Chapter Introduction
Photosynthesis in Higher Plants is Chapter 13 of the NCERT Class 11 Biology textbook and belongs to the Plant Physiology unit. This chapter covers the most fundamental biochemical process on Earth — the conversion of light energy into chemical energy. It spans early experiments, the structure and function of chloroplasts, light reactions, the Calvin cycle, C4 and CAM pathways, and photorespiration.
This is one of the highest-weightage chapters in CBSE and NEET Biology. Questions on the Z-scheme, Calvin cycle, C3 vs C4 comparison, and photorespiration appear every year.
Key Definitions
| Term | Definition |
|---|---|
| Photosynthesis | The process by which green plants synthesise organic compounds from CO₂ and water using sunlight, releasing O₂ |
| Chlorophyll | The green photosynthetic pigment that absorbs light, mainly in the red and blue regions |
| Photosystem | A light-harvesting complex of pigments and proteins; PS I (P700) and PS II (P680) |
| Photophosphorylation | Synthesis of ATP from ADP and Pi using light energy in the thylakoid membrane |
| Z-scheme | The non-cyclic electron flow from PS II to PS I through electron carriers, producing ATP and NADPH |
| RuBisCO | Ribulose-1,5-bisphosphate carboxylase/oxygenase — the most abundant protein on Earth; catalyses CO₂ fixation |
| Calvin cycle | The set of reactions in the stroma that fix CO₂ and produce sugar using ATP and NADPH from light reactions |
| Carboxylation | The fixation of CO₂ to RuBP catalysed by RuBisCO |
| Photorespiration | A wasteful process where RuBisCO acts as oxygenase, binding O₂ instead of CO₂ |
| Kranz anatomy | The arrangement of bundle sheath cells around vascular bundles in C4 leaves, surrounded by mesophyll |
| PEP carboxylase | Enzyme in C4 plants that fixes CO₂ to PEP forming oxaloacetate; has higher affinity for CO₂ than RuBisCO |
| Chemiosmosis | Synthesis of ATP driven by a proton gradient across a membrane (thylakoid in photosynthesis) |
Detailed Concepts
Early Experiments
| Scientist | Experiment | Finding |
|---|---|---|
| Priestley (1771) | Candle + mint plant in bell jar | Plant restores air that the candle damaged; plants produce O₂ |
| Ingenhousz (1779) | Exposed plants to sunlight vs dark | Only green parts in light produce O₂; sunlight is essential |
| Sachs (1854) | Tested leaves for starch before/after light | Glucose produced in light is stored as starch |
| Engelmann (1882) | Cladophora with aerobic bacteria, prism | Bacteria clustered in red and blue regions; these wavelengths are most effective for photosynthesis |
Site of Photosynthesis — Chloroplast
- Photosynthesis occurs in the chloroplasts of mesophyll cells.
- A chloroplast has a double membrane envelope, stroma (fluid), and thylakoids (disc-shaped membrane sacs).
- Thylakoids are stacked as grana; unstacked thylakoids are stroma lamellae.
- Light reactions occur in the thylakoid membranes; dark (Calvin) reactions occur in the stroma.
Photosynthetic Pigments
| Pigment | Colour | Absorption | Notes |
|---|---|---|---|
| Chlorophyll a | Blue-green | Red and blue | Reaction centre pigment; essential for all photosynthetic organisms |
| Chlorophyll b | Yellow-green | Red and blue | Accessory pigment; broadens absorption spectrum |
| Carotenoids | Orange-yellow | Blue and green | Accessory pigment; protects chlorophyll from photo-oxidation |
| Xanthophylls | Yellow | Blue and violet | Accessory pigment; type of carotenoid |
- Chlorophyll a is the only pigment that can convert absorbed light energy to chemical energy; all others are accessory pigments that pass energy to chlorophyll a.
- The absorption spectrum shows which wavelengths are absorbed; the action spectrum shows which wavelengths drive photosynthesis. They closely overlap, confirming that absorbed light drives the process.
Light Reactions
- Light reactions occur in the thylakoid membrane.
- They produce ATP, NADPH, and O₂ (from water splitting).
