated from water; not reduced in this context. Rubisco: catalytic CO2-fixing enzyme, not an electron carrier. Plastoquinone: a mobile intermediate; electrons move past it to PSI and ferredoxin. Common Pitfalls: Confusing the proton gradient (for ATP) with electron flow; assuming O2 is an electron acceptor here. Final Answer: NADPH (i.e., they reduce NADP+ to NADPH)

Question 4

Source of ATP for Carbon Fixation—Photophosphorylation Link Carbon fixation in the Calvin cycle requires ATP molecules. These ATP molecules are generated by which photosynthetic process?

Accepted Answer

Introduction / Context: The Calvin cycle needs energy (ATP) and reducing power (NADPH) to convert CO2 into carbohydrate. In chloroplasts, ATP for carbon fixation is supplied by photophosphorylation during the light reactions, not by the Calvin cycle itself or by mitochondrial oxidative phosphorylation under typical photosynthetic conditions. Given Data / Assumptions: Light reactions occur in thylakoid membranes and generate ATP and NADPH. The Calvin cycle operates in the stroma and consumes ATP and NADPH. Plant cells also possess mitochondria, but chloroplast ATP supports stromal reactions. Concept / Approach: Photon energy drives electron flow through PSII and PSI, establishing a proton gradient across the thylakoid membrane. ATP synthase uses this gradient to phosphorylate ADP to ATP (photophosphorylation). These ATP molecules are then available in the stroma to power the carboxylation and reduction steps of the Calvin cycle. Step-by-Step Solution: Light absorption → electron transfer chain → proton gradient formation.ATP synthase catalyzes ADP + Pi → ATP using the thylakoid proton motive force.Stromal ATP and NADPH feed Calvin cycle enzymes (e.g., RuBP regeneration requires ATP).Therefore, the ATP for carbon fixation is made by the photosynthetic ETS during the light reactions. Verification / Alternative check: Isolated chloroplast experiments show light-dependent ATP generation (no Calvin cycle required) and carbon fixation depends on externally supplied ATP/NADPH when light is absent. Why Other Options Are Wrong: Glucose formation does not generate ATP; it consumes ATP and NADPH. Chlorophyll replenishment is unrelated to ATP supply for fixation. Oxidative phosphorylation in mitochondria primarily supplies cytosolic/mitochondrial ATP, not the stromal pool during active photosynthesis. “None of the above” is incorrect because the ETS answer is correct. Common Pitfalls: Assuming ATP made anywhere in the cell is interchangeable; compartmentation matters—chloroplast-generated ATP is key for the Calvin cycle. Final Answer: Electron transfer system (ETS) during the light reactions, via photophosphorylation

Photosynthesis and Respiration Questions