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Counts: Unit 1: 15 · Unit 2: 15 · Unit 3: 20 · Unit 4: 15 · Unit 5: 18 · Unit MCQs total: 83 · Plus the 10 Dec 2025 PYQ MCQs below, clearly labelled.
Unit 1 — Plant-Water Relations and Mineral Nutrition
Q1. The water potential of pure water at atmospheric pressure is:
Correct answer: (b) 0 MPa. Pure water is the reference: its water potential is zero. Solutes make it negative.
Q2. Water potential (Ψ) of a plant cell is the sum of:
Correct answer: (c) Solute potential and pressure potential. Ψ = Ψs + Ψp. In most cells matric potential is small and often included with Ψs in simple treatments.
Q3. In a fully turgid cell, the pressure potential is:
Correct answer: (c) Positive. Turgor pressure pushes the wall outward, so Ψp is positive and balances the negative Ψs.
Q4. Most transpiration in a typical leaf occurs through:
Correct answer: (c) Stomata. Stomatal transpiration dominates in most leaves; cuticular loss is much smaller.
Q5. Transpiration pull in the ascent of sap depends mainly on:
Correct answer: (b) Cohesion and adhesion of water. Cohesion between water molecules and adhesion to xylem walls transmit the pull from leaves to roots.
Q6. Which factor increases the rate of transpiration?
Correct answer: (c) Strong wind. Wind removes the humid boundary layer around the leaf, steepening the vapour gradient.
Q7. Guttation differs from transpiration because guttation:
Correct answer: (b) Occurs through hydathodes as liquid. Guttation is liquid water forced out through hydathodes, usually at night when transpiration is low and root pressure is active.
Q8. Root pressure is best demonstrated by:
Correct answer: (b) Bleeding from a cut stem. Sap exuding from a cut stem shows positive pressure from roots; it is weak and cannot alone lift water in tall trees.
Q9. Which element is a macronutrient?
Correct answer: (c) Potassium. Macronutrients (N, P, K, Ca, Mg, S) are needed in large amounts; Zn, Mo and B are micronutrients.
Q10. Magnesium is a component of:
Correct answer: (b) Chlorophyll. Mg is the central atom of the chlorophyll molecule.
Q11. According to Arnon and Stout, an element is essential if the plant:
Correct answer: (b) Cannot complete its life cycle without it and it cannot be replaced. Essentiality requires indispensability, a specific role, and no complete substitute.
Q12. Iron deficiency in plants most commonly shows as:
Correct answer: (b) Interveinal chlorosis of young leaves. Fe is needed for chlorophyll synthesis; young leaves yellow between veins because Fe is relatively immobile.
Q13. Facilitated diffusion across a membrane:
Correct answer: (b) Uses a carrier or channel down a gradient. Facilitated diffusion is passive but carrier/channel-mediated, so it is faster and more selective than simple diffusion.
Q14. A channel differs from a carrier protein mainly in that a channel:
Correct answer: (b) Forms a continuous pore and transports much faster. Channels are pores; carriers bind, change shape and release, so they are slower.
Q15. Primary active transport is driven directly by:
Correct answer: (b) ATP hydrolysis by a pump. P-ATPases and similar pumps hydrolyse ATP. Secondary active transport instead uses a gradient made by such a pump.
Unit 2 — Translocation in Phloem
Q16. The main sugar transported in phloem sap is:
Correct answer: (c) Sucrose. Sucrose is non-reducing, soluble and stable for long-distance transport.
Q17. Phloem sap also contains, besides sugars:
Correct answer: (b) Amino acids, hormones and minerals. Sap carries amino acids, some minerals, hormones and signalling molecules, at alkaline pH.
Q18. Sieve tube elements are unusual because they are:
Correct answer: (b) Living but without a nucleus at maturity. Mature sieve elements lose their nucleus and rely on companion cells.
Q19. Companion cells are closely associated with:
Correct answer: (b) Sieve tube elements. Companion cells load solutes into sieve tubes and keep them alive.
Q20. In the girdling experiment, removing a ring of bark:
Correct answer: (b) Stops phloem flow and causes swelling above the ring. Bark removal interrupts phloem; food accumulates above the girdle and roots starve.
Q21. The pressure flow model was proposed by:
Correct answer: (b) Münch. Ernst Münch proposed mass flow driven by a turgor-pressure gradient in 1930.
Correct answer: (b) Active sucrose-H+ cotransport into sieve tubes. Sucrose is pumped against its gradient using a proton gradient made by H+-ATPase.
