☘ Botany · Physiology & Reproduction 🔥 Day 1

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Physiology & Reproduction — NEET Botany Notes

📗 Full Biology chapter: Plant Physiology 📗 Full Biology chapter: Reproduction

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Photosynthesis in Higher Plants

1. Introduction · 2. Why Learn This? · 3. Big Picture · 4. Story Intro

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1. Introduction & Objectives

Learn the molecular mechanism of solar energy harvesting in plants. By the end of this module, you will understand:

Photolysis of water, cyclic, and non-cyclic photophosphorylation.
Calvin cycle (\(C_3\)) and Hatch-Slack pathway (\(C_4\)) carbon fixation.
The ruinous photorespiration process (\(C_2\) cycle) and limiting factors of photosynthesis.

Estimated Reading Time: 30 minutes | Difficulty Level: Hard | Prerequisites: Cell organelles.

2. Why Should You Learn This?

Understanding photosynthesis helps agricultural scientists engineer high-yield \(C_4\) features into \(C_3\) crops (like rice) to boost food production under global warming.

3. Big Picture

Biosphere energy harvesting: \[\text{Light energy} \xrightarrow{\text{PS II \& PS I}} \text{Chemical Energy (ATP, NADPH)} \xrightarrow{\text{Calvin Cycle}} \text{Glucose (Carbohydrate)}\]

4. Story-Based Introduction

Imagine a leaf as a solar panel factory.

The **Chloroplast** is the assembly warehouse.
The **Thylakoids** are solar cells catching light rays to split water and charge batteries (ATP and NADPH).
The **Stroma** is the manufacturing unit where charged batteries are consumed to turn carbon dioxide into sugar.

5. Detailed Explanation & 10. Scientific Terminology

Photosynthesis is a physico-chemical process.

Light Reactions: Occur in thylakoid membranes. Photolysis of water occurs at PS II complex: \(2H_2O \rightarrow 4H^+ + O_2 + 4e^-\).
Dark Reactions (Light-independent): Occur in stroma. Carbon dioxide is fixed by RuBisCO. In \(C_3\) plants, the first stable product is 3-PGA (3 carbon). In \(C_4\) plants, it is OAA (4 carbon) in mesophyll cells, before moving to bundle sheath cells.

Light Reactions — Cyclic vs Non-Cyclic Photophosphorylation

Feature	Non-cyclic	Cyclic
Photosystems	PS II (P680) + PS I (P700)	PS I (P700) only
Location	Grana thylakoids	Stroma lamellae (no PS II, no NADP reductase)
Water photolysis / O₂	Yes (O₂ evolved)	No
Products	ATP + NADPH + O₂	ATP only

By the chemiosmotic hypothesis, a proton gradient builds up in the thylakoid lumen (from photolysis + plastoquinone pumping); protons flow back through the CF₀–CF₁ ATP synthase, driving ATP formation.

The Calvin Cycle (C3) — Three Stages

Carboxylation: RuBisCO adds CO₂ to the 5-C acceptor RuBP, forming two 3-C molecules of 3-PGA.
Reduction: 3-PGA → triose phosphate (G3P) using 2 ATP + 2 NADPH per CO₂.
Regeneration: RuBP is regenerated using 1 ATP.

Fixing 6 CO₂ (one glucose) needs 6 turns → 18 ATP + 12 NADPH in C3 plants (30 ATP + 12 NADPH in C4, due to the extra PEP-regeneration cost).

Cellular Respiration (Plants)

Respiration releases the energy stored by photosynthesis:

Glycolysis (cytoplasm): glucose → 2 pyruvate + 2 ATP + 2 NADH (common to aerobic and anaerobic paths).
Krebs cycle / TCA (mitochondrial matrix): pyruvate is oxidised, yielding NADH, FADH₂ and CO₂.
Electron transport & oxidative phosphorylation (inner membrane): NADH/FADH₂ drive ATP synthesis with O₂ as the final electron acceptor.

Anaerobic respiration/fermentation gives only 2 ATP (ethanol or lactate). The Respiratory Quotient (RQ) = CO₂ released / O₂ consumed: 1 for carbohydrates, <1 for fats, >1 for organic acids.

13. Important Tables & 14. Formula Box

C3 vs C4 Plants Comparison

Feature	C3 Plants	C4 Plants
Primary CO2 Acceptor	RuBP (5C)	PEP (3C)
Primary Carboxylation Enzyme	RuBisCO	PEP carboxylase (PEPcase)
Kranz Anatomy	Absent	Present
Photorespiration	High (wastes energy)	Absent

Calvin Cycle Chemical Equation: For synthesis of 1 molecule of Glucose: \[6CO_2 + 18ATP + 12NADPH \rightarrow C_6H_{12}O_6 + 18ADP + 12NADP^+ + 18P_i\] In \(C_4\) plants, due to transport steps, energy requirement increases to **30 ATP** per glucose.

