Mineral Nutrition

🎯 Key Points

• Macronutrients (large amounts): C,H,O,N,P,K,Ca,Mg,S; Micronutrients (trace): Fe,Mn,Zn,Cu,Mo,B,Cl,Ni — 17 essential elements total
• Mg is the CENTRE of chlorophyll — its deficiency causes interveinal chlorosis, same visible symptom as Fe deficiency but for a different reason (Fe is needed for chlorophyll SYNTHESIS, not as a structural component)
• N, P, K are mobile (exported from old to young leaves via phloem); Ca is immobile (stays put, causing distorted NEW growth when deficient, not old-leaf symptoms)
• Biological N-fixation uses nitrogenase (O₂-sensitive, needs anaerobic conditions); leghaemoglobin in root nodules protects nitrogenase from oxygen
• Nitrification: NH₃→NO₂⁻ (Nitrosomonas)→NO₃⁻ (Nitrobacter); Denitrification: NO₃⁻→N₂ (removes nitrogen from soil, opposite direction)

Essential Mineral Elements

17 elements are essential for plant growth:

• Macronutrients (needed in large amounts): C, H, O, N, P, K, Ca, Mg, S
• Micronutrients (needed in trace amounts): Fe, Mn, Zn, Cu, Mo, B, Cl, Ni

Key Functions

• Nitrogen (N): proteins, nucleic acids, chlorophyll; deficiency = yellowing (chlorosis), stunted growth
• Phosphorus (P): ATP, DNA, cell membranes; deficiency = purple/red coloration, poor root growth
• Potassium (K): stomatal opening, enzyme activation; deficiency = necrosis at leaf margins
• Magnesium (Mg): center of chlorophyll molecule; deficiency = chlorosis between veins (interveinal chlorosis)
• Iron (Fe): electron transport, chlorophyll synthesis; deficiency = interveinal chlorosis in young leaves
• Calcium (Ca): cell wall (calcium pectate), cell signaling; deficiency = distorted young leaves

Nitrogen Fixation

• Atmospheric N2 → usable NH3/NO3; enzyme = nitrogenase (requires anaerobic conditions)
• Biological fixation: Rhizobium (symbiotic in legume root nodules), Azotobacter (free-living), Nostoc/Anabaena (cyanobacteria)
• Nitrification: NH3 → NO2 (Nitrosomonas) → NO3 (Nitrobacter)
• Denitrification: NO3 → N2 (Pseudomonas, Thiobacillus): removes nitrogen from soil

Hydroponics

• Growing plants in nutrient solutions without soil
• Used to study mineral requirements; also commercial crop production

Insectivorous Plants

• Grow in nitrogen-poor soils; obtain N by trapping insects
• Examples: Nepenthes (pitcher plant), Drosera (sundew), Utricularia (bladderwort), Dionaea (Venus flytrap)

Criteria of Essentiality

• An element is considered essential if: the plant cannot complete its life cycle without it, its deficiency is specific and can be reversed only by supplying that element, and it is directly involved in plant metabolism
• Beneficial elements (not essential for all plants but helpful for some): sodium, silicon, cobalt, and selenium

Industrial (Chemical) Nitrogen Fixation

• The Haber-Bosch process combines atmospheric N2 with H2 under high temperature and pressure with a catalyst to industrially produce ammonia (used in chemical fertilisers); contrasts with biological fixation which occurs at normal temperature and pressure using the enzyme nitrogenase
• Nitrogenase is highly sensitive to free oxygen; in root nodules, the protein leghaemoglobin (pink pigment) protects nitrogenase by maintaining a low-oxygen environment

Toxicity of Micronutrients

• Excess of any mineral can be harmful; toxicity symptoms often appear as inhibition of uptake of other minerals (e.g. excess manganese can cause deficiency symptoms of calcium, magnesium, and iron by competing for uptake/translocation)
• The concentration of a mineral above which it becomes toxic varies for each plant and each mineral, so the critical concentration of toxicity is an important consideration in fertiliser application

Mechanism of Mineral Absorption and Translocation

• Minerals are absorbed by roots from the soil solution through two pathways: passive absorption (diffusion along a concentration gradient, no energy used) and active absorption (against the gradient, requires ATP and membrane transport proteins, can be inhibited by metabolic poisons)
• Some ions move into roots passively through diffusion via channels, while others require carrier proteins; uptake is selective, so the ion composition inside root cells differs from that in the surrounding soil solution
• Once inside the xylem, minerals are translocated upward along with the transpiration stream (passive movement in the transpiration pull); however, minerals can also move out of the xylem into living cells along the way
• Unlike water, minerals can be reused: elements like nitrogen, phosphorus, and potassium are mobile and exported from older, senescing leaves to younger growing parts through the phloem, while calcium is largely immobile and stays put

