Photosynthesis — Set 5

Correct Answer: A. Carbon

• **Carbon** = the element fixed during the Calvin cycle; ATP provides the phosphate energy currency needed for Rubisco and the subsequent reduction steps that convert inorganic CO₂ into organic G3P (a three-carbon sugar). • **Carbon fixation** — specifically, ATP is used in two steps: phosphorylating 3-PGA to 1,3-bisphosphoglycerate (with NADPH then reducing it to G3P), and regenerating RuBP from G3P to continue the cycle. • Every molecule of CO₂ fixed into sugar requires 3 ATP and 2 NADPH, making ATP the indispensable energy driver of carbon assimilation. • 💡 Option B (Oxygen) is wrong because oxygen is released (not fixed) during photolysis of water; Option C (Nitrogen) is wrong because nitrogen fixation is a completely separate process carried out by nitrogen-fixing bacteria, not chloroplasts; Option D (Phosphorus) is wrong because phosphorus in the form of phosphate groups is part of ATP itself and is not fixed by the Calvin cycle.

Correct Answer: A. Provide electrons/hydrogen

• **Provide electrons/hydrogen (reducing power)** = NADPH acts as the reducing agent in the Calvin cycle; it donates its high-energy electrons and hydrogen ions to convert 1,3-bisphosphoglycerate into the sugar G3P, a process called reduction. • **Electron carrier** — NADPH is produced in Photosystem I when NADP⁺ is reduced using electrons energised by light; it then carries that reducing power from the light reactions into the stroma where the Calvin cycle operates. • Two NADPH molecules are consumed for every CO₂ fixed into G3P, making it as essential as ATP for sugar synthesis. • 💡 Option B (Release oxygen) is wrong because oxygen release happens during photolysis of water in the light reactions, not in the Calvin cycle; Option C (Absorb light) is wrong because light absorption is the role of chlorophyll pigments in the thylakoid membranes; Option D (Split water) is wrong because water splitting (photolysis) is carried out by the oxygen-evolving complex associated with Photosystem II.

Correct Answer: C. 6CO2 + 6H2O -> C6H12O6 + 6O2

• **6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂** = the balanced overall equation for oxygenic photosynthesis; six molecules each of carbon dioxide and water are combined using light energy to produce one molecule of glucose and six molecules of oxygen. • **Reactants vs products** — CO₂ and H₂O are the raw materials absorbed from air and soil; glucose (C₆H₁₂O₆) is the energy-rich product stored for metabolism, and O₂ is the by-product released into the atmosphere. • This equation is endothermic — it absorbs about 2870 kJ of energy per mole of glucose from sunlight, storing it as chemical energy in the bonds of glucose. • 💡 Option A (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O) is wrong because this is the equation for aerobic respiration, the reverse process; Option B (6H₂O + 6O₂ → C₆H₁₂O₆ + 6CO₂) is wrong because it shows oxygen as a reactant, which makes no chemical sense for photosynthesis; Option D (6CO₂ + 6O₂ → C₆H₁₂O₆ + 6H₂O) is wrong because it incorrectly places O₂ as a reactant instead of H₂O.

Correct Answer: A. Reduce water loss

• **Reduce water loss** = the waxy cuticle is a hydrophobic layer secreted by the epidermal cells of leaves; it forms an almost impermeable barrier to liquid water and water vapour, drastically reducing transpiration through the leaf surface. • **Photosynthesis enabler** — by minimising water loss through the cuticle, the plant retains enough water in the mesophyll cells to sustain the photolysis reactions and keep turgor pressure in guard cells, which controls stomatal opening. • Plants in hot, dry environments (xerophytes) have exceptionally thick cuticles as an adaptation to survive in low-water conditions. • 💡 Option B (Absorb CO₂) is wrong because CO₂ enters the leaf through stomata, not through the cuticle — the waxy cuticle actually blocks gas exchange; Option C (Increase light) is wrong because the cuticle is transparent but does not concentrate or increase light intensity reaching the mesophyll; Option D (Store sugar) is wrong because sugar is stored as starch in chloroplasts and as sucrose transported in phloem, not in the cuticle.

Correct Answer: B. Stroma lamellae

• **Stroma lamellae** = flattened, tubular membrane extensions that extend through the stroma of the chloroplast and physically connect the stacked disc-like grana, forming a continuous internal membrane system. • **Integrated network** — stroma lamellae ensure that proton gradients, electrons, and membrane proteins can be shared across the entire thylakoid network, allowing the chloroplast to function as a coordinated unit during the light reactions. • Stroma lamellae are unstacked and lack PSII, functioning mainly in PSI-mediated cyclic electron flow. • 💡 Option A (Thylakoid) is wrong because individual thylakoids are the disc-like membrane sacs that stack to form grana — they are the units within each granum, not the connector between grana; Option C (Matrix) is wrong because 'matrix' refers to the fluid interior of mitochondria, not a chloroplast structure — the equivalent in chloroplasts is called the stroma; Option D (Cristae) is wrong because cristae are the infolded inner membrane structures found in mitochondria, not in chloroplasts.

