Plant Physiology
"Plants are the planet's FOOD FACTORIES. They convert SUNLIGHT into the ENERGY that sustains nearly ALL life on Earth."
1. Photosynthesis
The Equation
6CO₂ + 12H₂O →(Sunlight, Chlorophyll)→ C₆H₁₂O₆ + 6O₂ + 6H₂O
Where Does It Happen?
CHLOROPLASTS — organelles in the mesophyll cells of leaves. Each chloroplast contains CHLOROPHYLL (green pigment) that traps SUNLIGHT energy. 'A square millimetre of leaf surface contains ~500,000 chloroplasts.'
Raw Materials and Their Sources
- CO₂: From atmosphere. Enters through STOMATA (tiny pores, mainly on leaf underside). Guard cells control opening/closing.
- Water: From soil. Absorbed by ROOT HAIRS. Travels UP through XYLEM vessels.
- Sunlight: Trapped by chlorophyll in thylakoid membranes.
The Two Phases
Light Reaction (thylakoid membranes — REQUIRES light): Water is SPLIT (photolysis) → O₂ released. ATP and NADPH produced (energy carriers). Dark Reaction/Calvin Cycle (stroma — does NOT directly require light): CO₂ is FIXED using ATP and NADPH → Glucose produced.
Factors Affecting Photosynthesis Rate
Light intensity. CO₂ concentration. Temperature. Water availability.
2. Transpiration
Loss of WATER VAPOUR from aerial parts of the plant — mainly through STOMATA. Functions: Creates TRANSPIRATION PULL — draws water UP through xylem from roots (the main driving force for water transport). COOLING effect (evaporative cooling — protects leaves from scorching). Factors increasing transpiration: High temperature. Low humidity. Wind. High light intensity (stomata open).
3. Transport in Plants
| Feature | Xylem | Phloem |
|---|---|---|
| Transports | WATER + DISSOLVED MINERALS from roots | FOOD (sucrose) from leaves to all parts |
| Direction | ONE-WAY (roots → shoots) | TWO-WAY (source → sink) |
| Cells | DEAD at maturity (tracheids, vessels) | LIVING (sieve tubes — no nucleus. Companion cells control them) |
| Driving Force | TRANSPIRATION PULL (passive — no energy needed) | Active transport (needs ATP — energy) |
The Cohesion-Tension Theory
Water evaporates from leaves (transpiration) → creates NEGATIVE PRESSURE (tension) in leaf xylem → this tension is transmitted DOWN through the continuous water column → pulls water UP from roots. 'Cohesion (water molecules stick together) + Adhesion (water sticks to xylem walls) + Transpiration pull = THE mechanism of water transport.'
4. Common Mistakes
- 'Plants only photosynthesise, they don't respire' — Plants PHOTOSYNTHESISE (day) AND RESPIRE (day AND night).
- 'Xylem is living' — Xylem vessels and tracheids are DEAD at maturity. Phloem sieve tubes are LIVING.
- 'Transpiration is wasteful' — It's ESSENTIAL — drives water transport AND cools the plant.
5. AP SSC Exam Focus
| Topic | Marks |
|---|---|
| Photosynthesis — equation, site, raw materials | 4-5 |
| Xylem vs Phloem | 3-4 |
| Transpiration — factors and significance | 3-4 |
Photosynthesis in AP Context
'Andhra Pradesh is the RICE BOWL of India. The Krishna-Godavari delta produces MILLIONS of tonnes of rice annually — all powered by PHOTOSYNTHESIS. AP farmers depend on sunlight, water from the Godavari and Krishna rivers, and CO₂ from the atmosphere. Understanding photosynthesis is not just academic — it is understanding the PROCESS that FEEDS our state.'
Factors Limiting Photosynthesis — Agricultural Applications
- In winter (Rabi) : Lower temperatures can LIMIT photosynthesis rate. Farmers choose wheat over rice (wheat needs less warmth).
- Water stress: In drought-prone Rayalaseema, water is the LIMITING FACTOR. Even with abundant sunlight, photosynthesis STOPS when stomata close to conserve water.
- CO₂ enrichment in greenhouses: Adding CO₂ increases photosynthesis rate — a technique used in commercial horticulture.
Transpiration — A Necessary Evil?
Transpiration is sometimes called a 'NECESSARY EVIL' — it loses precious water, BUT it DRIVES water transport and COOLS the plant. Without transpiration: no water transport, no mineral delivery, no photosynthesis. 'The plant has EVOLVED to balance water loss against the benefits of transpiration. Stomata CLOSE when water is scarce — even though this STOPS photosynthesis. Survival trumps growth.'
Photosynthesis — Deeper Dive into Experiments
Experiment — Testing for Starch (Evidence of Photosynthesis) : Take a variegated leaf (green + white patches). Draw its outline. Boil in water (kill cells). Boil in alcohol (remove chlorophyll — leaf turns white). Wash in water. Add iodine solution. Observation: GREEN parts turn BLUE-BLACK (starch present). WHITE parts remain BROWN (no chlorophyll → no photosynthesis → no starch). 'This experiment proves TWO things: (1) chlorophyll is NECESSARY for photosynthesis, (2) starch is a PRODUCT of photosynthesis.'