Q24. Water enters sieve tubes at the source because:
Correct answer: (a) Solute concentration rises, lowering water potential. High solute concentration lowers Ψ, water enters by osmosis, and turgor pressure rises.
Q25. At the sink, unloading causes:
Correct answer: (b) Water to leave the sieve tube and pressure to fall. Solute removal raises Ψ, water exits, and pressure falls — maintaining the source-to-sink gradient.
Q26. Evidence for phloem transport from aphid stylets is that:
Correct answer: (b) Stylets left in sieve tubes exude sap under pressure. Cut aphid stylets exude phloem sap, showing it is under positive pressure.
Correct answer: (b) Plasmodesmata from cell to cell. Symplastic loading passes through plasmodesmata; apoplastic loading crosses membranes via transporters.
Q28. A growing fruit is an example of a:
Correct answer: (b) Sink. Sinks import food: growing fruits, roots, tubers and young leaves.
Q29. Which observation supports mass flow rather than simple diffusion?
Correct answer: (b) Sap moves rapidly over long distances and in bulk. Observed rates and bulk movement over metres are far too fast for diffusion alone.
Q30. If a sink is removed, phloem flow to that region:
Correct answer: (b) Stops or falls sharply. Flow depends on sink demand; without unloading, the pressure gradient collapses.
Correct answer: (b) P700. P700 is the PSI reaction-centre chlorophyll a.
Q33. Antenna molecules in photosynthesis:
Correct answer: (b) Harvest light and pass energy to the reaction centre. Antenna pigments absorb light and funnel excitation energy to the reaction centre.
Q34. Photolysis of water in photosynthesis occurs at:
Correct answer: (b) PSII. The oxygen-evolving complex of PSII splits water, releasing O2, protons and electrons.
Q35. In non-cyclic electron flow, the final electron acceptor is:
Correct answer: (c) ADP. Electrons end on NADP+, forming NADPH for the Calvin cycle.
Q36. Cyclic photophosphorylation produces:
Correct answer: (b) ATP only. Cyclic flow around PSI makes ATP without NADPH or O2, balancing the ATP:NADPH ratio.
Q37. Chemiosmosis in chloroplasts uses a proton gradient across the:
Correct answer: (b) Thylakoid membrane. Protons accumulate in the thylakoid lumen; flow back through CFo/CF1 makes ATP.
Q38. The first stable product of the Calvin (C3) cycle is:
Correct answer: (b) 3-phosphoglycerate. CO2 + RuBP gives two molecules of 3-phosphoglycerate, catalysed by RuBisCO.
Q39. RuBisCO is:
Correct answer: (b) Ribulose-1,5-bisphosphate carboxylase/oxygenase. RuBisCO carboxylates RuBP in C3 fixation and can also oxygenate it in photorespiration.
Q40. In C4 plants, CO2 is first fixed in mesophyll cells into:
Correct answer: (b) Oxaloacetate. PEP carboxylase fixes CO2 into oxaloacetate in mesophyll cells; the C4 acid moves to bundle-sheath cells.
Q41. Kranz anatomy is characteristic of:
Correct answer: (b) C4 plants. C4 leaves show a wreath of bundle-sheath cells around veins.
Q42. CAM plants open their stomata mainly:
Correct answer: (b) At night. CAM plants fix CO2 at night into malic acid and close stomata by day, saving water.
Q43. Photorespiration begins when RuBisCO acts as an oxygenase, producing:
Correct answer: (b) Phosphoglycolate. Phosphoglycolate is salvaged through chloroplast, peroxisome and mitochondrion cooperation, losing CO2.
Q44. Glycolysis takes place in the:
Correct answer: (b) Cytoplasm. Glycolysis is cytosolic and does not need oxygen.
Q45. In glycolysis, the net gain per glucose is:
Correct answer: (a) 2 ATP and 2 NADH. Gross 4 ATP are made, 2 ATP are invested, so net = 2 ATP, plus 2 NADH and 2 pyruvate.
Q46. The TCA (Krebs) cycle operates in the:
Correct answer: (b) Mitochondrial matrix. Pyruvate enters mitochondria, forms acetyl-CoA, and is oxidised in the matrix.
Q47. The electron transport chain of respiration is located in the:
Correct answer: (b) Inner mitochondrial membrane. Complexes I–IV and ATP synthase sit in the inner mitochondrial membrane.
Q48. The glyoxylate cycle occurs in:
Correct answer: (b) Glyoxysomes. Glyoxysomes in germinating oil seeds convert fat-derived acetyl-CoA toward sugars, bypassing CO2-loss steps of the TCA cycle.