Correct Answer: D. Mesophyll

• **Mesophyll** = the internal photosynthetic tissue of the leaf, sandwiched between the upper and lower epidermis; it contains the highest concentration of chloroplast-rich cells in the leaf and is the primary site of photosynthesis. • **Two layers** — the palisade mesophyll (tightly packed, columnar cells just below the upper epidermis) contains the most chloroplasts and performs the bulk of photosynthesis; the spongy mesophyll (loosely arranged with large air spaces) facilitates gas exchange of CO₂ and O₂ through stomata. • The proximity of mesophyll cells to leaf veins (xylem and phloem) ensures rapid supply of water and removal of sugars during active photosynthesis. • 💡 Option A (Phloem) is wrong because phloem is the vascular tissue that transports sucrose and amino acids away from the leaf, not the site of photosynthesis; Option B (Xylem) is wrong because xylem conducts water and minerals upward from roots to leaves, serving as supply tissue; Option C (Epidermis) is wrong because the epidermis is the outer protective cell layer and typically lacks chloroplasts (except in guard cells).

Correct Answer: A. Open and close stomata

• **Open and close stomata** = guard cells are pairs of kidney-shaped (in dicots) or dumbbell-shaped (in grasses) cells that flank each stomatal pore; by gaining or losing water through osmosis, they change shape to widen or narrow the pore. • **Turgor mechanism** — when guard cells take up K⁺ ions and water during the day (triggered by light and low CO₂), they become turgid and bow outward, opening the stoma to allow CO₂ in for photosynthesis; at night or under water stress, they lose water and the pore closes. • This precise regulation allows the plant to balance CO₂ uptake for photosynthesis against water loss through transpiration. • 💡 Option B (Produce chlorophyll) is wrong because while guard cells do contain some chloroplasts, chlorophyll production is not their primary function and is common to all green cells; Option C (Protect the leaf) is wrong because physical protection is provided by the tough epidermal cells and cuticle, not guard cells; Option D (Store water) is wrong because water storage is the role of specialised cells in succulents (e.g., vacuolated hydrenchyma), not guard cells.

Correct Answer: C. Anthocyanins

• **Anthocyanins** = a group of water-soluble flavonoid pigments dissolved in the cell vacuoles; they produce red, purple, and blue colours depending on the pH of the cellular environment — acidic vacuoles give red tones, neutral to basic give purple/blue. • **Ecological role** — anthocyanins act as sunscreens absorbing UV and high-intensity light to protect leaves and ripening fruits; in fruits they also attract animals for seed dispersal by signalling ripeness. • They are synthesised in response to high sugar concentrations, bright light, cold temperatures, and UV radiation, which is why red colour intensifies in apples on the sun-facing side. • 💡 Option A (Xanthophyll) is wrong because xanthophylls are yellow-orange pigments, not red or purple; Option B (Chlorophyll) is wrong because chlorophyll is the green pigment responsible for photosynthesis; Option D (Carotene) is wrong because carotenes are orange-red fat-soluble pigments (like β-carotene in carrots) — they contribute to orange tones but are distinct from the water-soluble anthocyanins responsible for vivid red and purple.

Correct Answer: D. ATP and NADPH

• **ATP and NADPH** = the two energy-carrying molecules generated in the light-dependent reactions (via the electron transport chain and chemiosmosis in the thylakoid membrane) that are then consumed in the light-independent reactions (Calvin cycle) in the stroma. • **Chemical link** — ATP provides phosphate-bond energy for driving the Calvin cycle's phosphorylation steps, while NADPH donates the reducing power (electrons + H⁺) needed to reduce 3-PGA to G3P; together they are the currency that couples the two stages of photosynthesis. • Without a continuous supply of ATP and NADPH from the light reactions, the Calvin cycle would halt even if CO₂ and Rubisco were abundant. • 💡 Option A (Glucose) is wrong because glucose is the final product of the Calvin cycle (dark reactions), not the light reactions; Option B (Oxygen) is wrong because oxygen is a by-product of the light reactions released into the atmosphere — it is not used in the dark reactions; Option C (Water) is wrong because water is a reactant split in the light reactions, not a product passed to the dark reactions.

Correct Answer: C. Chemosynthesis

• **Chemosynthesis** = a metabolic process used by certain bacteria and archaea around deep-sea hydrothermal vents and cold seeps, where sunlight cannot penetrate; they oxidise inorganic chemicals such as hydrogen sulfide (H₂S), methane, or ammonia to generate the ATP needed to fix CO₂ into organic matter. • **Hydrothermal vent ecosystems** — communities of tube worms, giant clams, and crabs thrive in total darkness at depths of 2,000–4,000 m, sustained entirely by chemosynthetic bacteria at the base of the food chain, with no dependence on solar energy. • Chemosynthesis is scientifically significant because it shows life can exist without sunlight, expanding the search for life on other planets with subsurface oceans (e.g., Europa, Enceladus). • 💡 Option A (Nitrogen fixation) is wrong because nitrogen fixation converts atmospheric N₂ into ammonia using nitrogenase enzymes — it builds nitrogen compounds, not food from CO₂; Option B (Fermentation) is wrong because fermentation is an anaerobic process that breaks down existing organic molecules (like glucose) for energy, not a process that makes food from inorganic sources; Option D (Photosynthesis) is wrong because photosynthesis requires sunlight, which is absent in the deep sea.