NCERT Solutions for Class 11 Biology Chapter 11 Photosynthesis in Higher Plants
NCERT Solutions for Class 11 Biology Chapter 11 Photosynthesis in Higher Plants provides structured revision notes and comprehensive textbook solutions covering early historical experiments, chloroplast pigments, photochemical light reactions, chemiosmotic ATP synthesis, C3 and C4 biosynthetic pathways, photorespiration, and factors governing photosynthetic rate.
Overview of Topics & Subtopics (Click to Jump)
1. Early Landmark Experiments & Site of Photosynthesis
Short Note:
• Photosynthesis: Physico-chemical process by which green plants use light energy to synthesize organic compounds from CO2 and H2O, releasing O2 as a byproduct.
• Key Historical Contributions:
- Joseph Priestley (1770): Bell jar experiment showing plants restore air consumed by breathing mice or burning candles; discovered Oxygen in 1774.
- Jan Ingenhousz (1779): Showed that sunlight is essential and green aquatic plants release oxygen bubbles in light.
- Julius von Sachs (1854): Proved green parts produce glucose, stored as starch granules.
- T.W. Engelmann (1843–1909): Placed Cladophora in a prism light spectrum with aerobic bacteria; plotted the first action spectrum matching absorption of Chlorophyll a and b (blue & red regions).
- Cornelis van Niel (1897–1985): Demonstrated O2 evolves from water (H2O), not carbon dioxide (CO2), using purple/green sulfur bacteria.
• Chloroplast Structure: Consists of a membrane system (grana, stroma lamellae) for light reactions and fluid stroma for dark reactions.
2. Photosynthetic Pigments & Light Reaction
Short Note:
• Photosynthetic Pigments (Separated by Paper Chromatography):
- Chlorophyll a: Bright or blue-green (Chief photosynthetic reaction center pigment).
- Chlorophyll b: Yellow-green (Accessory pigment).
- Xanthophylls: Yellow pigments.
- Carotenoids: Yellow to yellow-orange (Protect chlorophyll a from photo-oxidation).
• Light Harvesting Complexes (LHC): Antenna molecules absorb photons and transfer energy to a reaction center.
- Photosystem I (PS I): Reaction center P700 (absorbs light at 700 nm).
- Photosystem II (PS II): Reaction center P680 (absorbs light at 680 nm).
• Splitting of Water (Photolysis): Associated with PS II on the inner thylakoid membrane: 2H2O → 4H+ + O2 + 4e-.
3. Chemiosmotic Hypothesis & Photophosphorylation
Short Note:
• Non-Cyclic Photophosphorylation (Z-Scheme): Involves both PS II and PS I. Electrons flow from H2O → PS II → Electron Transport Chain → PS I → NADP+, yielding both ATP and NADPH + H+.
• Cyclic Photophosphorylation: Involves PS I only when wavelength > 680 nm or NADPH accumulates. Electrons circulate within PS I, producing only ATP (no NADPH or O2 evolved).
• Chemiosmotic Hypothesis (Peter Mitchell): ATP synthesis requires a proton gradient across the thylakoid membrane:
1. H+ ions accumulate inside the thylakoid lumen due to photolysis and stroma H+ pumping.
2. Breakdown of proton gradient through CF0-CF1 ATP Synthase channels activates the catalytic headpiece (CF1) to synthesize ATP from ADP and inorganic phosphate (Pi).
4. Biosynthetic Phase: C3, C4 Pathways & Photorespiration
Short Note:
• Calvin Cycle (C3 Pathway): Occurs in stroma of all photosynthetic plants. Primary CO2 acceptor is 5-carbon RuBP (Ribulose-1,5-bisphosphate).
- 3 Main Steps: Carboxylation (catalyzed by RuBisCO to form 3-PGA) → Reduction (uses 2 ATP and 2 NADPH per CO2) → Regeneration (uses 1 ATP per CO2).
• C4 Pathway (Hatch & Slack Pathway): Adapted to dry tropical climates (e.g., Maize, Sugarcane, Sorghum).
- Characterized by Kranz Anatomy (bundle sheath cells around vascular bundles with large chloroplasts lacking grana).
- Primary CO2 acceptor in mesophyll cells is 3-carbon PEP (Phosphoenolpyruvate), catalyzed by PEPcase (no RuBisCO in mesophyll).
• Photorespiration (C2 Cycle): Wasteful process in C3 plants when RuBisCO binds O2 instead of CO2 at high O2/low CO2 ratios and high temperatures. Consumes ATP and releases CO2 without synthesizing sugars or ATP. Absent in C4 plants.
Intext & Conceptual Questions
RuBisCO (Ribulose-1,5-bisphosphate carboxylase-oxygenase) possesses active sites that can bind to both carbon dioxide (CO2) and oxygen (O2). When CO2 concentration is high relative to O2, it acts as a carboxylase to initiate the Calvin cycle. However, when O2 concentration is high or temperatures rise, it acts as an oxygenase, combining RuBP with O2 to form phosphoglycolate and initiating wasteful photorespiration.
• Anatomical Feature: Kranz Anatomy, characterized by large bundle sheath cells arranged in wreath-like layers around vascular bundles with thick walls impervious to gas exchange and abundant chloroplasts.
• Advantage: It concentrates CO2 at the RuBisCO site in bundle sheath cells, preventing oxygenation of RuBP and completely eliminating photorespiration, thereby improving photosynthetic yield at high temperatures.
Textbook Chapter End Exercise Questions
| Feature | C3 Pathway | C4 Pathway |
|---|---|---|
| Primary CO2 Acceptor | RuBP (5-Carbon sugar) | PEP (3-Carbon compound) |
| First Stable Product | 3-PGA (3-Carbon acid) | OAA (Oxaloacetic acid - 4-Carbon) |
| Kranz Anatomy | Absent | Present in bundle sheath cells |
| Photorespiration Loss | High at high temp and light | Negligible / Completely absent |
| Optimum Temperature | 20°C to 25°C | 30°C to 45°C |
| Feature | Cyclic Photophosphorylation | Non-Cyclic Photophosphorylation |
|---|---|---|
| Photosystems Involved | Only PS I (P700) | Both PS I (P700) and PS II (P680) |
| Electron Pathway | Cyclic (returns to PS I) | Non-cyclic (linear Z-scheme) |
| Photolysis of Water | Does not occur | Occurs associated with PS II |
| End Products | Only ATP synthesized | ATP, NADPH, and O2 evolved |
Read More & Chapter Resources
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