Class 11 Biology Notes Chapter 12 (Mineral nutrition) – Biology Book

Detailed Notes with MCQs of Chapter 12: Mineral Nutrition from your NCERT Class 11 Biology textbook. This chapter is crucial not just for your board exams but also forms a significant part of the syllabus for various government exams where Biology is a subject. We'll break down the essential concepts systematically.

Chapter 12: Mineral Nutrition - Detailed Notes for Government Exam Preparation

1. Introduction

• Plants, like all living organisms, require certain chemical elements for their growth, development, and physiological processes.
• These elements are obtained primarily from the soil in the form of inorganic ions and are called mineral nutrients.
• Carbon, Hydrogen, and Oxygen are mainly obtained from CO₂ and H₂O and are considered non-mineral elements, though essential.

2. Methods to Study Mineral Requirements of Plants

• Hydroponics:
  • Developed by Julius von Sachs (German botanist) in 1860.
  • Technique of growing plants in a nutrient solution (water containing dissolved essential minerals) in the complete absence of soil.
  • Significance:
    • Allows identification of essential elements.
    • Helps determine the deficiency symptoms of specific elements.
    • Used for commercial production of vegetables (e.g., tomato, seedless cucumber, lettuce).
  • Requires careful control of nutrient concentration, pH, and aeration of the solution.

3. Essential Mineral Elements

• Over 60 elements are found in different plants, but not all are essential.

• Criteria for Essentiality (Arnon & Stout, 1939): An element is considered essential if:

  1. It is absolutely necessary for supporting normal growth and reproduction. The plant cannot complete its life cycle without it.
  2. The requirement is specific and cannot be replaced by another element.
  3. The element is directly involved in the plant's metabolism (e.g., as a component of an enzyme or metabolite).

• Based on these criteria, 17 elements are currently considered essential for most plants.

4. Classification of Essential Elements

• Based on Quantitative Requirement:

  • Macronutrients: Required in relatively large amounts (generally > 10 mmole kg⁻¹ of dry matter).
    • Includes: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), Potassium (K), Sulphur (S), Calcium (Ca), Magnesium (Mg). (Remember: C, H, O are mainly non-mineral). Mnemonic hint: C HOPKiNS Ca Mg (C, H, O, P, K, N, S, Ca, Mg).
  • Micronutrients (Trace Elements): Required in very small amounts (generally < 10 mmole kg⁻¹ of dry matter).
    • Includes: Iron (Fe), Manganese (Mn), Copper (Cu), Molybdenum (Mo), Zinc (Zn), Boron (B), Chlorine (Cl), Nickel (Ni). Mnemonic hint: Fe Mn Cu Mo Zn B Cl Ni (Pronounce like: 'Femin Coomo Zin Bekal Ni').

• Based on Function: Essential elements can be grouped into four broad categories:

  1. Structural Components: Components of biomolecules (e.g., C, H, O, N).
  2. Energy-Related Compounds: Components of energy-related chemical compounds (e.g., Mg in chlorophyll, P in ATP).
  3. Enzyme Activation/Inhibition: Activating or inhibiting enzymes (e.g., Mg²⁺ activates RuBisCO and PEPcase; Zn²⁺ activates alcohol dehydrogenase; Mo activates nitrogenase).
  4. Osmotic Potential: Altering the osmotic potential of cells (e.g., K⁺ plays a role in opening and closing of stomata).

5. Role of Macro- and Micronutrients (Key Functions & Deficiency Symptoms)

(Focus on specific roles and deficiency symptoms - highly testable)

6. Deficiency Symptoms of Essential Elements

• Critical Concentration: The concentration of an essential element below which plant growth is retarded.
• Deficiency: When the supply of an essential element becomes limited, plant growth is retarded.
• Mobility of Elements:
  • Actively Mobilized Elements: (N, P, K, Mg) - Deficiency symptoms appear first in older (senescent) leaves because these elements are moved to younger, growing tissues.
  • Relatively Immobile Elements: (Ca, S) - Deficiency symptoms tend to appear first in younger tissues as they cannot be remobilized from older parts.
• Common Deficiency Symptoms: Chlorosis (loss of chlorophyll), Necrosis (death of tissues), Stunted growth, Premature leaf/fruit fall, Inhibition of cell division.

