56 free MCQs on Mineral Nutrition with worked answers and explanations. Essential mineral elements, their roles and deficiency symptoms. Nitrogen fixation and soil nutrition.
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Atmospheric N₂ is fixed into ammonia by nitrogen-fixing bacteria (protected from oxygen by leghaemoglobin in root nodules), converted to nitrate via nitrification for plant uptake, and eventually returned to the atmosphere as N₂ via denitrification, completing the cycle.
Q1.
Which element is present at the center of the chlorophyll molecule?
Answer: C. Magnesium
Why: Magnesium (Mg) is the central atom in the chlorophyll molecule. Deficiency of Mg causes interveinal chlorosis (yellowing between veins) especially in older leaves.
Q2.
Nitrogen fixation means converting atmospheric nitrogen (N2) into
Answer: B. Ammonia or nitrates usable by plants
Why: Nitrogen fixation converts N2 gas (inert) into ammonia (NH3) or nitrates that plants can absorb. Without this process, atmospheric N2 is unavailable to most organisms.
Q3.
Which bacterium lives in root nodules of legumes and fixes nitrogen?
Answer: B. Rhizobium
Why: Rhizobium is a symbiotic nitrogen-fixing bacterium that lives in root nodules of legumes (pea, bean, soybean). It has a mutualistic relationship with the plant - it gets sugar, plant gets fixed nitrogen.
Q4.
What does a chlorotic plant look like?
Answer: B. Yellow leaves due to lack of chlorophyll
Why: Chlorosis is yellowing of leaves due to reduced chlorophyll production. It is caused by deficiency of nitrogen, magnesium, iron, sulfur, or manganese. It indicates mineral deficiency.
Q5.
Which element is essential for cell wall formation (as calcium pectate)?
Answer: C. Calcium
Why: Calcium forms calcium pectate in the middle lamella of plant cell walls, cementing adjacent cells together. Calcium deficiency causes distorted and failed development of growing tips (terminal bud death).
Q6.
Necrosis in plants refers to
Answer: B. Death of plant tissue (brown/black dead patches)
Why: Necrosis means localized death of tissue, appearing as brown or black patches. Potassium deficiency typically causes necrosis at leaf margins and tips.
Q7.
Which macronutrient is essential for making ATP, DNA, and cell membranes?
Answer: D. Phosphorus
Why: Phosphorus is a key component of ATP (energy), DNA/RNA (genetic material), and phospholipids (cell membranes). Deficiency causes poor root growth, purple discoloration of leaves, and delayed maturity.
Q8.
Growing plants in nutrient solutions without soil is called
Answer: C. Hydroponics
Why: Hydroponics is the technique of growing plants in defined nutrient solutions without soil. It was used by scientists (Knop, Arnon) to discover which mineral elements are essential for plant growth.
Q9.
Which free-living soil bacterium fixes atmospheric nitrogen?
Answer: C. Azotobacter
Why: Azotobacter is a free-living (non-symbiotic) aerobic nitrogen-fixing bacterium in soil. Other free-living fixers: Clostridium (anaerobic), Beijerinckia. Unlike Rhizobium, it does not need a plant host.
Q10.
Nitrogen deficiency in plants causes
Answer: B. Chlorosis (yellowing) especially in older leaves first
Why: Nitrogen deficiency causes yellowing (chlorosis) starting in older leaves (nitrogen is mobile and moves to younger leaves). Growth is stunted. Nitrogen is a component of proteins and chlorophyll.
Q11.
Which element activates stomatal opening by accumulating in guard cells?
Answer: C. Potassium
Why: Potassium (K+) ions actively accumulate in guard cells in the light, increasing osmotic pressure. Water enters by osmosis, making guard cells turgid and opening stomata.
Q12.
Insectivorous plants grow in soils deficient in which mineral?
Answer: C. Nitrogen
Why: Insectivorous plants (Venus flytrap, Drosera, Nepenthes) grow in nitrogen-poor soils (bogs, wet rocks). They trap insects to supplement their nitrogen supply. This is an adaptation to low-nutrient environments.
Q13.
Which nutrient is needed in the largest amount by plants?
Answer: C. Nitrogen
Why: Nitrogen is the macronutrient needed in the largest quantity by plants. It is a component of amino acids, proteins, nucleic acids, and chlorophyll. It is the most common limiting nutrient in agriculture.
Q14.
Iron deficiency in plants causes
Answer: B. Interveinal chlorosis in young leaves
Why: Iron deficiency causes interveinal chlorosis (yellowing between veins while veins remain green) first in young leaves because iron is immobile in plants. Iron is needed for chlorophyll synthesis and electron transport.