Soil as the Reservoir of Essential Elements

• Soil is the chief reservoir of mineral nutrients; the weathering and breakdown of rocks releases ions, while soil also supplies water, air, humus (organic matter) and a variety of living organisms, all of which influence nutrient availability
• Mineral ions are held on the surface of clay and humus particles and are released into the soil solution, from where the roots absorb them; a deficiency or excess of any element in the soil is soon reflected in the growth of the plant
• Fertilisers are added to replenish the elements (especially N, P and K) removed by repeated cropping, thereby restoring soil fertility for continued cultivation

Role of Sulphur and the Micronutrients

• Sulphur (S): present in the amino acids cysteine and methionine, and in several coenzymes, vitamins (thiamine, biotin) and ferredoxin; its deficiency causes chlorosis of the young leaves
• Manganese (Mn): activates many enzymes of respiration, photosynthesis and nitrogen metabolism, and is essential for the splitting of water that releases O₂ during photosynthesis
• Zinc (Zn): activates several enzymes, chiefly carboxylases, and is needed for the synthesis of the hormone auxin (IAA)
• Copper (Cu) and Molybdenum (Mo): Cu is associated with certain redox (electron-transfer) enzymes; Mo is a component of the enzymes nitrogenase and nitrate reductase, making it vital for nitrogen fixation and assimilation
• Boron (B): involved in the uptake and utilisation of Ca²⁺, membrane functioning, pollen germination, cell elongation and carbohydrate translocation; Chlorine (Cl): with Na and K helps determine the solute concentration and the anion-cation balance in cells, and is essential in the water-splitting reaction of photosynthesis

Root Nodule Formation and nif Genes

• Nodule formation in legumes proceeds through a stepwise interaction between Rhizobium and the roots: the bacteria multiply near and attach to the root hairs, which curl; the bacteria then invade through an infection thread and reach the cortex, where they induce cell divisions that give rise to the nodule
• The mature nodule remains connected to the vascular tissue of the root so that it can exchange the fixed nitrogen for nutrients from the plant; the nodule houses leghaemoglobin (which scavenges oxygen) and the enzyme nitrogenase to carry out fixation
• The genes that govern nitrogen fixation, the nif genes, code for the nitrogenase enzyme complex; nitrogenase reduces atmospheric N₂ to 2NH₃, a highly energy-demanding reaction requiring large amounts of ATP

Fate of Absorbed Ammonia: Amination and Amides

• The ammonia formed by biological fixation or by the reduction of nitrate is toxic if it accumulates, so it is rapidly incorporated into organic compounds by two principal routes
• Reductive amination: ammonia reacts with alpha-ketoglutaric acid to form the amino acid glutamic acid (glutamate)
• Transamination: the amino group is transferred from one amino acid (usually glutamate) to the keto group of a keto acid, generating a new amino acid; the enzyme transaminase catalyses all such reactions
• Amides (asparagine and glutamine) are formed by adding a second amino group to the amino acids aspartic acid and glutamic acid; because amides carry more nitrogen than the parent amino acids, they are the form in which nitrogen is transported through the xylem to the growing parts of the plant

🚀 NEET Advanced Edge

Distinguishing mobile vs immobile element deficiency by leaf position: Deficiency symptoms of MOBILE elements (N, P, K, Mg) appear FIRST in OLDER leaves (the plant recalls the nutrient from old tissue, sacrificing it); deficiency of IMMOBILE elements (Ca, S, Fe) appears FIRST in YOUNGER leaves/growing points (nothing to recall from elsewhere) — a classic NEET diagnostic logic question.

Why excess micronutrients cause symptoms resembling deficiency of OTHER elements: Ions compete for the same membrane transport proteins/binding sites — excess Mn can physically block Fe, Mg, or Ca uptake, so the visible symptom may look like an Fe/Mg/Ca deficiency even though the actual problem is Mn toxicity, not a true shortage of those elements.

Hydroponics as a research tool: Growing plants with one nutrient deliberately withheld from the solution is exactly how the "criteria of essentiality" were experimentally established — if withholding element X causes the plant to fail to complete its life cycle, AND symptoms are reversed only by re-adding X, then X is essential (not just beneficial).