7. Toxicity of Micronutrients

• Micronutrients are required in low concentrations; a moderate increase can cause toxicity.
• The concentration range between adequacy and toxicity is narrow.
• Toxicity Symptoms: Difficult to identify, often involve interference with the absorption/function of other nutrients.
• Example: Manganese (Mn) Toxicity:
  • Symptoms: Appearance of brown spots surrounded by chlorotic veins.
  • Mechanism:
    • Inhibits Fe and Mg uptake.
    • Inhibits Ca translocation to the shoot apex.
  • Therefore, Mn toxicity symptoms can actually reflect deficiencies of Fe, Mg, and Ca.

8. Mechanism of Absorption of Elements

• Occurs through roots.
• Process involves two phases:
  1. Initial Phase (Passive): Rapid uptake into the apoplast (outer space - cell walls, intercellular spaces). Does not require metabolic energy. Driven by diffusion/ion exchange.
  2. Second Phase (Active): Slow uptake into the symplast (inner space - cytoplasm, vacuoles). Requires metabolic energy (ATP). Occurs against concentration gradient, involving specific membrane proteins (pumps/carriers).
• Flux: Movement of ions (Influx = inward movement into cells; Efflux = outward movement).

9. Translocation of Solutes

• Mineral salts are primarily translocated upwards through the xylem along with the ascending stream of water (transpiration pull).
• Analysis of xylem sap confirms the presence of mineral salts.
• Some exchange between xylem and phloem occurs; elements can be redistributed (especially mobile ones).

10. Metabolism of Nitrogen

• Nitrogen is a crucial macronutrient, often limiting plant growth.

• Plants absorb N mainly as NO₃⁻, but also as NO₂⁻ and NH₄⁺.

• Nitrogen Cycle:

  • Nitrogen Fixation: Conversion of atmospheric N₂ gas into ammonia (NH₃). This is done by certain prokaryotes (bacteria, cyanobacteria).
  • Ammonification: Decomposition of organic nitrogen (proteins, nucleic acids in dead organisms) into ammonia by microbes.
  • Nitrification: Conversion of ammonia to nitrate.
    • NH₃ → NO₂⁻ (Nitrite) by Nitrosomonas / Nitrococcus.
    • NO₂⁻ → NO₃⁻ (Nitrate) by Nitrobacter.
    • These bacteria are chemoautotrophs. Nitrate is the form most readily absorbed by plants.
  • Denitrification: Conversion of nitrate back to N₂ gas by bacteria like Pseudomonas and Thiobacillus, returning N to the atmosphere.

• Biological Nitrogen Fixation (BNF):

  • Reduction of N₂ to NH₃ by living organisms using the enzyme nitrogenase.
  • Nitrogenase is highly sensitive to oxygen.
  • N₂ + 8e⁻ + 8H⁺ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi (Requires significant energy).
  • Nitrogen-fixing microbes:
    • Free-living:
      • Aerobic: Azotobacter, Beijerinckia
      • Anaerobic: Rhodospirillum
      • Cyanobacteria: Anabaena, Nostoc
    • Symbiotic:
      • Rhizobium with legumes (soybean, clover, alfalfa, beans).
      • Frankia with non-leguminous plants (e.g., Alnus).

• Symbiotic Nitrogen Fixation - Nodule Formation (in Legumes):

  1. Rhizobia multiply near roots and attach to epidermal/root hair cells.
  2. Root hairs curl, bacteria invade.
  3. An infection thread forms, carrying bacteria into the root cortex.
  4. Bacteria are released into cortical cells, differentiate into bacteroids (specialized N-fixing forms).
  5. Cortical cells divide rapidly, forming the nodule.
  6. Vascular connection established with the host for nutrient exchange.
  7. Nodules contain nitrogenase enzyme and Leghemoglobin.
  8. Leghemoglobin: An oxygen scavenger (pink pigment), protects nitrogenase from O₂ by creating anaerobic conditions.

