Mineral Nutrition — NCERT Class 11 Biology

Hydroponics is the technique of growing plants in a nutrient solution (water containing dissolved mineral salts) without soil. The method was developed by Julius von Sachs (1860).

Usefulness in identifying essential minerals: 1. Plants are grown in a complete nutrient solution (containing all known essential elements) as a control. 2. Then, one mineral element at a time is omitted from the solution. 3. If the plant shows deficiency symptoms (stunted growth, chlorosis, necrosis, etc.), the omitted element is essential for that plant. 4. If the plant grows normally even without that element, it is non-essential. 5. By systematically removing each element and observing the plant's response, all essential mineral elements can be identified.

This method also helps determine the specific role of each element and the critical concentration below which deficiency occurs.

Criteria for essentiality (Arnon and Stout, 1939): 1. The element must be absolutely necessary for normal growth, development, and reproduction — the plant cannot complete its life cycle without it. 2. The requirement must be specific — the element cannot be replaced by another element. 3. The element must be directly involved in the metabolism of the plant (as a component of an essential molecule or as an enzyme activator).

Essential elements (17):

• Macronutrients (9): Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sulphur (S).
• Micronutrients (8): Iron (Fe), Manganese (Mn), Boron (B), Zinc (Zn), Copper (Cu), Molybdenum (Mo), Chlorine (Cl), Nickel (Ni).

Nitrogen fixation is the process of converting atmospheric nitrogen (N₂) into ammonia (NH₃) or other nitrogen compounds that can be assimilated by plants. N₂ is inert (triple bond N≡N, bond energy 942 kJ/mol) and must be converted to a usable form.

Biological nitrogen fixation is carried out by nitrogen-fixing organisms:

• Symbiotic: Rhizobium (in legume root nodules), Frankia (in actinorhizal plants).
• Free-living: Azotobacter (aerobic), Clostridium (anaerobic), Beijerinckia (aerobic).
• Cyanobacteria (blue-green algae): Nostoc, Anabaena, Oscillatoria — some have heterocysts (specialised N₂-fixing cells).

The enzyme nitrogenase catalyses the reaction: N₂ + 8e⁻ + 8H⁺ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi

Nitrogenase consists of two components: 1. Fe-protein (dinitrogenase reductase): Accepts electrons from ferredoxin and transfers them to MoFe-protein; hydrolyses ATP. 2. MoFe-protein (dinitrogenase): Contains Mo and Fe; reduces N₂ to NH₃.

Nitrogenase is extremely sensitive to oxygen and functions only under anaerobic conditions. In legume root nodules, leghaemoglobin (a pink iron-containing pigment) scavenges free O₂ to create a microaerophilic environment for nitrogenase.

Rhizobium is a symbiotic nitrogen-fixing bacterium that forms root nodules in leguminous plants (pea, bean, clover, lentil). It fixes atmospheric N₂ into NH₃, which the plant uses to synthesise amino acids and proteins. In return, the plant provides the bacterium with carbohydrates and a protected environment.

Nodule formation (step by step): 1. Recognition: Rhizobium recognises the correct legume host through chemical signals — the plant secretes flavonoids and the bacterium produces Nod factors (lipochitooligosaccharides). 2. Attachment: Rhizobium attaches to the root hairs using specific adhesins. 3. Curling and infection thread: The root hair curls, and the bacterium enters through an infection thread (a tubular structure formed by the plant). 4. Cortical cell division: The infection thread reaches the inner cortex, and cortical cells start dividing to form the nodule primordium. 5. Nodule formation: Bacteria are released into the cortical cells, multiply, and differentiate into bacteroids (nitrogen-fixing form). The nodule matures with a vascular connection to the plant. 6. Nitrogen fixation: Inside the nodule, bacteroids express nitrogenase and fix N₂ → NH₃. Leghaemoglobin (pink pigment) regulates O₂ supply — it provides just enough O₂ for bacterial respiration while protecting nitrogenase from O₂ inactivation.

Hydroponics is the technique of growing plants in a nutrient solution (water containing all essential mineral elements in proper proportions) without soil. It was first demonstrated by Julius von Sachs (1860). In this method, plant roots are suspended in the nutrient solution, which is aerated to provide oxygen. Hydroponics is used to: (a) identify essential mineral elements, (b) study deficiency symptoms, (c) grow crops commercially in controlled environments.

The three criteria for essentiality (Arnon and Stout, 1939): 1. The element must be absolutely necessary for normal growth, development, and reproduction — the plant cannot complete its life cycle without it. 2. The requirement must be specific — no other element can substitute for it. 3. The element must be directly involved in plant metabolism (as a structural component, enzyme cofactor, or participant in metabolic reactions).

Macronutrients (9): Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sulphur (S). These are required in concentrations greater than 10 mmol/kg dry weight.

Micronutrients (8): Iron (Fe), Manganese (Mn), Boron (B), Zinc (Zn), Copper (Cu), Molybdenum (Mo), Chlorine (Cl), Nickel (Ni). These are required in concentrations less than 10 mmol/kg dry weight.