Experiment — CO₂ Is Necessary: Take two potted plants. Place one in a bell jar with KOH (absorbs CO₂). Place the other in a bell jar without KOH. Keep both in sunlight. After 24 hours, test leaves for starch. KOH jar leaf: NO starch. Control leaf: starch present. Conclusion: CO₂ is NECESSARY for photosynthesis.
Experiment — O₂ Is Released: Place Hydrilla (aquatic plant) in a beaker of water. Cover with a funnel. Invert a test tube over the funnel. Expose to sunlight. Bubbles rise from the plant and collect in the test tube. Test the gas: glowing splint REKINDLES → gas is OXYGEN.
The Calvin Cycle — Simplified
Melvin Calvin traced the path of carbon in photosynthesis using radioactive C-14 (Nobel Prize, 1961). The cycle: CO₂ + RuBP (5-carbon) → 2 PGA (3-carbon) → (ATP + NADPH from light reaction) → PGAL (3-carbon) → Glucose (6-carbon) + RuBP regenerated. 'The Calvin cycle turns 6 times to produce ONE glucose molecule, consuming 18 ATP and 12 NADPH per glucose.'
C₃ vs C₄ Plants — AP-Relevant Context
| Feature | C₃ Plants | C₄ Plants |
|---|---|---|
| First product | 3-carbon compound (PGA) | 4-carbon compound (oxaloacetate) |
| Photorespiration | Significant | Negligible |
| Water use efficiency | Lower | Higher |
| Examples | Rice, wheat, soybean | Maize, sugarcane, sorghum |
| AP relevance | Rice is C₃ — dominant crop in AP | Sugarcane is C₄ — grown in coastal AP |
Minerals and Plant Nutrition
Macronutrients (needed in larger amounts): Nitrogen (N — proteins, chlorophyll. Deficiency: yellowing of old leaves), Phosphorus (P — ATP, DNA. Deficiency: purple leaves, stunted growth), Potassium (K — stomatal opening, enzyme activation. Deficiency: scorched leaf margins).
Micronutrients (needed in trace amounts): Iron (Fe — chlorophyll synthesis), Zinc (Zn — enzyme cofactor), Boron (B — pollen tube growth), Manganese (Mn — water splitting in photosynthesis).
Transport — Root Pressure and Guttation
Besides transpiration pull, two other forces move water: Root Pressure: Active pumping of ions into xylem → water follows by osmosis → positive pressure pushes water up. Visible as 'guttation' — water droplets on leaf margins in small plants on humid mornings (NOT dew — this is water EXUDED by the plant). Capillarity: Narrow xylem vessels exert capillary force — water rises in thin tubes due to adhesion.
Phloem Transport — The Pressure Flow Hypothesis
At the SOURCE (leaves): Sucrose is actively LOADED into sieve tubes → high solute concentration → water ENTERS from adjacent xylem by osmosis → HIGH PRESSURE. At the SINK (roots, fruits, growing tips): Sucrose is actively UNLOADED → water EXITS → LOW PRESSURE. The pressure difference drives bulk flow from source to sink. 'Phloem transport requires ENERGY (ATP) at BOTH ends — for active loading and unloading. Xylem transport is passive — driven by transpiration.'
Stomatal Mechanism — Potassium Ion Pump
Guard cells OPEN stomata by actively pumping K⁺ ions in → water follows by osmosis → guard cells become TURGID and curve (due to uneven wall thickness) → pore OPENS. Guard cells CLOSE by pumping K⁺ out → water exits → guard cells become FLACCID → pore CLOSES. 'Light triggers the K⁺ pump. In darkness, stomata close. This is why most transpiration occurs during the day.'
Quick Self-Test
- Write the photosynthesis equation. (Answer: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O, requires sunlight and chlorophyll.)
- Why is transpiration called a 'necessary evil'? (Answer: It loses water but drives water transport and cools the plant.)
- Xylem transports ___. Phloem transports ___. (Answer: Water+minerals; food/sucrose.)
- Which plant organelle carries out photosynthesis? (Answer: Chloroplast.)
- What gas is released during photosynthesis? (Answer: Oxygen.)
- Name two factors that increase transpiration rate. (Answer: High temperature, low humidity, wind, high light intensity.)
Comparison — Photosynthesis vs Respiration in Plants
| Aspect | Photosynthesis | Respiration |
|---|---|---|
| When | DAY only (requires light) | DAY AND NIGHT (continuous) |
| Where | Chloroplasts | Mitochondria (and cytoplasm) |
| CO₂ | Consumed (raw material) | Released (product) |
| O₂ | Released (product) | Consumed (raw material) |
| Glucose | Produced | Broken down |
| Energy | STORED (endergonic) | RELEASED (exergonic) |
| 'During the day, plants do BOTH — photosynthesis produces O₂, respiration consumes it. Net O₂ release = photosynthesis O₂ − respiration O₂. At night, only respiration occurs → plants take in O₂ and release CO₂. This is why hospitals remove plants from wards at night.' |
AP Exam Diagram Practice
Draw and label: (a) T.S. of leaf showing chloroplasts in mesophyll cells. (b) Stomata — open and closed states. (c) Xylem and phloem in a stem cross-section. 'Diagrams are a RELIABLE source of marks. Spend 5 minutes, use a sharp pencil, label clearly with horizontal indicator lines.'