• Fate of Ammonia (NH₃):

  • Ammonia is protonated to form ammonium ion (NH₄⁺), which is toxic in high concentrations.
  • Plants assimilate NH₄⁺ into amino acids through two main pathways:
    1. Reductive Amination: NH₄⁺ reacts with α-ketoglutaric acid (from Krebs cycle) to form glutamic acid (glutamate). Enzyme: Glutamate Dehydrogenase.
       α-ketoglutaric acid + NH₄⁺ + NADPH → Glutamate + H₂O + NADP⁺
    2. Transamination: Transfer of amino group (-NH₂) from one amino acid (usually glutamate) to the keto group of a keto acid. Enzyme: Transaminases. Glutamate is the main amino donor. This forms various other amino acids.
       Example: Glutamic acid + Oxaloacetic acid ⇌ Aspartic acid + α-ketoglutaric acid
  • Nitrogen is transported to other plant parts mainly as amides (e.g., asparagine, glutamine), which have a higher N:C ratio than amino acids. Formed by adding another amino group to aspartic acid and glutamic acid, respectively.

MCQs for Practice (Based on NCERT Chapter 12)

1. The technique of growing plants in a nutrient solution without soil is known as:
   a) Aeroponics
   b) Hydroponics
   c) Aquaculture
   d) Tissue culture

2. Which of the following criteria is NOT used to determine the essentiality of an element for plants?
   a) The element must be necessary for normal growth and reproduction.
   b) The requirement must be specific and not replaceable by another element.
   c) The element must be directly involved in plant metabolism.
   d) The element must be present in the soil in large quantities.

3. Which of the following is a Macronutrient?
   a) Iron (Fe)
   b) Manganese (Mn)
   c) Calcium (Ca)
   d) Zinc (Zn)

4. Deficiency symptoms of mobile elements like Nitrogen (N) and Potassium (K) first appear in:
   a) Young leaves
   b) Roots
   c) Older leaves
   d) Flowers

5. The central metal ion in the chlorophyll pigment is:
   a) Iron (Fe)
   b) Magnesium (Mg)
   c) Manganese (Mn)
   d) Copper (Cu)

6. The enzyme nitrogenase, crucial for biological nitrogen fixation, requires which element as its component/activator?
   a) Zinc (Zn)
   b) Boron (B)
   c) Molybdenum (Mo)
   d) Copper (Cu)

7. Leghemoglobin, found in root nodules of legumes, functions as:
   a) An oxygen scavenger
   b) A nitrogen carrier
   c) An energy source for nitrogenase
   d) A structural component of the nodule

8. The conversion of ammonia (NH₃) to nitrite (NO₂⁻) during nitrification is carried out by bacteria like:
   a) Nitrobacter
   b) Pseudomonas
   c) Nitrosomonas
   d) Rhizobium

9. Manganese (Mn) toxicity in plants often leads to the deficiency symptoms of:
   a) Nitrogen, Phosphorus, Potassium
   b) Iron, Magnesium, Calcium
   c) Sulphur, Boron, Zinc
   d) Molybdenum, Chlorine, Nickel

10. The primary assimilation of ammonium (NH₄⁺) into amino acids via reaction with α-ketoglutaric acid is called:
    a) Transamination
    b) Denitrification
    c) Reductive Amination
    d) Ammonification

Answer Key:

1. b) Hydroponics
2. d) The element must be present in the soil in large quantities.
3. c) Calcium (Ca)
4. c) Older leaves
5. b) Magnesium (Mg)
6. c) Molybdenum (Mo)
7. a) An oxygen scavenger
8. c) Nitrosomonas
9. b) Iron, Magnesium, Calcium
10. c) Reductive Amination

Remember to revise these points thoroughly. Pay special attention to the specific roles and deficiency symptoms of each element, the process of nitrogen fixation, and the key enzymes involved. Good luck with your preparation!