Photosystem I vs Photosystem II:
| Feature | Photosystem II (PS II) | Photosystem I (PS I) |
|---|---|---|
| Reaction centre | P680 | P700 |
| Absorption peak | 680 nm | 700 nm |
| Location | Grana thylakoids | Grana and stroma lamellae |
| Function | Splits water, provides electrons | Reduces NADP⁺ to NADPH |
| First/Second | Operates first in non-cyclic flow | Operates second |
Z-scheme (Non-cyclic electron flow): 1. PS II absorbs light → P680 is excited → electrons are emitted. 2. Electrons pass through electron acceptor → plastoquinone (PQ) → cytochrome b₆f complex → plastocyanin (PC). 3. Lost electrons from PS II are replaced by photolysis of water: 2H₂O → 4H⁺ + 4e⁻ + O₂ (water splitting by Mn-containing oxygen-evolving complex). 4. PS I absorbs light → P700 is excited → electrons emitted. 5. Electrons pass through ferredoxin (Fd) → NADP⁺ reductase → reduces NADP⁺ to NADPH. 6. This is called the Z-scheme because the energy level of electrons rises, falls, and rises again, forming a Z-shaped diagram.
Cyclic vs Non-cyclic photophosphorylation:
| Feature | Non-cyclic | Cyclic |
|---|---|---|
| Involves | PS II + PS I | PS I only |
| Electron flow | Unidirectional (PS II → PS I → NADP⁺) | Circular (PS I → Fd → Cyt b₆f → PC → back to PS I) |
| Products | ATP + NADPH + O₂ | ATP only |
| Water splitting | Yes | No |
| NADPH formed | Yes | No |
| Occurs when | Normal conditions | When NADPH accumulates and NADP⁺ is unavailable |
Chemiosmotic hypothesis (Peter Mitchell):
- Protons accumulate inside the thylakoid lumen from: (1) water splitting, (2) proton pumping by Cyt b₆f, (3) removal of H⁺ from stroma when NADP⁺ is reduced.
- This creates a proton gradient across the thylakoid membrane.
- Protons flow back to the stroma through ATP synthase (CF₀–CF₁ particle), driving ATP synthesis from ADP + Pi.
- CF₀ is the transmembrane channel; CF₁ is the catalytic head on the stroma side.
Dark Reactions — The Calvin Cycle (C₃ Pathway)
- Discovered by Melvin Calvin, Benson, and Bassham using radioactive ¹⁴C and Chlorella (a green alga).
- Occurs in the stroma; does not directly require light but depends on ATP and NADPH from light reactions.
Three stages of the Calvin cycle:
| Stage | Process | Details |
|---|---|---|
| Carboxylation | CO₂ + RuBP → 2 × 3-PGA | RuBisCO catalyses this; RuBP (5C) + CO₂ → unstable 6C intermediate → 2 × 3-PGA (3C) |
| Reduction | 3-PGA → G3P (glyceraldehyde-3-phosphate) | 3-PGA is phosphorylated by ATP → 1,3-bisphosphoglycerate; then reduced by NADPH → G3P |
| Regeneration | G3P → RuBP | For every 3 CO₂ fixed (producing 6 G3P), 5 G3P are used to regenerate 3 RuBP using ATP; 1 G3P is net output |
- Per turn of cycle: 1 CO₂ fixed, 3 ATP + 2 NADPH used.
- Per glucose (6 CO₂): 18 ATP + 12 NADPH consumed.
- Net equation for Calvin cycle: 6CO₂ + 18ATP + 12NADPH → C₆H₁₂O₆ + 18ADP + 12NADP⁺ + 18Pi
C₄ Pathway (Hatch-Slack Pathway)
- Discovered by Hatch and Slack in sugarcane (a tropical grass).
- An adaptation to avoid photorespiration in hot, dry conditions.
Mechanism: 1. In mesophyll cells: CO₂ + PEP (phosphoenolpyruvate, 3C) → oxaloacetate (4C), catalysed by PEP carboxylase. 2. Oxaloacetate → malate (in some plants) or aspartate (in others). 3. Malate/aspartate is transported to bundle sheath cells. 4. In bundle sheath cells: malate → pyruvate (3C) + CO₂ (decarboxylation releases CO₂). 5. Released CO₂ enters the Calvin cycle in bundle sheath chloroplasts (via RuBisCO). 6. Pyruvate returns to mesophyll → phosphorylated to PEP using ATP.