Q15.
The process of converting nitrates back to atmospheric nitrogen by bacteria is called
Answer: D. Denitrification
Why: Denitrification is the conversion of nitrates (NO3-) to molecular nitrogen (N2) by denitrifying bacteria (Pseudomonas, Thiobacillus). It occurs in waterlogged anaerobic soil and removes nitrogen from the soil.
Q16.
What does NPK stand for in fertilizers?
Answer: D. Nitrogen-Phosphorus-Kalium (potassium)
Why: NPK fertilizers contain Nitrogen (N), Phosphorus (P), and Potassium (K, from Kalium). These are the three macronutrients most commonly deficient in agricultural soils and most important for plant growth.
Q17.
What is the role of sulfur in plants?
Answer: B. Component of some amino acids (cysteine, methionine) and proteins
Why: Sulfur is a component of amino acids cysteine and methionine, and thus of many proteins. It is also part of coenzymes (CoA) and vitamins. Deficiency causes chlorosis in young leaves (similar to N deficiency).
Q18.
Which cyanobacterium fixes nitrogen and is found in rice fields?
Answer: C. Anabaena
Why: Anabaena (and Nostoc) are cyanobacteria (blue-green algae) that fix atmospheric nitrogen. Anabaena azollae grows in symbiosis with Azolla (water fern) in rice paddies, naturally fertilizing the water.
Q19.
Boron is classified as a
Answer: B. Micronutrient
Why: Boron is a micronutrient (trace element) essential in small amounts. It is important for pollen germination, cell wall formation, and sugar transport. Deficiency causes death of growing tips.
Q20.
What is nitrification?
Answer: B. Converting NH3 to nitrites and then nitrates by bacteria
Why: Nitrification is the two-step oxidation of ammonia: NH3 to NO2- (by Nitrosomonas) and then NO2- to NO3- (by Nitrobacter). It increases the availability of nitrogen in a form most plants prefer to absorb.
Q21.
Nitrogen fixation is done by:
Answer: B. Rhizobium and other nitrogen-fixing bacteria
Why: Nitrogen fixation: N2 gas → NH3 (ammonia) by nitrogen-fixing bacteria like Rhizobium (in legume root nodules) and free-living Azotobacter.
Q22.
Root nodules in legumes contain:
Answer: B. Rhizobium bacteria (nitrogen fixers)
Why: Rhizobium bacteria live symbiotically in root nodules of legumes. They fix atmospheric N2 into ammonia used by the plant.
Q23.
What criteria must an element meet to be considered essential for plant growth?
Answer: B. It must be required for plant to complete its life cycle, irreplaceable by another element, and have direct physiological role
Why: Criteria (Arnon and Stout, 1939): (1) The plant cannot complete its life cycle without it; (2) No other element can substitute for it; (3) It must be directly involved in plant metabolism. 17 elements meet these criteria.
Q24.
What is the role of the enzyme nitrogenase in nitrogen fixation?
Answer: B. It converts atmospheric N2 to NH3; requires anaerobic conditions and ATP
Why: Nitrogenase is the enzyme complex that reduces N2 to NH3 (N2 + 8H+ + 8e- + 16ATP → 2NH3 + H2 + 16ADP + 16Pi). It is irreversibly inhibited by O2. Legume nodules maintain low O2 using leghemoglobin.
Q25.
What is leghemoglobin and what is its role in root nodules?
Answer: B. A pink oxygen-scavenging protein in root nodules that maintains anaerobic conditions for nitrogenase
Why: Leghemoglobin is a pink/red protein found in legume root nodules. It scavenges free O2 in the nodule, maintaining the anaerobic microenvironment required by nitrogenase (which is O2-sensitive). Its genes come from both the plant and Rhizobium.
Q26.
What are the differences between macronutrients and micronutrients?
Answer: B. Macronutrients are needed in large amounts (>0.1% dry weight); micronutrients are needed in trace amounts (<0.1%)
Why: Macronutrients (needed >0.1% dry weight): C, H, O, N, P, K, Ca, Mg, S. Micronutrients (needed <0.01% dry weight): Fe, Mn, Zn, Cu, Mo, B, Cl, Ni. Essential for specific enzymatic and structural functions.
Q27.
Zinc deficiency causes which symptom in plants?
Answer: B. Little leaf disease (shortened internodes, small malformed leaves)
Why: Zinc deficiency causes 'little leaf' or 'rosette' disease - very short internodes and small, narrow, chlorotic leaves that cluster. Zinc is needed for synthesis of auxin (IAA) and for several enzymes. Common in maize and rice.
Q28.
What is the importance of molybdenum in plant nutrition?
