Biology · BotanyBOT 07
Mineral Nutrition
Essential elements, deficiency symptoms, nitrogen fixation, nitrification cycle
Concept Core
Essential theory — everything NCERT tests on Mineral Nutrition
ESSENTIAL ELEMENTS
Criteria (Arnon & Stout): (1) Indispensable for life cycle completion, (2) specific — cannot be substituted, (3) directly involved in metabolism.
Macronutrients (>10 mmol/kg DW): C H O N P K Ca Mg S
Micronutrients (<10 mmol/kg): Fe Mn Cu Mo Zn B Cl Ni
DEFICIENCY SYMPTOMS
Mobile elements → older leaf symptoms: N P K Mg
Immobile → younger leaf symptoms: Ca Fe S B
Chlorosis: N, K, Mg, S, Fe, Mn, Zn. Necrosis: Ca, Mg, K, Cu, Fe. Stunted growth: N, P, K.
NITROGEN FIXATION
Rhizobium: symbiotic with legumes (root nodules). Azotobacter: free-living aerobe. Clostridium: anaerobe. Anabaena, Nostoc: cyanobacteria.
Enzyme: Nitrogenase (O₂-sensitive). Leghemoglobin (pink) maintains anaerobic microenvironment in nodules.
N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂
NITROGEN CYCLE
1. Ammonification: organic N→NH₄⁺ (decomposers)
2. Nitrification: NH₄⁺→NO₂⁻ (Nitrosomonas) → NO₃⁻ (Nitrobacter)
3. Assimilation: plants absorb NH₄⁺/NO₃⁻
4. Denitrification: NO₃⁻→N₂ (Pseudomonas, anaerobic)
SPECIAL ELEMENT ROLES
Mg: chlorophyll core. K: stomatal guard cells, enzyme activation. Ca: middle lamella, spindle fibre. Mo: nitrogenase + nitrate reductase. B: pollen germination, cell wall. Fe: cytochrome, nitrogenase. Mn: photolysis of water (PSII).
Fact & Formula Vault
High-yield facts, numbers, and formulas
Macronutrients
C H O N P K Ca Mg S
N: most limiting in agriculture
K: most abundant in plant ash
Mg: centre of chlorophyll
Nitrogen Fixation
16 ATP consumed per N₂
Leghemoglobin = O₂ buffer (pink)
Nitrogenase = O₂-sensitive
Only prokaryotes fix N₂
Deficiency Quick Ref
Old leaf symptoms: N P K Mg
Young leaf: Ca Fe S B
Chlorosis = N Mg Fe Mn Zn
Necrosis = Ca Mg K Cu
Worked Examples
NEET-style questions solved step-by-step
EASYLeghemoglobin in root nodules functions to:▾
Leghemoglobin in root nodules functions to:
Scavenge oxygen and maintain anaerobic conditions required by nitrogenase. Nitrogenase is irreversibly inhibited by O₂. Leghemoglobin does NOT supply O₂ to nitrogenase.
MEDIUMA plant shows chlorosis in older leaves and stunted growth. Most likely deficiency:▾
A plant shows chlorosis in older leaves and stunted growth. Most likely deficiency:
Old leaf chlorosis = mobile element. Mobile + chlorosis + stunted = Nitrogen deficiency. N is highly mobile — remobilised from old leaves to new growth first.
HARDWhich element is a constituent of nitrogenase AND nitrate reductase?▾
Which element is a constituent of nitrogenase AND nitrate reductase?
Molybdenum (Mo). Mo is part of the molybdenum-iron cofactor in nitrogenase and is also required for nitrate reductase enzyme function.
Mistake DNA
Common NEET traps for this chapter
⚠ Nitrogenase and O₂
Nitrogenase is O₂-sensitive — it is INHIBITED by O₂. Leghemoglobin buffers O₂ to protect it.
✓ Fix: Leghemoglobin scavenges O₂; does not supply it to nitrogenase
⚠ Nitrosomonas vs Nitrobacter order
NH₄⁺→NO₂⁻ = Nitrosomonas (step 1). NO₂⁻→NO₃⁻ = Nitrobacter (step 2).
✓ Fix: Alphabetical N-S before N-B = step 1 before step 2
⚠ Mobile vs immobile elements
Mobile elements show deficiency in OLD leaves. Immobile show in YOUNG/new leaves.
✓ Fix: N P K Mg = mobile = old leaf. Ca Fe S B = immobile = young leaf
Chapter Intelligence
Exam data and last-minute strategy
NEET Frequency
1–2 Q/year. Leghemoglobin function, 16 ATP nitrogenase, deficiency symptoms (old vs young leaf), Mo roles are highest yield.
Key Numbers
16 ATP per N₂. Mg = chlorophyll core. Mo = nitrogenase + nitrate reductase. Leghemoglobin = pink. 17 essential elements (C H O excluded by some).
Strategy
Make a table: Element → Role → Deficiency symptom → Mobile/Immobile. This table covers 90% of NEET questions on this chapter.
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