Aspirant Academy

Science and Technology

Plant Biology

Biology: Cell, Plant Parts, Nutrition/Reproduction, Human Physiology, Food/Nutrition, Immunity/Diseases, Microbes, Fermentation, Biotech/Genetic Engineering, GMO Ethics, Vaccines/CRISPR/mRNA, Artificial Organs

Paper II · Unit 2 Section 4 of 13 0 PYQs 31 min
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Plant Biology

3.1 Plant Nutrition

Autotrophs synthesise their own food. Heterotrophs depend on other organisms.

Photosynthesis equation: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

Two Stages of Photosynthesis

  1. Light-dependent reactions (Light stage): In thylakoid membranes. Chlorophyll absorbs light → water splits (photolysis) → O₂ released → ATP and NADPH produced.
  2. Calvin cycle (Dark stage): In stroma. CO₂ fixed using ATP and NADPH → glucose synthesised.

Plant Nutrients

Macronutrients (needed in large amounts): N, P, K, Ca, Mg, S

  • Nitrogen for proteins and chlorophyll
  • Phosphorus for DNA and ATP
  • Potassium for stomatal regulation

Micronutrients (needed in trace amounts): Fe, Mn, Zn, Cu, Mo, B, Cl

  • Iron for chlorophyll synthesis
  • Manganese for photosynthesis

Nitrogen Fixation

Nitrogen fixation converts atmospheric N₂ to NH₃ (ammonia) that plants can use.

  • Rhizobium — symbiotic with legumes (soybean, pea, clover) in root nodules
  • Azotobacter — free-living in soil
  • Cyanobacteria (blue-green algae) — nitrogen fixation in paddy fields (e.g., Anabaena)

3.2 Plant Reproduction

Type Mode Examples
Vegetative reproduction Asexual — through modified stems, roots, leaves Potato (tuber), ginger (rhizome), Bryophyllum (leaves), Dahlia (tuberous roots)
Spore formation Asexual Mosses, ferns, fungi
Sexual (Flower → Fruit → Seed) Pollination + fertilisation All flowering plants (angiosperms)

Double fertilisation is unique to angiosperms. One sperm fertilises the egg (→ embryo); another sperm fuses with polar nuclei (→ endosperm). Discovered by Nawaschin (1898); extensively studied by Panchanan Maheshwari (Delhi University — PYQ 2024).

3.3 Plant Hormones (Phytohormones)

Hormone Function Application
Auxin (IAA) Cell elongation, apical dominance, rooting Rooting powder for cuttings
Gibberellin Stem elongation, seed germination, fruit development Increasing fruit size (seedless grapes)
Cytokinin Cell division, delay of senescence Tissue culture
Ethylene Fruit ripening, abscission (leaf fall) Ripening chambers for bananas
Abscisic Acid (ABA) Stress response, stomatal closure, dormancy Drought tolerance