Q49. The oxidative pentose phosphate pathway mainly provides:
Correct answer: (b) NADPH and pentose phosphates. It supplies NADPH for biosynthesis and ribose-5-phosphate for nucleotides.
Q50. Fermentation differs from aerobic respiration in that it:
Correct answer: (b) Regenerates NAD+ without an electron transport chain. Fermentation recycles NADH to NAD+ in the cytoplasm, yielding only the glycolytic ATP.
Unit 4 — Enzymes and Nitrogen Metabolism
Q51. Enzymes are mainly:
Correct answer: (b) Proteins. Most enzymes are proteins; a few catalytic RNAs (ribozymes) are the exception.
Q52. The part of an enzyme where the substrate binds is the:
Correct answer: (b) Active site. The active site binds substrate and stabilises the transition state.
Q53. Induced-fit theory states that:
Correct answer: (b) The active site changes shape on substrate binding. Binding induces a conformational change that fits substrate better than a rigid lock and key.
Q54. Km (Michaelis constant) is the substrate concentration at which the rate is:
Correct answer: (b) Half-maximum. Low Km means high affinity; Km is a useful comparative measure, not a direct binding constant.
Q55. A competitive inhibitor:
Correct answer: (b) Competes with substrate for the active site. Competitive inhibition can be overcome by more substrate; apparent Km rises, Vmax is unchanged.
Q56. A non-competitive inhibitor typically:
Correct answer: (b) Lowers Vmax. It binds elsewhere (often enzyme and enzyme-substrate complex), so Vmax falls while Km is often unchanged.
Q57. Feedback inhibition in a pathway usually acts on:
Correct answer: (b) The first committed enzyme. End-product inhibition of an early step prevents wasteful overproduction.
Q58. Nitrogenase is inactivated by:
Correct answer: (b) Oxygen. Nitrogenase is extremely O2-sensitive; leghaemoglobin helps keep free O2 low in nodules.
Q59. In a legume nodule, leghaemoglobin:
Correct answer: (b) Buffers O2: supplies respiration while protecting nitrogenase. It binds O2 reversibly, keeping free O2 low but delivering enough for bacteroid respiration.
Q60. The nitrogenase reaction needs, per N2 reduced:
Correct answer: (c) 16 ATP. The standard stoichiometry is N2 + 8H+ + 8e- + 16 ATP → 2NH3 + H2 + 16 ADP + 16 Pi.
Q61. A well-known free-living nitrogen fixer is:
Correct answer: (b) Azotobacter. Azotobacter fixes nitrogen freely in soil; Rhizobium is symbiotic in legumes.
Q62. Nitrate reductase in plants is located in the:
Correct answer: (b) Cytoplasm. Nitrate → nitrite occurs in the cytoplasm; nitrite → ammonia occurs in chloroplasts/plastids.
Q63. Nitrite reductase is located in the:
Correct answer: (b) Chloroplast/plastid. This is the classic trap: nitrate reduction is cytosolic, nitrite reduction is plastidic.
Q64. Ammonia is assimilated into amino acids mainly through:
Correct answer: (b) GS-GOGAT (glutamine synthetase–glutamate synthase). GS adds ammonia to glutamate making glutamine; GOGAT transfers the amide group to make two glutamates.
Q65. Glutamate dehydrogenase provides:
Correct answer: (b) An alternative, low-affinity route at high ammonia levels. GDH can aminate alpha-ketoglutarate directly, but GS-GOGAT is the main route at normal ammonia levels.
Unit 5 — Plant Growth Regulators and Light/Temperature Responses
Q66. The principal natural auxin is:
Correct answer: (b) Indole-3-acetic acid (IAA). IAA is the main auxin, made mainly in shoot tips, young leaves and developing seeds.
Q67. Apical dominance is maintained mainly by auxin from the:
Correct answer: (b) Selective weed killer (synthetic auxin). At herbicidal doses, synthetic auxins cause uncontrolled growth in broad-leaved weeds.
Q69. Bakanae (foolish seedling) disease led to the discovery of:
Correct answer: (b) Gibberellins. Kurosawa (1926) linked the tall rice seedlings to a product of Gibberella fujikuroi.
Q70. Gibberellins promote seed germination largely by inducing:
Correct answer: (b) Alpha-amylase in the aleurone layer. GA from the embryo induces aleurone cells to make hydrolases that mobilise endosperm starch.