Deficiency symptoms are the visible morphological changes that appear in a plant when an essential element is absent or below the critical concentration. Common symptoms include chlorosis, necrosis, stunted growth, and delayed flowering.

(a) Chlorosis: Yellowing of leaves due to loss of chlorophyll. Caused by deficiency of N, Mg, Fe, S, Mn, Zn. It is the most common deficiency symptom.

(b) Necrosis: Death of tissue (typically leaf tissue), appearing as brown or black spots/patches. Caused by deficiency of K, Ca, Mg, Cu. Can also result from pathogen infection.

(c) Active absorption: Uptake of mineral ions against the concentration gradient using energy (ATP). Involves carrier proteins (pumps) in the plasma membrane. Example: uptake of ions by root hair cells from dilute soil solution.

(d) Toxicity: Any concentration of a mineral element that reduces plant growth and development. Micronutrients are required in very small amounts and become toxic at even slightly higher concentrations. For example, manganese toxicity causes brown spots surrounded by chlorotic veins and can induce iron deficiency.

The nitrogen cycle is the continuous cycling of nitrogen between the atmosphere, soil, and living organisms. It involves the following steps:

1. Nitrogen fixation: Conversion of atmospheric N₂ to NH₃.

• Biological: By Rhizobium (symbiotic), Azotobacter, Nostoc, Anabaena (free-living).
• Industrial: Haber-Bosch process (N₂ + 3H₂ → 2NH₃ at high temperature and pressure).
• Atmospheric: By lightning (N₂ + O₂ → NOₓ).

2. Ammonification: Decomposition of organic nitrogen (proteins, amino acids, urea) in dead organisms and waste into NH₃/NH₄⁺ by decomposer bacteria and fungi.

3. Nitrification: Oxidation of ammonia to nitrate.

• Step 1: NH₃ → NO₂⁻ (nitrite) by Nitrosomonas and Nitrosococcus.
• Step 2: NO₂⁻ → NO₃⁻ (nitrate) by Nitrobacter.

4. Assimilation: Uptake of NH₄⁺ and NO₃⁻ by plants; conversion to amino acids, proteins, nucleotides. Animals obtain nitrogen by eating plants.

5. Denitrification: Reduction of nitrate (NO₃⁻) back to N₂ gas by Pseudomonas and Thiobacillus in waterlogged, anaerobic conditions. This returns nitrogen to the atmosphere.

(See Intext Page 204 Q1 for detailed answer.)

Biological nitrogen fixation converts N₂ → NH₃ using the enzyme nitrogenase found in:

• Symbiotic: Rhizobium (legume nodules), Frankia (non-legume nodules).
• Free-living: Azotobacter, Clostridium, Beijerinckia (soil bacteria).
• Cyanobacteria: Nostoc, Anabaena (some with heterocysts).

Reaction: N₂ + 8e⁻ + 8H⁺ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi

Nitrogenase components:

• Fe-protein (dinitrogenase reductase): transfers electrons, hydrolyses ATP.
• MoFe-protein (dinitrogenase): reduces N₂ to NH₃.

Key points: Nitrogenase is oxygen-sensitive; leghaemoglobin protects it in nodules; the process requires 16 ATP per N₂ molecule; the NH₃ produced is protonated to NH₄⁺ and assimilated into amino acids.

(See Intext Page 204 Q2 for detailed answer.)

Nodule formation involves: (1) Rhizobium recognition via flavonoids and Nod factors, (2) root hair curling and infection thread formation, (3) cortical cell division forming nodule primordium, (4) bacteroid differentiation and nodule maturation with vascular connections, (5) nitrogen fixation inside nodule protected by leghaemoglobin.

(a) Nitrogen (N):

• Component of amino acids, proteins, nucleic acids (DNA, RNA), chlorophyll, ATP, NADH, coenzymes.
• Required for vegetative growth.
• Deficiency causes chlorosis (starting from older leaves), stunted growth, late flowering.

(b) Phosphorus (P):

• Component of nucleic acids (DNA, RNA), ATP, phospholipids (cell membranes).
• Essential for energy transfer (ATP), root development, and seed/fruit formation.
• Deficiency causes poor root growth, delayed maturity, purple colouration of leaves.

(c) Potassium (K):

• Involved in protein synthesis, stomatal opening/closing, activation of enzymes (50+ enzymes).
• Maintains turgor pressure and osmotic balance.
• Deficiency causes chlorosis in interveinal areas, necrosis at leaf margins, weak stems.

(d) Calcium (Ca):

• Required for cell wall formation (calcium pectate in middle lamella).
• Activates enzymes, essential for cell division and cell elongation.
• Deficiency causes stunted growth, meristem damage, curling of young leaves.

(e) Magnesium (Mg):

• Central atom of the chlorophyll molecule.
• Activates enzymes of photosynthesis and respiration (RUBP carboxylase, PEP carboxylase).
• Maintains ribosome structure.
• Deficiency causes chlorosis in older leaves (interveinal).

(f) Iron (Fe):

• Required for synthesis of chlorophyll (not a component, but essential for its formation).
• Component of cytochromes (electron transport chain), ferredoxin, and catalase.
• Deficiency causes interveinal chlorosis in young leaves (iron is immobile).