Class 11 Biology Notes Chapter 12 (Chapter 12) – Examplar Problems (English) Book

Detailed Notes with MCQs of Chapter 12, Mineral Nutrition. This is a crucial chapter, not just for your Class 11 understanding, but also frequently tested in various government exams involving Biology. Pay close attention to the specific roles of elements, deficiency symptoms, and the nitrogen cycle.

Chapter 12: Mineral Nutrition - Detailed Notes

1. Methods to Study Mineral Requirements of Plants

• Hydroponics:
  • Definition: Technique of growing plants in a nutrient solution (without soil) in complete absence of soil. Developed by Julius von Sachs (1860).
  • Significance: Allows identification of essential elements, study of deficiency symptoms, and determination of specific roles of elements. Nutrient solutions must be aerated for root respiration.
  • Applications: Commercial production of vegetables (tomato, seedless cucumber, lettuce). Helps conserve water and soil resources.

2. Essential Mineral Elements

• Criteria for Essentiality (Arnon & Stout): 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. It is directly involved in the metabolism of the plant (e.g., component of an enzyme, structural molecule).
• Classification based on Quantitative Requirements:
  • Macronutrients: Required in large amounts (generally > 10 mmole kg⁻¹ of dry matter).
    • Includes: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), Potassium (K), Sulfur (S), Calcium (Ca), Magnesium (Mg).
    • (C, H, O are mainly obtained from CO₂ and H₂O).
  • 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).
• Classification based on Function:
  • Structural Components: C, H, O, N (Components of biomolecules like proteins, carbohydrates, lipids, nucleic acids).
  • Energy-Related Compounds: Mg (in Chlorophyll), P (in ATP).
  • Enzyme Activation/Inhibition: Mg²⁺ (activator for RuBisCO, PEPcase, kinases), Zn²⁺ (activator for alcohol dehydrogenase, carbonic anhydrase), Mo (nitrogenase, nitrate reductase), Mn²⁺ (photolysis of water).
  • Osmotic Potential Regulation: K⁺ (opening/closing of stomata, turgidity).

3. Role of Macro- and Micronutrients

(Focus on key roles and deficiency symptoms)

4. Deficiency Symptoms of Essential Elements

• Critical Concentration: The concentration of an essential element below which plant growth is retarded.
• Deficiency Symptoms: Morphological changes indicative of element shortage. Symptoms vary depending on the element and plant species.
• Mobility and Symptoms:
  • Mobile Elements (N, P, K, Mg): Deficiency symptoms appear first in older/senescent leaves as elements are mobilized to younger tissues.
  • Immobile Elements (Ca, S, Fe, Mn, Zn, B, Cu): Deficiency symptoms appear first in younger leaves/tissues as elements cannot be remobilized from older parts.
• Common Symptoms:
  • Chlorosis: Loss of chlorophyll leading to yellowing. Caused by deficiency of N, K, Mg, S, Fe, Mn, Zn, Mo.
  • Necrosis: Death of tissues, particularly leaf tissue. Caused by deficiency of Ca, Mg, Cu, K.
  • Inhibition of Cell Division: Lack of N, K, S, Mo.
  • Delayed Flowering: Deficiency of N, S, Mo.
  • Stunted Growth: General symptom for many deficiencies.

5. Toxicity of Micronutrients

• Micronutrients are required in low amounts; a moderate increase can cause toxicity.
• Toxicity levels vary for different micronutrients and plants.
• Manganese (Mn) Toxicity:
  • Symptoms: Appearance of brown spots surrounded by chlorotic veins.
  • Mechanism: Mn competes with Fe and Mg for uptake and with Mg for binding to enzymes. Mn also inhibits Ca translocation to the shoot apex.
  • Therefore, Mn toxicity symptoms can actually manifest as deficiency symptoms of Fe, Mg, and Ca.

