🌱 Biology — Plant Physiology
1. Photosynthesis
DEFINITION
- Process by which plants utilize CO₂ and H₂O to prepare food (carbohydrate) in presence of sunlight and chlorophyll
- Equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
PHOTOSYNTHETIC PIGMENTS
- Chlorophyll a and b: Main photosynthetic pigments
- Accessory pigments: Carotenoids, Xanthophylls
- Found in chloroplasts
PHOTOSYNTHETIC ACTIVE RADIATION (PAR)
- Wavelength range: 400-700 nm
- Maximum photosynthesis in blue and red regions of light
- Minimum in green region (green light is reflected)
2. Respiration in Plants
AEROBIC RESPIRATION
- Occurs in presence of O₂
- Complete breakdown of glucose → CO₂ + H₂O + Energy (ATP)
- Equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
- Occurs in mitochondria
- High energy production
ANAEROBIC RESPIRATION
- Occurs in absence of O₂
- Incomplete breakdown of glucose → C₂H₅OH + H₂O + CO₂ + Energy
- Occurs in yeast cells
- Low energy production
ATP (Adenosine Triphosphate)
- Energy currency of the cell
- Hydrolysis of 1 ATP → ~7.6 kcal or 31.8 kJ energy
3. Transportation in Plants
XYLEM
- Conducts water and minerals from roots to leaves
- Made of: Vessels, Tracheids, Xylem parenchyma, Xylem fibres
- Movement is upward (root → stem → leaves)
PHLOEM
- Conducts food (photosynthetic products) from leaves to all parts
- Made of: Sieve tubes, Companion cells, Phloem parenchyma, Phloem fibres
- Movement is bidirectional (upward and downward)
TRANSPIRATION
- Evaporative loss of water from plants
- Occurs mainly through stomata in leaves
- Xerophytes: Desert plants with adaptations to reduce transpiration (thick cuticle, thorns, no leaves)
ROOT PRESSURE
- Force by which water moves upward into tracheary elements in root region
- Measured by manometer
- Cohesion-Tension theory: Given by Dixon and Jolly
4. Mineral Nutrition
MACRONUTRIENTS
- N, P, K, Ca, Mg, S — required in large quantities
- Nitrogen (N): In proteins, nucleic acids (absorbed as NO₃⁻ or NH₄⁺)
- Phosphorus (P): In ATP, nucleic acids (absorbed as H₂PO₄⁻ or HPO₄²⁻)
- Potassium (K): Enzyme activation, water balance, stomatal opening
- Magnesium (Mg): Present in chlorophyll
- Calcium (Ca): Affects cytoskeleton, membranes, acts as secondary messenger
MICRONUTRIENTS
- Fe, Mn, B, Zn, Cu, Mo, Cl — required in small quantities
- Iron (Fe): Present in active site of redox enzymes; electron carriers in chlorophyll synthesis
- Zinc (Zn): Enzyme activation, auxin synthesis
- Boron (B): Pollen germination, cell elongation, carbohydrate translocation
- Molybdenum (Mo): Nitrate reduction
NITROGEN CYCLE
- Ammonification: Dead organic matter → NH₃ (decomposing microbes)
- Nitrification: NH₄⁺ → NO₂⁻ → NO₃⁻ (Nitrosomonas, Nitrobacter)
- Denitrification: NO₃⁻ → N₂ (Pseudomonas)
- Biological Nitrogen Fixation: By Rhizobium in root nodules of legumes
5. Plant Growth and Development
PLANT HORMONES
| Hormone | Typical Activities | Location |
|---|---|---|
| Auxins | Stem elongation, root initiation, fruit growth; inhibit axillary bud outgrowth | Apical meristems |
| Cytokinins | Inhibit leaf senescence; promote cell division, axillary bud growth | Root apical meristems |
| Gibberellins | Seed germination, stem growth, fruit development; break seed dormancy | Root and shoot tips, seeds |
| Abscisic acid | Maintains seed dormancy; closure of stomata; stress hormone | Leaves, fruits, root caps |
| Ethylene | Promotes fruit ripening, leaf abscission; inhibits stem elongation | Roots, shoot apical meristems |
PHOTOPERIODISM
- Long Day Plants (LDP): Require light > critical duration (e.g., Spinach, Sugarbeet)
- Short Day Plants (SDP): Require light < critical duration (e.g., Xanthium, Dahlia)
- Day Neutral Plants (DNP): Light duration does not affect flowering (e.g., Tomato, Cotton)
VERNALISATION
- Low temperature treatment to induce flowering
- e.g., Chilling treatment to wheat varieties reduces vegetative growth and induces early flowering
Summary: Plant Physiology at a Glance
| Topic | Key Point |
|---|---|
| Photosynthesis | 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O |
| PAR | 400-700 nm; max in blue & red |
| Aerobic Respiration | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy |
| Xylem | Water transport (upward) |
| Phloem | Food transport (bidirectional) |
| N₂ Fixation | Rhizobium in legume root nodules |
| Ethylene | Promotes fruit ripening |
| Abscisic Acid | Stress hormone; stomatal closure |
KEY TAKEAWAYS FOR UPSC
- Photosynthesis: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
- PAR: 400-700 nm; maximum in blue and red light
- Aerobic respiration: In mitochondria; high energy; complete oxidation
- Anaerobic respiration: In yeast; low energy; produces ethanol
- Xylem: Water (upward); Phloem: Food (bidirectional)
- Transpiration: Water loss through stomata; xerophytes have adaptations
- Nitrogen fixation: Rhizobium in legumes; ammonification, nitrification, denitrification
- Plant hormones: Auxin (growth), Ethylene (ripening), ABA (stress)
- Photoperiodism: LDP, SDP, DNP
- Vernalisation: Cold treatment for flowering
High-Yield Interactive UPSC Quiz (Biology — Plant Physiology)
Q1. What is the photosynthetic equation?
Correct Answer: B. The correct photosynthetic equation is 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O.
Q2. What is the wavelength range of PAR?
Correct Answer: B. Photosynthetic Active Radiation (PAR) has wavelength range of 400-700 nm.
Q3. Where does aerobic respiration occur in plants?
Correct Answer: B. Aerobic respiration occurs in the mitochondria.
Q4. Which tissue conducts water in plants?
Correct Answer: B. Xylem conducts water and minerals from roots to leaves.
Q5. Which bacteria is associated with nitrogen fixation in legumes?
Correct Answer: C. Rhizobium bacteria are associated with nitrogen fixation in leguminous plants.
Q6. Which plant hormone promotes fruit ripening?
Correct Answer: C. Ethylene promotes fruit ripening and leaf abscission.
Q7. What is the "stress hormone" in plants?
Correct Answer: C. Abscisic acid (ABA) is the stress hormone; it causes stomatal closure and maintains dormancy.
Q8. Which process converts nitrate to nitrogen gas?
Correct Answer: C. Denitrification converts nitrate (NO₃⁻) to nitrogen gas (N₂) by Pseudomonas bacteria.
Q9. Which element is present in chlorophyll?
Correct Answer: B. Magnesium (Mg²⁺) is present in the chlorophyll molecule.
Q10. Which plants are called Long Day Plants?
Correct Answer: B. Long Day Plants (LDP) require light exposure exceeding a critical duration for flowering (e.g., Spinach, Sugarbeet).