Answer: B. Essential cofactor for nitrogenase (N2 fixation) and nitrate reductase (NO3- to NO2-)
Why: Molybdenum (Mo) is a micronutrient needed in the smallest amount of all essential elements. It is a cofactor for nitrogenase and nitrate reductase. Deficiency causes nitrogen deficiency symptoms even with adequate N supply ('whiptail' in cauliflower).
Q29.
What is ammonification?
Answer: B. Converting dead organic matter (proteins) back to ammonia by decomposers
Why: Ammonification is the decomposition of organic nitrogen (from dead organisms and waste) back to NH3/NH4+ by putrefying bacteria and fungi. It is part of the nitrogen cycle that returns organic nitrogen to the soil.
Q30.
Deficiency of which element causes 'blossom end rot' in tomatoes?
Answer: C. Calcium
Why: Blossom end rot of tomatoes is caused by calcium deficiency in the developing fruit. Calcium is poorly mobile in plants; irregular watering disrupts Ca transport. Affected tissue becomes dark, leathery, and sunken at the flower end.
Q31.
What is the significance of mycorrhizal association for plant mineral nutrition?
Answer: B. Mycorrhizal hyphae dramatically increase root surface area, especially enhancing phosphorus uptake from soil
Why: Arbuscular mycorrhizal (AM) fungi form extensive hyphal networks that greatly increase the effective absorptive surface of roots. They are particularly efficient at acquiring phosphorus (poorly mobile in soil). Over 90% of land plants form mycorrhizal associations.
Q32.
What is the role of manganese in plant physiology?
Answer: B. It activates enzymes and is essential for water splitting in photosynthesis (oxygen-evolving complex)
Why: Manganese activates many enzymes (decarboxylases, dehydrogenases) and is essential for the oxygen-evolving complex (OEC) of Photosystem II that splits water, releasing O2. Deficiency causes interveinal chlorosis similar to iron deficiency.
Q33.
What is the difference between essential and beneficial elements for plants?
Answer: B. Essential elements are required by all plants and have specific metabolic roles; beneficial elements improve growth in some plants but are not universally required
Why: Essential: required by all plants, specific metabolic role. Beneficial: improve growth/yield in some plants or under specific conditions but are not universally required. Examples of beneficial: Na (for C4 plants), Si (strengthens cell walls), Co (for legumes for N2 fixation).
Q34.
Phosphorus deficiency symptoms differ from nitrogen deficiency. How?
Answer: B. P deficiency often causes purple/red coloration (anthocyanin accumulation) due to sugar buildup; N deficiency causes chlorosis
Why: Phosphorus deficiency causes accumulation of sugars (P is needed for sugar metabolism/export) which promotes anthocyanin pigment synthesis, giving purple or bronze coloration. N deficiency causes yellow-green chlorosis starting in oldest leaves.
Q35.
What is active transport in mineral absorption by roots?
Answer: B. Movement of minerals against concentration gradient using ATP and carrier proteins
Why: Most mineral ions are absorbed by active transport - against their concentration gradient, requiring ATP and specific carrier proteins (permeases) in the plasma membrane. This allows plants to accumulate minerals at concentrations far higher than in soil.
Q36.
What are the symptoms of copper deficiency in plants?
Answer: B. Wilting and dark green leaves; young leaves may show chlorosis; lignification reduced
Why: Copper is a component of plastocyanin (photosynthesis electron transport), cytochrome c oxidase, and lignin synthesis. Deficiency causes dark green, wilted young leaves, die-back of shoot tips, and reduced pollen viability.
Q37.
What is meant by a mobile vs. immobile nutrient in plants?
Answer: B. Mobile nutrients are redistributed from older to younger leaves during deficiency; immobile nutrients remain in older leaves
Why: Mobile nutrients (N, P, K, Mg) are retranslocated from older to younger tissues during deficiency; deficiency symptoms appear first in OLD leaves. Immobile nutrients (Ca, Fe, B, Cu) cannot be moved; deficiency symptoms appear in YOUNG leaves first.
Q38.
What is the ecological role of nitrogen-fixing organisms in natural ecosystems?
Answer: B. They are the primary source of new biologically available nitrogen, supporting all life in the ecosystem
Why: Nitrogen-fixing organisms (cyanobacteria, Azotobacter, Rhizobium) introduce new fixed nitrogen into ecosystems. Without them, nitrogen would gradually be lost from soil by denitrification. In natural ecosystems without fertilizers, N2-fixers are critical for maintaining soil fertility.
Q39.
What is biofertilizer and give two examples?