6. Mechanism of Absorption of Elements

• Mineral absorption occurs primarily through the roots.
• Two Phases:
  • Initial Phase (Passive): Rapid uptake into the 'outer space' (apoplast - intercellular spaces and cell walls). Does not require metabolic energy. Driven by diffusion/ion exchange.
  • Second Phase (Active): Slow uptake into the 'inner space' (symplast - cytoplasm and vacuole). Requires metabolic energy (ATP). Occurs against concentration gradient via membrane proteins (pumps).
• Pathways:
  • Apoplast Pathway: Movement through adjacent cell walls, does not cross cell membranes until the endodermis (Casparian strip blocks it).
  • Symplast Pathway: Movement from cell to cell through plasmodesmata, crossing cell membranes.
• Role of Endodermis: The Casparian strip (suberin) in the endodermis is impermeable to water and solutes, forcing water and minerals entering via the apoplast to cross the endodermal cell membrane (symplast pathway) before reaching the xylem. This allows for selective uptake.

7. Translocation of Solutes

• Minerals are transported upwards to stems, leaves, and growing regions primarily through the xylem along with water (transpiration stream).
• Analysis of xylem sap confirms the presence of mineral salts.
• Some exchange between xylem and phloem occurs.

8. Soil as Reservoir of Essential Elements

• Soil provides anchorage and essential minerals, water, and air to plants.
• Minerals originate from the weathering of rocks.
• Factors affecting mineral availability: Soil pH, aeration, temperature, microbial activity, soil texture.

9. Metabolism of Nitrogen

• Nitrogen is a crucial limiting nutrient for both natural and agricultural ecosystems.

• Nitrogen Cycle: The circulation of nitrogen between the atmosphere, soil, and living organisms.

  • Nitrogen Fixation: Conversion of atmospheric N₂ gas (inert) into ammonia (NH₃). This is the primary way N enters the ecosystem.
    • Atmospheric: Lightning, UV radiation (minor).
    • Industrial: Haber-Bosch process.
    • Biological: Accomplished by certain prokaryotes (bacteria, cyanobacteria) called diazotrophs using the enzyme Nitrogenase.
  • Ammonification: Decomposition of organic nitrogen (proteins, nucleic acids from dead plants/animals) back into ammonia (NH₃) by ammonifying bacteria and fungi.
  • Nitrification: Conversion of ammonia first into nitrite (NO₂⁻) and then into nitrate (NO₃⁻). This is done by chemoautotrophic nitrifying bacteria in the soil.
    • NH₃ → NO₂⁻ (by Nitrosomonas / Nitrococcus)
    • NO₂⁻ → NO₃⁻ (by Nitrobacter / Nitrocystis)
    • Nitrate (NO₃⁻) is the primary form absorbed by plants.
  • Denitrification: Conversion of nitrate (NO₃⁻) back into N₂ gas, which returns to the atmosphere. Carried out by denitrifying bacteria (e.g., Pseudomonas, Thiobacillus) under anaerobic conditions.

• Biological Nitrogen Fixation (BNF):

  • Nitrogenase Enzyme: Mo-Fe protein complex. Highly sensitive to oxygen. Requires anaerobic conditions and a large amount of energy (16 ATP per N₂ fixed).
    • N₂ + 8e⁻ + 8H⁺ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi
  • Types of Nitrogen-Fixing Microbes:
    • Free-living:
      • Aerobic: Azotobacter, Beijerinckia
      • Anaerobic: Rhodospirillum, Clostridium
      • Cyanobacteria (Blue-green algae): Anabaena, Nostoc (often have heterocysts for anaerobic conditions).
    • Symbiotic: Form mutualistic relationships.
      • Rhizobium: With roots of legumes (e.g., peas, beans, alfalfa). Forms root nodules.
      • Frankia: With roots of non-leguminous plants (e.g., Alnus, Casuarina). Forms root nodules.
  • Nodule Formation (Rhizobium-Legume):
    1. Roots secrete chemicals (flavonoids) attracting Rhizobium.
    2. Bacteria attach to root hairs, causing curling.
    3. Bacteria invade root hair, forming an "infection thread".
    4. Infection thread carries bacteria into the root cortex.
    5. Bacteria are released into cortical cells and differentiate into bacteroids (N₂-fixing form).
    6. Nodule development involves cell division in cortex and pericycle. Vascular connection established.
    7. Leghemoglobin: A pink-colored O₂-scavenging pigment produced by the host plant cells in the nodule. Protects nitrogenase from oxygen while ensuring aerobic respiration for energy.

