Photosynthesis in Higher Plants
'Photosynthesis is the only significant solar energy storage process on Earth.' — Plant Physiology
1. Chapter Overview
PHOTOSYNTHESIS is the MOST IMPORTANT biochemical process on Earth — it PRODUCES food and OXYGEN, and DRIVES the global carbon cycle. This chapter covers the PIGMENTS involved (chlorophylls, carotenoids), the LIGHT REACTIONS (photosystems, electron transport, photophosphorylation), the CALVIN CYCLE (C3 pathway), the C4 PATHWAY (Kranz anatomy), PHOTORESPIRATION, and the FACTORS affecting photosynthesis.
2. Early Experiments
| Scientist | Experiment | Discovery |
|---|---|---|
| Van Helmont (1648) | Willow tree experiment (soil weight unchanged after 5 years) | Plant mass comes from WATER, not soil |
| Joseph Priestley (1770) | Candle + mint plant experiment | Plants RELEASE O₂ |
| Jan Ingenhousz | Priestley's experiment WITH light | Light is ESSENTIAL for O₂ release |
| Julius von Sachs | Starch test with iodine | Chloroplasts PRODUCE starch |
| T.W. Engelmann | Bacteria + algae + prism | RED and BLUE light drive MOST photosynthesis |
Overall Equation
6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
Where Does Photosynthesis Happen?
- Mesophyll cells of leaves (chloroplasts)
- Chloroplast structure: Thylakoids (grana) → Light reactions; Stroma → Dark reactions (Calvin cycle)
3. Photosynthetic Pigments
| Pigment | Colour | Function |
|---|---|---|
| Chlorophyll a | Blue-green | PRIMARY pigment (in reaction centres) |
| Chlorophyll b | Yellow-green | ACCESSORY pigment (transfers energy to Chl a) |
| Carotenoids | Yellow/orange | ACCESSORY + PROTECTION (prevent photooxidation) |
| Xanthophylls | Yellow | Accessory + photoprotection |
Absorption Spectrum vs Action Spectrum
- Absorption spectrum: Wavelengths ABSORBED by a pigment (peaks in blue and red for chlorophyll)
- Action spectrum: Rate of photosynthesis at DIFFERENT wavelengths (Engelmann's experiment)
- Both show MAXIMUM photosynthesis in RED and BLUE wavelengths
4. Light Reactions (Photochemical Phase)
Photosystems
| Photosystem | Location | Reaction Centre | Key Function |
|---|---|---|---|
| PS II | Thylakoid (grana) | P680 | SPLITS H₂O → O₂; passes e⁻ to PS I |
| PS I | Thylakoid (stroma lamellae) | P700 | NADP⁺ → NADPH |
Electron Transport (Z-Scheme)
- Light strikes PS II → P680 excited → e⁻ ejected
- e⁻ passes through electron transport chain (plastoquinone, cytochrome b6f, plastocyanin)
- Energy released in chain → ATP SYNTHESIS (photophosphorylation)
- e⁻ reaches PS I → P700 excited → e⁻ ejected
- Ferredoxin → NADP⁺ → NADPH
Photolysis of Water
- 2H₂O → 4H⁺ + 4e⁻ + O₂ (requires PS II, Mn²⁺, Cl⁻)
- OXYGEN released comes from WATER, not CO₂ (proved by Ruben and Kamen using ¹⁸O)
Photophosphorylation
| Type | Mechanism | Products |
|---|---|---|
| Non-cyclic | Linear e⁻ flow through PS II → PS I | ATP + NADPH + O₂ |
| Cyclic | e⁻ from PS I cycles BACK | ONLY ATP (no NADPH, no O₂) |
5. Calvin Cycle (C3 Pathway / Dark Reactions)
Location
- STROMA of chloroplast
- SPLITS into 3 phases: CARBOXYLATION, REDUCTION, REGENERATION
Steps
- Carboxylation: CO₂ + RuBP (5C) → 2 molecules of 3-PGA (3C)
- Enzyme: RUBISCO (RuBP carboxylase) — MOST abundant enzyme on Earth
- Reduction: 3-PGA → G3P (3-phosphoglyceraldehyde)
- Uses ATP and NADPH (from light reactions)
- Regeneration: G3P → RuBP (using ATP)
Yield
- 6 turns of Calvin cycle → 1 molecule of GLUCOSE (C₆H₁₂O₆)
- Requires: 18 ATP + 12 NADPH
6. C4 Pathway (Hatch-Slack Pathway)
Why C4?
