BA03 — Botany — Plant Biology

📖 BA03 · AFCAT General Awareness — Biology ★ Moderate Yield

Plants sustain every ecosystem. AFCAT tests the photosynthesis equation, which gas comes from which source, plant hormones, and the xylem-phloem distinction. The key trap: oxygen comes from water, not CO₂.

✈ AFCAT Focus: 6CO₂+6H₂O+light→C₆H₁₂O₆+6O₂; O₂ released comes from WATER; chlorophyll absorbs red+blue light; ethylene = fruit ripening; ABA = stress/drought; auxin = phototropism; xylem = water upward (dead cells); phloem = food bidirectional (living cells).

1. Photosynthesis

Overall Equation: 6CO₂ + 6H₂O + Light energy → C₆H₁₂O₆ + 6O₂

Light Reactions (thylakoid): Water + Light + Chlorophyll → ATP + NADPH + O₂
Water is split (photolysis): 2H₂O → 4H⁺ + 4e⁻ + O₂ — O₂ comes from WATER not CO₂
Dark Reactions / Calvin Cycle (stroma): CO₂ + ATP + NADPH → Glucose

💡 AFCAT Trap: "The oxygen released in photosynthesis comes from ___?" → Always Water. CO₂ provides only carbon atoms. Appears in multiple AFCAT papers.

2. Transport in Plants

Feature	XYLEM	PHLOEM
What it transports	Water + dissolved minerals from roots upward	Food (sugars/sucrose) made in leaves
Direction	Unidirectional (upward only)	Bidirectional (up and down)
Cell type	Dead cells; thick lignified walls	Living cells; sieve tubes + companion cells
Driving force	Transpiration pull (suction from leaf water loss)	Pressure flow (source to sink)
Process name	Ascent of sap	Translocation

3. Plant Hormones

Hormone	Produced in	Primary Function	AFCAT Fact
Auxin (IAA)	Shoot apex	Cell elongation; phototropism (bending toward light)	Plant bends toward light
Gibberellin	Young leaves, seeds	Stem elongation; seed germination; fruit growth	Dwarf plants grow tall
Cytokinin	Root apex	Cell division; delays leaf senescence (ageing)	Keeps leaves green longer
ABA	Leaves, seeds	Stomatal closure (water stress); seed dormancy	Stress hormone; only inhibitor
Ethylene	Ripening fruits	Fruit ripening; leaf and flower drop	Only gaseous hormone

📝 AFCAT PYQs — Botany

Q1. Oxygen released during photosynthesis comes from: AFCAT PYQ

(a) CO₂(b) Glucose(c) Water(d) Chlorophyll

✔ (c) Water

Water is split by photolysis: 2H₂O → O₂ + 4H⁺ + 4e⁻. CO₂ provides only carbon atoms for glucose. Proven by ¹⁸O isotope experiments.

Q2. Which plant hormone causes bending toward light? AFCAT PYQ

(a) Gibberellin(b) Cytokinin(c) Auxin(d) ABA

✔ (c) Auxin

Auxin migrates to the shaded side, causing more elongation there → shoot bends toward light (phototropism). ABA is the inhibitory stress hormone.

🧠 Quick Memory Chart — BA03

🌿 Photosynthesis

6CO₂+6H₂O+light → glucose+6O₂
O₂ from WATER (photolysis)
Chlorophyll: absorbs red + blue
Light rxn: thylakoid → ATP+NADPH+O₂
Dark rxn: stroma → glucose

☃ Plant Hormones

Auxin: phototropism (bends to light)
Gibberellin: stem elongation
Cytokinin: cell division, anti-ageing
ABA: stomata close; stress; inhibitor
Ethylene: fruit ripening (only gas)

💦 Transport

Xylem: water+minerals upward (dead)
Phloem: food bidirectional (living)
Transpiration: water loss via stomata
Transpiration pull drives xylem flow

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