11. Misconceptions, 12. Memory Tricks & 20. Doubt Section

11. Common Misconceptions

❌ Misconception: The "Dark Reaction" only happens at night.
✔ Correction: It is light-independent but relies on the short-lived products of the light reaction (ATP, NADPH), so it occurs simultaneously with light reactions during daytime.

20. Doubt Section

Doubt: Why does RuBisCO perform photorespiration?
Answer: RuBisCO has affinity for both \(CO_2\) and \(O_2\). When \(CO_2 : O_2\) ratio falls (high heat, closed stomata), RuBisCO binds oxygen, breaking down RuBP to phosphoglycolate in a wasteful process yielding no ATP.

6. Visual Learning (Z-Scheme)

Figure 1.2: Z-scheme flow showing light reactions redox potential levels

18. PYQs & 19. Practice Questions

1. (NEET PYQ) In \(C_4\) plants, the primary \(CO_2\) acceptor is located in which cells?

Correct Answer: C. Mesophyll cells

Explanation: In \(C_4\) plants, phosphoenolpyruvate (PEP), the primary \(CO_2\) acceptor, is located in the mesophyll cells. The carboxylation is catalyzed by PEPcase.

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Plant Growth Regulators

1. Introduction · 2. Why Learn This? · 3. Big Picture

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1. Introduction, 2. Why & 3. Big Picture

Auxin, Gibberellin, Cytokinin, Ethylene, and Abscisic Acid coordinate plant responses. They determine shoot direction, seed germination, cell division, and drought survival.

5. Detailed Explanation & 10. Terminology

Auxin: Apical dominance, rooting in stem cuttings, prevents abscission.
Gibberellins: Bolting (internode elongation) in rosette plants, breaks dormancy.
Cytokinins: Cell division, delays senescence, overcomes apical dominance.
Abscisic Acid (ABA): Stress hormone. Closes stomata during water stress, inhibits germination.

Growth, Development & Photoperiodism

Plant growth has three phases — meristematic, elongation and maturation. Differentiation, dedifferentiation and redifferentiation let cells take on and change roles. Flowering can depend on daylength (photoperiodism — short-day, long-day and day-neutral plants, sensed by phytochrome) and on cold exposure (vernalisation). Seed germination and dormancy are set by the balance of gibberellins (promote) and ABA (inhibit).

13. PGR Classification Table

Hormone	Primary Role	NEET High-Yield Bioassay
Auxin	Apical dominance, cell elongation	Avena curvature test
Gibberellin	Internode elongation, malting	Dwarf maize / Barley endosperm test
Cytokinin	Cell division, shoot initiation	Tobacco pith culture
Ethylene	Fruit ripening, senescence	Triple response test

11. Misconceptions & 12. Memory Tricks

❌ Misconception: Abscisic acid promotes fruit ripening.
✔ Correction: Ethylene is the primary ripening gas; ABA is primarily a growth inhibitor and stress coordinator.

18. PYQs

1. (NEET PYQ) Which phytohormone is known as the stress hormone due to its role in stomatal closure during water deficit?

Correct Answer: D. Abscisic Acid

Explanation: Abscisic acid (ABA) acts as a stress hormone by inducing stomatal closure in response to water stress to prevent transpiration loss.

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Sexual Reproduction in Flowering Plants

Microsporogenesis, Megasporogenesis and Embryo Sac structures

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5. Detailed Explanation & 10. Terminology

Pollen Structure: Exine made of **Sporopollenin** (most resistant organic material known; lacks degrading enzymes).
Embryo Sac: 7-celled, 8-nucleate structure at maturity. Contains egg apparatus, polar nuclei, and antipodals.
Double Fertilization: Syngamy (egg + sperm \(\rightarrow\) zygote) and Triple Fusion (polar nuclei + sperm \(\rightarrow\) PEN). Exclusive to angiosperms.

Pollination & Post-Fertilisation Events

Pollination: transfer of pollen to the stigma — autogamy (same flower), geitonogamy (another flower, same plant), or xenogamy (different plant). Agents include wind (anemophily), water (hydrophily) and animals/insects (zoophily).
Outbreeding devices (to prevent self-pollination): dichogamy, self-incompatibility, unisexual flowers.
After fertilisation: the ovule → seed, the ovary → fruit; the zygote develops into the embryo and the PEN into nutritive endosperm.
Special modes: apomixis (seed without fertilisation), parthenocarpy (fruit without fertilisation), polyembryony (more than one embryo per seed).

14. Ploidy Levels

- Pollen grain / Male gamete = \(n\) - Antipodals / Synergids = \(n\) - Zygote / Embryo = \(2n\) - Primary Endosperm Nucleus (PEN) = \(3n\) (Triple fusion product)

18. PYQs

1. (NEET PYQ) The pollen exine layer is composed of which highly resistant organic substance?

Correct Answer: C. Sporopollenin

Explanation: Sporopollenin is highly resistant to heat, strong acids, and bases. No enzyme that degrades sporopollenin is currently known.