Q71. Cytokinins are best known for promoting:
Correct answer: (b) Cell division and delaying leaf senescence. Cytokinins (e.g. kinetin, zeatin) promote cytokinesis and counteract auxin in shoot/root balance.
Q72. Abscisic acid (ABA) is called a stress hormone mainly because it:
Correct answer: (b) Closes stomata during water stress and induces dormancy. ABA rises in water stress, closes stomata and maintains seed dormancy.
Q73. Ethylene is unusual among plant hormones because it is a:
Correct answer: (b) Gas. Ethylene (C2H4) diffuses as a gas through intercellular spaces.
Q74. The triple response of dark-grown seedlings to ethylene includes:
Correct answer: (b) Shortening and thickening, exaggerated hook, horizontal growth. Neljubow's triple response is the classic ethylene bioassay.
Q75. Ethylene biosynthesis runs: methionine →
Correct answer: (a) ACC → ethylene. ACC (1-aminocyclopropane-1-carboxylic acid) is the immediate precursor; ACC synthase is rate-limiting.
Q76. A climacteric fruit shows:
Correct answer: (b) A sharp rise in respiration and ethylene during ripening. Banana, apple, tomato and avocado are climacteric; citrus and grape are non-climacteric.
Q77. Short-day plants flower when the night is:
Correct answer: (b) Longer than a critical length (uninterrupted). It is the long uninterrupted night, not the short day as such, that induces flowering in SDPs.
Q78. A long-day plant flowers when:
Correct answer: (b) Days exceed a critical length / nights are shorter than critical. LDPs need a short night; a light break in a long night can prevent SDP flowering and promote LDP flowering.
Q79. Phytochrome Pr absorbs mainly:
Correct answer: (b) Red light (660 nm). Pr absorbs red light and converts to Pfr; Pfr absorbs far-red (730 nm) and reverts to Pr.
Q80. The physiologically active form of phytochrome is generally:
Correct answer: (b) Pfr. Pfr is the active form that triggers photomorphogenic responses such as de-etiolation.
Q81. Vernalization is:
Correct answer: (b) Promotion of flowering by a period of low temperature. Cold treatment (e.g. winter cereals) makes the plant competent to flower when days lengthen.
Q82. Photoperiodism and vernalization differ because:
Correct answer: (b) Photoperiodism responds to light/dark cycles, vernalization to cold. One is a light-duration cue, the other a temperature cue; both can control flowering time.
Q83. Ethephon is used in agriculture because it:
Correct answer: (b) Releases ethylene in plant tissues. Ethephon (2-chloroethylphosphonic acid) breaks down to release ethylene, promoting ripening and abscission.
Dec 2025 PYQ MCQs — clearly labelled as PYQ
These are the actual Section A questions from booklet NEE-3107-A, December 2025, with answers.
PYQ Q1. The movement of water from cell to cell is mainly due to differences in:
Correct answer: (b) Water potential. PYQ Dec 2025 Q1. Water moves from higher to lower water potential.
PYQ Q2. Guttation occurs through:
Correct answer: (b) Hydathodes. PYQ Dec 2025 Q2.
PYQ Q3. Which of the following is a micronutrient?
Correct answer: (d) Zinc. PYQ Dec 2025 Q3. N, P and Mg are macronutrients.
PYQ Q4. Which pigment acts as the primary electron donor in photosynthesis?
Correct answer: (c) Chlorophyll a. PYQ Dec 2025 Q4.
PYQ Q5. In glycolysis, one molecule of glucose produces:
Correct answer: (a) 2 ATP. PYQ Dec 2025 Q5. Net gain 2 ATP; gross is 4 ATP.
PYQ Q6. Assimilation of ammonia into amino acids is catalyzed by:
Correct answer: (b) Glutamate dehydrogenase and glutamine synthetase. PYQ Dec 2025 Q6.
PYQ Q7. Short-day plants flower when:
Correct answer: (a) Day length is shorter than the critical period. PYQ Dec 2025 Q7, as framed by the paper. Modern framing: a long uninterrupted night.
PYQ Q8. Phloem loading involves:
Correct answer: (a) Active transport of sucrose into sieve tubes. PYQ Dec 2025 Q8.
PYQ Q9. Enzymes are mainly composed of:
Correct answer: (a) Proteins. PYQ Dec 2025 Q9.
PYQ Q10. Nitrate reduction in plants takes place in:
Correct answer: (a) Cytoplasm. PYQ Dec 2025 Q10. Nitrite reduction, the next step, is in the chloroplast.