• Fate of Ammonia (NH₃):

  • Ammonia is toxic at high concentrations. It is rapidly converted into amino acids in the plant cell.
  • Protonation: At physiological pH, NH₃ is protonated to form ammonium ion (NH₄⁺).
  • Assimilation: NH₄⁺ is used to synthesise amino acids via two main pathways:
    1. Reductive Amination: Ammonium reacts with α-ketoglutaric acid (from Krebs cycle) to form glutamate. Enzyme: Glutamate Dehydrogenase (GDH).
       α-ketoglutarate + NH₄⁺ + NAD(P)H → Glutamate + H₂O + NAD(P)⁺
    2. Transamination: Transfer of amino group (-NH₂) from an amino acid (like glutamate) to the keto group of a keto acid. Enzyme: Transaminases (require pyridoxal phosphate - PLP). Forms various other amino acids.
       Glutamate + Oxaloacetic acid ⇌ Aspartic acid + α-ketoglutaric acid
  • Amide Formation: Nitrogen is transported to other plant parts mainly as amides (e.g., Asparagine, Glutamine). Amides contain more nitrogen than amino acids and are formed from aspartic acid and glutamic acid, respectively, by adding another amino group.

Multiple Choice Questions (MCQs)

1. The technique of growing plants in a defined nutrient solution, in the complete absence of soil, is known as:
   (a) Aeroponics
   (b) Hydroponics
   (c) Soil culture
   (d) Tissue culture

2. Which of the following is NOT a criterion for the essentiality of a mineral element for a plant?
   (a) The element must be absolutely necessary for normal growth and reproduction.
   (b) The requirement of the element must be specific and not replaceable by another element.
   (c) The element must be directly involved in the plant's metabolism.
   (d) The element must be present in the soil in large quantities.

3. Identify the macronutrient from the following list:
   (a) Iron (Fe)
   (b) Manganese (Mn)
   (c) Calcium (Ca)
   (d) Zinc (Zn)

4. Which mineral element is a central component of the chlorophyll molecule?
   (a) Iron (Fe)
   (b) Magnesium (Mg)
   (c) Manganese (Mn)
   (d) Copper (Cu)

5. Deficiency symptoms of immobile elements like Calcium (Ca) and Iron (Fe) first appear in:
   (a) Older leaves
   (b) Younger leaves
   (c) Roots
   (d) Flowers

6. The enzyme nitrogenase, crucial for biological nitrogen fixation, contains which metallic elements?
   (a) Iron and Magnesium (Fe, Mg)
   (b) Molybdenum and Iron (Mo, Fe)
   (c) Manganese and Chlorine (Mn, Cl)
   (d) Copper and Zinc (Cu, Zn)

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

8. The function of leghemoglobin in the root nodules of legumes is to:
   (a) Fix atmospheric nitrogen directly
   (b) Provide energy (ATP) for nitrogen fixation
   (c) Protect nitrogenase from oxygen
   (d) Transport fixed nitrogen to the host plant

9. The primary assimilation of ammonium ions (NH₄⁺) into amino acids involves the enzyme:
   (a) Nitrogenase
   (b) Nitrate reductase
   (c) Glutamate dehydrogenase
   (d) Transaminase

10. Toxicity symptoms like brown spots surrounded by chlorotic veins, often related to competitive inhibition of Fe and Mg uptake, are characteristic of excess:
    (a) Zinc (Zn)
    (b) Boron (B)
    (c) Manganese (Mn)
    (d) Copper (Cu)

Answer Key for MCQs:

1. (b)
2. (d)
3. (c)
4. (b)
5. (b)
6. (b)
7. (c)
8. (c)
9. (c)
10. (c)

Make sure you revise these concepts thoroughly, especially the specific roles, deficiency symptoms (linked to mobility), and the entire nitrogen cycle including the organisms and enzymes involved. Good luck with your preparation!