Answer: B. Living microorganisms that enhance soil nutrient availability: Rhizobium, Azospirillum, mycorrhizal fungi, cyanobacteria
Why: Biofertilizers are preparations of living microorganisms that improve soil nutrient availability: (1) Rhizobium - N2 fixation in legumes; (2) Azospirillum/Azotobacter - free-living N2 fixation; (3) Mycorrhizal fungi - P solubilization; (4) Anabaena/Nostoc - N2 fixation in paddy fields.
Q40.
What is the impact of acid rain on plant mineral nutrition?
Answer: B. Acid rain leaches essential cations (Ca2+, Mg2+, K+) from soil and mobilizes toxic aluminum ions, disrupting plant nutrition
Why: Acid rain (pH < 5.6) leaches base cations (Ca2+, Mg2+, K+) from soil, making them unavailable. It also mobilizes toxic Al3+ which inhibits root growth and nutrient uptake. Combined, this causes widespread forest decline in affected areas.
Q41.
The nitrogen cycle includes the process of nitrification, which converts:
Answer: B. NH3 to NO2- and then NO3- (by Nitrosomonas and Nitrobacter)
Why: Nitrification: Nitrosomonas converts NH3 to NO2- (nitrite); Nitrobacter converts NO2- to NO3- (nitrate). Both are chemoautotrophs.
Q42.
What is the biochemical basis of nitrogen fixation by nitrogenase? Include the role of ferredoxin.
Answer: B. Ferredoxin (reduced by photosynthesis or respiration) donates electrons to dinitrogenase reductase (Fe protein) which then reduces dinitrogenase (MoFe protein) that reduces N2 to NH3
Why: Electron flow: reduced ferredoxin/flavodoxin (from PS I or pyruvate oxidation) → nitrogenase reductase (Fe protein, dinitrogenase reductase) → nitrogenase (MoFe protein) → N2 → NH3. Each N2 requires 8 electrons, 8 H+, 16 ATP. The MoFe protein has the FeMo-cofactor active site.
Q43.
What is the GS-GOGAT pathway for nitrogen assimilation in plants?
Answer: B. Glutamine synthetase (GS) incorporates NH4+ into glutamine; GOGAT transfers the amide group to alpha-ketoglutarate, forming two glutamates
Why: Primary NH4+ assimilation: GS catalyzes glutamate + NH4+ + ATP → glutamine. GOGAT (glutamine oxoglutarate aminotransferase) then catalyzes glutamine + alpha-ketoglutarate → 2 glutamate (using reduced ferredoxin or NADH). This is the main pathway; the older GDH pathway is secondary.
Q44.
How do carnivorous plants digest their prey and what nutrients do they gain?
Answer: B. They secrete protease and other hydrolytic enzymes to digest prey, primarily gaining nitrogen and phosphorus to supplement growth in nutrient-poor environments
Why: Carnivorous plants secrete digestive enzymes (proteases, esterases, nucleases) onto captured prey. They absorb the digestion products - mainly nitrogen (as amino acids/NH4+) and phosphorus - to supplement their photosynthetically produced carbon. This adaptation allows survival in N-poor, acidic, waterlogged soils.
Q45.
What is the significance of the nod factors produced by Rhizobium?
Answer: B. They are lipochitooligosaccharides that signal root hair curling and initiation of nodule development in host plant
Why: Nod (nodulation) factors are lipochitin oligosaccharides secreted by Rhizobium in response to flavonoids from legume roots. They bind specific receptors on root hair cells, triggering root hair curling, infection thread formation, and initiation of the nodulation developmental program.
Q46.
What is luxury consumption of minerals in plants and why can it be harmful?
Answer: B. Plants absorb minerals in excess of what is needed for optimal growth when soil levels are high; excess can disrupt ion balance and cause toxicity
Why: Luxury consumption occurs when plants absorb minerals in excess of metabolic needs (especially K, N, P). While temporarily tolerated (stored in vacuoles), extreme excess causes ionic imbalances, osmotic effects, and toxicity. Excess nitrogen causes excessive vegetative growth at the expense of reproduction.
Q47.
What is Hoagland solution and what is its importance in plant nutrition research?
Answer: B. A defined nutrient solution containing all essential minerals in known concentrations, used to study mineral requirements and grow plants without soil
Why: Hoagland solution (developed by D.R. Hoagland) is a precisely defined aqueous solution containing all essential mineral nutrients. It is the standard medium for hydroponic experiments studying mineral nutrition, deficiency symptoms, and uptake mechanisms in plant physiology research.
Q48.
What is the role of boron in plant cell wall formation?
