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BC04 — Plant Biology (Botany)

📖 BC04  ·  CDS General Knowledge — Biology ★ High Priority

Plants are the primary producers of all ecosystems — without photosynthesis, there would be no oxygen, no food chain, and no life as we know it. CDS tests the biochemistry of photosynthesis, the role of plant hormones, and the classification of the plant kingdom directly.

📌 CDS Focus: Photosynthesis equation: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂; chlorophyll absorbs red and blue light (reflects green); auxins control phototropism; abscisic acid = stress hormone (causes leaf fall); Bt cotton = GM crop; angiosperms = largest plant group. These facts repeat directly in CDS.

1. Photosynthesis

Photosynthesis is the process by which green plants convert carbon dioxide and water into glucose using light energy — simultaneously producing oxygen as a by-product.

Fig. 1 — Photosynthesis: Light-Dependent and Light-Independent Reactions
PHOTOSYNTHESIS — Two Stages in the Chloroplast OVERALL EQUATION: 6CO₂ + 6H₂O + Light energy → C₆H₁₂O₆ (Glucose) + 6O₂ LIGHT REACTIONS (Thylakoid membranes of chloroplast) Requires: Light, water, chlorophyll Produces: ATP + NADPH + O₂ Water is split (photolysis of water): 2H₂O → 4H⁺ + 4e⁻ + O₂↑ O₂ released here (by-product) Chlorophyll absorbs red + blue light DARK REACTIONS (Calvin Cycle) (Stroma of chloroplast) Requires: CO₂, ATP, NADPH (from light rxn) Produces: Glucose (C₆H₁₂O₆) CO₂ fixed by RuBisCO enzyme. CO₂ + RuBP → 3-PGA → Glucose Does NOT need direct light (uses energy from ATP/NADPH)

2. Plant Hormones

HormoneSite of ProductionKey FunctionExample Effect Auxin (IAA)Shoot apexCell elongation; phototropism; apical dominanceBending towards light; rooting powder uses auxin GibberellinYoung leaves, seedsStem elongation; seed germination; fruit growthDwarf plants treated with gibberellin grow tall CytokininRoot apexCell division; delays senescence (leaf ageing)Keeps leaves green longer; used in tissue culture Abscisic Acid (ABA)Leaves, seedsStomatal closure; seed dormancy; leaf abscissionPlant wilts in drought; ABA closes stomata to save water EthyleneRipening fruitsFruit ripening; leaf and flower dropBananas ripen faster near other ripe fruit (ethylene gas)
💡 ABA = Stress Hormone: Abscisic acid is nicknamed the "stress hormone" or "inhibitory hormone." It closes stomata during drought (conserves water), induces seed dormancy (prevents germination in unfavourable conditions), and triggers leaf fall (abscission). The only hormone that generally inhibits growth. This function is directly tested in CDS.

3. Plant Kingdom Classification

Fig. 2 — Plant Kingdom: Five Groups, Complexity and Key Features
PLANT KINGDOM — From Simple to Complex Group Features Habitat & Examples Seeds? Thallophyta No root/stem/leaf; thallus body No vascular tissue Algae (Spirogyra, Ulva, Chara) No Bryophyta No true roots/vascular tissue Amphibians of plant kingdom Mosses, Liverworts (Funaria) No Pteridophyta First vascular plants; true roots and leaves (no seeds yet) Ferns, Horsetails (Equisetum) No Gymnosperms Naked seeds (not enclosed in fruit); cone-bearing Pine, Cycas, Pinus, Cedrus Naked Angiosperms Seeds enclosed in fruit; flowering plants (LARGEST group) Rice, Wheat, Mango, Rose Enclosed

📝 CDS PYQs — Plant Biology

Q1. The oxygen released during photosynthesis comes from: CDS PYQ
(a) Carbon dioxide(b) Glucose(c) Water(d) Chlorophyll
✔ Answer: (c) Water
During the light reactions, water molecules are split by photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂. The oxygen released into the atmosphere comes entirely from water, NOT from CO₂. This was proven by isotope labelling experiments using ¹⁸O-labelled water. CO₂ provides the carbon atoms that end up in glucose. This exact question appears in multiple CDS papers.
Q2. Which plant hormone causes fruit ripening? CDS PYQ
(a) Auxin(b) Gibberellin(c) Cytokinin(d) Ethylene
✔ Answer: (d) Ethylene
Ethylene (also written ethene, C₂H₄) is the only gaseous plant hormone. It accelerates fruit ripening by increasing respiration rate, breaking down chlorophyll, and softening cell walls. This is why fruits ripen faster when stored together — they produce and share ethylene gas. Commercially, ethylene or calcium carbide (which releases ethylene) is used to ripen bananas and tomatoes for transport.
Q3. Which group of plants is called "amphibians of the plant kingdom"? CDS PYQ
(a) Algae(b) Ferns(c) Bryophytes(d) Gymnosperms
✔ Answer: (c) Bryophytes
Bryophytes (mosses, liverworts) are called the "amphibians of the plant kingdom" because they live in moist terrestrial habitats but require water for sexual reproduction (sperm swim to the egg through water — just like amphibians need water to breed). They lack true vascular tissue (no xylem/phloem), so they cannot grow tall. This analogy is a repeated CDS direct question.

🧠 Quick Memory Chart — BC04

🌿 Photosynthesis
  • 6CO₂+6H₂O+light→C₆H₁₂O₆+6O₂
  • O₂ comes from water (not CO₂)
  • Chlorophyll: absorbs red+blue light
  • Light rxn: thylakoid (ATP+NADPH+O₂)
  • Dark rxn: stroma (CO₂ fixed→glucose)
🌻 Plant Hormones
  • Auxin: bending toward light (phototropism)
  • Gibberellin: stem elongation, germination
  • Cytokinin: cell division, anti-ageing
  • ABA: stomata closure (stress hormone)
  • Ethylene: fruit ripening (only gas)
🍁 Plant Groups
  • Thallophyta: algae (no roots/stem)
  • Bryophyta: amphibians of plant kingdom
  • Pteridophyta: first vascular; no seeds
  • Gymnosperms: naked seeds (conifers)
  • Angiosperms: seeds in fruit; largest group

📝 Practice Exercise

E1. Transpiration occurs mainly through:
(a) Roots(b) Stomata on leaves(c) Bark(d) Flowers
E2. Xylem is responsible for:
(a) Transport of food (organic solutes)(b) Transport of water and minerals from root to leaf(c) Gas exchange(d) Photosynthesis
E3. Which plant group was the first to develop vascular tissue (xylem and phloem)?
(a) Bryophyta(b) Gymnosperms(c) Thallophyta(d) Pteridophyta
Answers:
E1 → (b) Stomata [~90% of water loss; stomata on leaf surface regulated by guard cells]  |  E2 → (b) Water and minerals [Xylem = water + minerals upward; Phloem = food (sugars) in all directions]  |  E3 → (d) Pteridophyta [first true vascular plants; ferns have xylem and phloem; bryophytes do not]
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