Quick Summary
Plants need 17 essential mineral elements for growth. These are classified as macro- and micronutrients based on quantity required. Nitrogen metabolism — fixation, assimilation, and the cycle — is the most important topic in this chapter. Deficiency symptoms are tested directly in NEET.
Essential Mineral Elements
Criteria (Arnon & Stout, 1939): 1. Deficiency causes growth impairment 2. Cannot be replaced by another element 3. Involved directly in plant metabolism
17 essential elements: C, H, O, N, P, K, Ca, Mg, S (macronutrients) + Fe, Mn, Zn, Cu, Mo, B, Cl, Ni (micronutrients)
C, H, O come from air and water — all others from soil.
Macronutrients and Their Functions
| Element | Key Function | Deficiency Symptom |
|---|---|---|
| Nitrogen (N) | Amino acids, proteins, nucleic acids, chlorophyll | Chlorosis (older leaves first) |
| Phosphorus (P) | ATP, nucleic acids, phospholipids | Purple/red colour (anthocyanin accumulation) |
| Potassium (K) | Stomatal regulation, enzyme activation | Scorching of leaf tips and margins |
| Calcium (Ca) | Cell wall (middle lamella as calcium pectate), cell signalling | Curling of young leaves, death of meristems |
| Magnesium (Mg) | Central atom of chlorophyll, enzyme activator | Interveinal chlorosis (older leaves) |
| Sulphur (S) | Amino acids (cysteine, methionine), coenzymes | Yellowing of young leaves |
Micronutrients and Their Functions
| Element | Key Function | Deficiency |
|---|---|---|
| Iron (Fe) | Chlorophyll synthesis (not part of chlorophyll), cytochromes | Interveinal chlorosis of young leaves |
| Manganese (Mn) | Photosystem II (water splitting), enzyme activation | Interveinal chlorosis |
| Zinc (Zn) | Auxin synthesis, enzyme activity | Little leaf, rosette disease |
| Copper (Cu) | Electron transport, enzyme cofactor | Wilting of young leaves |
| Molybdenum (Mo) | Nitrogen fixation (nitrogenase), nitrate reduction | Whiptail disease (cauliflower) |
| Boron (B) | Cell wall formation, pollen germination | Death of meristems, hollow stem |
Mobile nutrients (deficiency in older leaves): N, P, K, Mg Immobile nutrients (deficiency in young leaves): Ca, Fe, Mn, B
Nitrogen Metabolism
Nitrogen Fixation
Biological Nitrogen Fixation: N₂ → NH₃
Enzyme: Nitrogenase (requires Mo, Fe; inhibited by O₂) Equation: N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi
| Type | Organism |
|---|---|
| Free-living aerobic | Azotobacter, Beijerinckia |
| Free-living anaerobic | Clostridium, Anabaena |
| Symbiotic (legumes) | Rhizobium (in root nodules) |
| Symbiotic (non-legumes) | Frankia (in Alnus root nodules) |
Leghemoglobin: Pink pigment in root nodules; protects nitrogenase from O₂ while providing O₂ to bacteroid metabolism.
Nitrification and Ammonification
- Ammonification: Decomposers convert organic N → NH₄⁺
- Nitrification: NH₄⁺ → NO₂⁻ (Nitrosomonas) → NO₃⁻ (Nitrobacter)
- Denitrification: NO₃⁻ → N₂ (Pseudomonas, Thiobacillus) — returns N to atmosphere
Nitrogen Assimilation
NH₄⁺ + α-ketoglutarate → Glutamate (via GS-GOGAT pathway) Glutamate → other amino acids via transamination
Special Modes of Nutrition
Insectivorous plants: Supplement N-poor environments by trapping insects
- Nepenthes (pitcher plant) — leaf modification
- Drosera (sundew) — sticky tentacles
- Dionaea (Venus flytrap) — snap traps
- Utricularia (bladderwort) — aquatic bladder traps
Parasitic plants:
- Cuscuta (dodder) — complete stem parasite, no chlorophyll
- Striga — partial root parasite
- Viscum (mistletoe) — partial stem parasite (has chlorophyll)
Hydroponics
Growing plants in nutrient solution (no soil). Used to study essential mineral requirements (Hoagland's nutrient solution).
Important Questions
One Mark
- Name the enzyme responsible for biological nitrogen fixation.
- What is leghemoglobin? Where is it found?
- Which mineral is the central atom of the chlorophyll molecule?
- Name one free-living nitrogen-fixing bacterium.
Short Answer (3 marks)
- Distinguish between macronutrients and micronutrients with two examples each.
- Explain the role of Rhizobium in nitrogen fixation.
- What are mobile and immobile mineral nutrients? Give two examples of each.
Long Answer (5 marks)
- Describe biological nitrogen fixation. Name the organisms involved and the enzyme responsible. Write the equation.
- List any six macronutrients with their key functions and deficiency symptoms.
Common Mistakes
- Fe is NOT a constituent of chlorophyll — Mg is. Fe is needed for chlorophyll synthesis.
- Mobile nutrient deficiency: older leaves affected (N, P, K, Mg). Immobile: young leaves affected (Ca, Fe, B).
- Nitrogenase is inhibited by O₂ — leghemoglobin buffers O₂ at safe level (doesn't eliminate O₂ entirely).
- Cuscuta is a complete parasite (holoparasite) with no chlorophyll; Viscum is a partial parasite (hemiparasite) with chlorophyll.
Board Exam Tips
- Deficiency symptoms table (6 macronutrients) is a standard 5-mark question.
- Biological nitrogen fixation — organisms, enzyme, equation — appears every year.
- Insectivorous plants and their trap mechanisms are reliable 3-mark questions.
- NEET: Nitrogen fixation, nitrogenase, leghemoglobin, and mobile/immobile nutrient concepts appear regularly.