- In hot, dry conditions: RUBISCO does BOTH carboxylation (CO₂) and oxygenation (O₂)
- Oxygenation leads to PHOTORESPIRATION (wasteful)
- C4 plants OVERCOME photorespiration by CONCENTRATING CO₂ at the RUBISCO site
Kranz Anatomy (C4 plants)
- Bundle sheath cells + Mesophyll cells — ARRANGED in a WREATH pattern
- Mesophyll: Initial CO₂ fixation (PEP carboxylase) → OAA (4C, oxaloacetate)
- OAA → Malate/Aspartate → transported to BUNDLE SHEATH cells
- Bundle sheath: CO₂ RELEASED → Calvin cycle (RUBISCO) → Sugar
C3 vs C4 Plants
| Feature | C3 Plants | C4 Plants |
|---|---|---|
| First product | 3-PGA (3C) | OAA (4C) |
| CO₂ fixation enzyme | RUBISCO | PEP carboxylase (first), RUBISCO (second) |
| Leaf anatomy | Normal | KRANZ anatomy |
| Photorespiration | HIGH | VERY LOW |
| Optimal temperature | 15-25°C | 30-45°C |
| Examples | Wheat, Rice, Soybean | Maize, Sugarcane, Sorghum |
7. Photorespiration
- Definition: Uptake of O₂ and release of CO₂ in LIGHT (wasteful process)
- Mechanism: RUBISCO oxygenase activity → 2-PG (2C) → peroxisome/mitochondria → CO₂ released
- NO ATP produced
- More SEVERE in hot, dry conditions (stomata close → CO₂ low, O₂ high)
- C4 plants MINIMISE photorespiration by concentrating CO₂ in bundle sheath
8. Factors Affecting Photosynthesis
| Factor | Effect |
|---|---|
| Light intensity | Increases up to LIGHT SATURATION point; then plateaus |
| CO₂ concentration | Increases up to CO₂ SATURATION; CO₂ compensation point varies (C3 > C4) |
| Temperature | Optimum 25-30°C (C3), 30-45°C (C4) |
| Water | Stomatal CLOSURE under water stress → reduces CO₂ uptake |
| Oxygen | HIGH O₂ → PHOTORESPIRATION (Warburg effect — ALMOST exclusive to C3) |
Blackman's Law of Limiting Factors
- When MULTIPLE factors affect a process, the rate is LIMITED by the FACTOR CLOSEST to its MINIMUM
9. Common Mistakes
- Oxygen released comes from WATER, NOT CO₂: Confirmed by isotope labelling (¹⁸O)
- RUBISCO does BOTH reactions: Carboxylase (good — makes 3-PGA) and Oxygenase (bad — photorespiration)
- Calvin cycle is NOT completely 'dark': It can operate in light; some enzymes are LIGHT-ACTIVATED
- C4 plants fix CO₂ TWICE: First by PEP carboxylase in mesophyll, then by RUBISCO in bundle sheath
- Light reaction happens in THYLAKOID, dark in STROMA: These are SPATIALLY separated within chloroplast
10. CBSE Exam Focus
- Light reaction — photosystems, Z-scheme (5-mark)
- Calvin cycle — 3 phases, yield (5-mark)
- C3 vs C4 plants — differences (5-mark)
- Photorespiration — causes and consequences (3-mark)
- Factors affecting photosynthesis — experiments (3/5-mark)
- Absorption vs action spectrum (3-mark)
11. Self-Test (5+ Q&A)
Q1: What is the Z-scheme of electron transport in photosynthesis? A: Linear electron flow from P680 (PS II) → plastoquinone → cytochrome b6f → plastocyanin → P700 (PS I) → ferredoxin → NADP⁺. Forms ATP (photophosphorylation) and NADPH.
Q2: Differentiate between C3 and C4 plants. A: C3: First product 3-PGA (3C); RUBISCO in mesophyll; HIGH photorespiration. C4: First product OAA (4C); Kranz anatomy; PEP carboxylase in mesophyll; LOW photorespiration.
Q3: Why is photorespiration considered wasteful? A: It uses ATP and NADPH, releases FIXED CO₂ (no net gain), and produces NO useful energy or carbon compounds.
Q4: Name the enzyme that fixes CO₂ in the Calvin cycle. What is special about it? A: RUBISCO (RuBP carboxylase-oxygenase). It is the MOST ABUNDANT enzyme on Earth and can fix BOTH CO₂ (carboxylation) and O₂ (oxygenation → photorespiration).
Q5: What is the significance of Kranz anatomy? A: The WREATH-like arrangement of bundle sheath and mesophyll cells in C4 plants ALLOWS spatial SEPARATION of initial CO₂ fixation (mesophyll) and Calvin cycle (bundle sheath), CONCENTRATING CO₂ for RUBISCO.
12. Conclusion
Photosynthesis is the FUNDAMENTAL process that SUPPORTS nearly all life on Earth. Light reactions CONVERT solar energy to CHEMICAL energy (ATP + NADPH). The Calvin cycle USES this energy to FIX CO₂ into organic compounds. C4 plants are EVOLUTIONARY adaptations to HOT, DRY environments — they minimise photorespiration by USING a CO₂-concentrating mechanism. Understanding photosynthesis is CRITICAL for addressing GLOBAL challenges — food security (improving crop yield) and climate change (carbon sequestration).