Answer: B. Boron cross-links pectin (as borate-diol ester) in the primary cell wall, maintaining wall integrity and cell-to-cell adhesion
Why: Boron forms borate ester bonds that cross-link rhamnogalacturonan II (RG-II) in the pectin fraction of primary cell walls. This borate-RG-II complex is essential for wall architecture and porosity. Boron deficiency disrupts pollen germination (borate needed for pollen tube growth) and causes death of growing tips.
Q49.
How does silicon benefit plant growth even though it is not universally essential?
Answer: B. Silicon deposits in cell walls increase rigidity and pest resistance, improve water use efficiency, and reduce toxic metal stress; essential for rice and other grasses
Why: Silicon (Si) is considered beneficial/quasi-essential especially for grasses and rice. Si deposits as silica in epidermal cell walls, increasing physical strength, reducing fungal penetration, decreasing insect damage, improving drought tolerance, and reducing uptake of toxic metals like Al and Mn.
Q50.
What is the Haber-Bosch process and how does it compare to biological nitrogen fixation?
Answer: B. Haber-Bosch is an industrial process (N2 + H2 at high T/P with Fe catalyst) that produces ammonia; it requires 400-500 degrees C and 200 atm; biological fixation occurs at room temperature using nitrogenase
Why: Haber-Bosch (1909): N2 + 3H2 at 400-500C, 200 atm with Fe catalyst produces NH3. It consumes ~1-2% of global energy. Biological nitrogen fixation by nitrogenase occurs at ambient temperature and pressure but requires ~16 ATP per N2. Both produce the same NH3 product.
Q51.
What is phytoremediation and which plants are used for it?
Answer: B. Using plants to absorb and accumulate toxic heavy metals from contaminated soils (hyperaccumulators like Thlaspi, Alyssum, sunflower) to clean up pollution
Why: Phytoremediation uses plants that hyperaccumulate heavy metals (Zn, Cd, Ni, Pb) in their shoots. Thlaspi caerulescens hyperaccumulates Zn and Cd; Alyssum species accumulate Ni. After harvest, the metal-rich biomass is disposed of safely. It is a low-cost, ecological approach to soil decontamination.
Q52.
What is the role of chloride ions (Cl-) in plant physiology?
Answer: B. Chloride is a micronutrient essential for photosynthesis (water-splitting in PSII), stomatal regulation (counter-ion for K+ in guard cells), and osmoregulation
Why: Chloride is a micronutrient: (1) Required as cofactor for PSII water-splitting complex; (2) Serves as counter-ion to K+ during stomatal opening; (3) Involved in turgor regulation. Plants need very small amounts (<0.01% dry weight) but deficiency disrupts photosynthesis and wilting occurs.
Q53.
How does pH of the soil affect nutrient availability to plants?
Answer: B. Soil pH dramatically affects nutrient solubility: most nutrients are available at pH 6-7; iron and manganese become insoluble above pH 7; aluminum and manganese become toxic below pH 5
Why: Optimal nutrient availability is pH 6-7. Below pH 5: Al3+, Mn2+, and Fe2+ become soluble and toxic; P binds to Al and Fe. Above pH 7: Fe, Mn, Zn, Cu, B become insoluble (deficient). N-fixing bacteria prefer neutral pH. Liming raises acidic soil pH; sulfur applications lower alkaline soil pH.
Q54.
What is the difference between ectomycorrhizae and arbuscular mycorrhizae?
Answer: B. Ectomycorrhizae form a sheath around roots and penetrate between cortical cells (Hartig net) but not into cells; arbuscular mycorrhizae penetrate into cortical cells forming arbuscules (tree-like structures)
Why: Ectomycorrhizae: hyphae form a mantle around roots and grow between cortical cells (Hartig net); found in many trees (oak, pine). Arbuscular (endomycorrhizae/AM fungi): penetrate into cortical cells forming arbuscules for nutrient exchange and vesicles for storage; most common type, found in ~80% of plant species.
Q55.
In legume-Rhizobium symbiosis, nodule formation involves:
Answer: B. Nod factor signaling triggering root hair curling, infection thread formation, and differentiation of bacteroids expressing nitrogenase
Why: Nod factors (lipochitooligosaccharides) from Rhizobium trigger calcium spiking in root hair cells, curling, infection thread formation, and differentiation into nitrogen-fixing bacteroids.
Q56.
Non-symbiotic hemoglobin in plant root nodules functions to:
Answer: B. Scavenge O2 to very low levels, protecting nitrogenase from inactivation while supplying O2 for bacteroid respiration
Why: Leghemoglobin (plant hemoglobin in legume nodules): O2 buffer. Maintains O2 at very low concentration (O2 nM range) -- enough for bacteroid respiration but insufficient to inactivate nitrogenase (O2-labile).