C₃ vs C₄ plants:
| Feature | C₃ Plants | C₄ Plants |
|---|---|---|
| CO₂ fixation enzyme | RuBisCO | PEP carboxylase (first), then RuBisCO |
| First stable product | 3-PGA (3C) | Oxaloacetate (4C) |
| Leaf anatomy | Normal mesophyll | Kranz anatomy (bundle sheath + mesophyll) |
| Photorespiration | High (at high temperature) | Negligible |
| CO₂ compensation point | High (21–150 ppm) | Low (0–10 ppm) |
| Optimum temperature | 20–25 °C | 30–40 °C |
| Water use efficiency | Lower | Higher |
| Examples | Wheat, rice, potato, soybean | Maize, sugarcane, sorghum, millet |
Kranz anatomy:
- Bundle sheath cells are large, with thick walls, many chloroplasts (agranal in some species), and surround the vascular bundle.
- Mesophyll cells are smaller, with fewer chloroplasts (granal), and surround the bundle sheath.
- This arrangement creates a CO₂-concentrating mechanism: mesophyll pumps CO₂ to bundle sheath, where RuBisCO operates in a high-CO₂, low-O₂ environment, minimising photorespiration.
Photorespiration (C₂ Cycle)
- Occurs when RuBisCO acts as oxygenase instead of carboxylase.
- Triggered by: high temperature, high O₂, low CO₂.
- RuBP + O₂ → 1 PGA (3C) + 1 phosphoglycolate (2C).
- The phosphoglycolate pathway involves chloroplasts → peroxisomes → mitochondria and back.
- Products: CO₂ is released, but no ATP or sugar is synthesised — hence it is called a wasteful process.
- C₄ plants largely avoid photorespiration because PEP carboxylase concentrates CO₂ in bundle sheath cells.
Factors Affecting Photosynthesis
| Factor | Effect | Details |
|---|---|---|
| Light | Rate increases with light intensity up to a saturation point | Light saturation occurs at ~10% full sunlight for C₃ plants; C₄ plants saturate at higher intensities |
| CO₂ concentration | Rate increases with CO₂ up to saturation | Current atmospheric CO₂ (~0.04%) is limiting for C₃ plants; C₄ plants are more efficient at low CO₂ |
| Temperature | Optimum 25–35 °C for most plants | At high temp, enzymes denature; photorespiration increases in C₃ plants |
| Water | Essential as reactant and for stomatal opening | Water stress closes stomata, reducing CO₂ uptake |
- Law of limiting factors (Blackman, 1905): If a process depends on multiple factors, the rate is determined by the factor that is closest to its minimum value (the limiting factor).
Important Diagrams to Practise
1. Z-scheme — electron flow from PS II → Cyt b₆f → PC → PS I → Fd → NADP⁺ reductase, with energy levels. 2. Calvin cycle — carboxylation, reduction, regeneration with molecules, ATP/NADPH used at each step. 3. C₄ pathway — mesophyll cell (PEP carboxylase) → bundle sheath cell (RuBisCO/Calvin cycle) with transport of malate/pyruvate. 4. Chloroplast structure — grana, stroma lamellae, stroma, double membrane, thylakoid. 5. ATP synthase — CF₀ channel + CF₁ head, proton flow from lumen to stroma.
Common Mistakes to Avoid
1. Calling dark reactions "light-independent" — while they do not directly use light, they depend on ATP and NADPH from light reactions; in nature they usually occur in light. 2. Confusing PS I and PS II order — PS II operates first in non-cyclic flow despite the numbering; PS I was discovered first. 3. Confusing cyclic and non-cyclic photophosphorylation — cyclic involves only PS I and produces only ATP; non-cyclic involves both photosystems and produces ATP + NADPH + O₂. 4. Forgetting that water is the source of O₂ in photosynthesis — O₂ comes from photolysis of water, not from CO₂. 5. Saying C₄ plants do not have the Calvin cycle — C₄ plants use the Calvin cycle in bundle sheath cells; the C₄ pathway is an additional CO₂-concentrating mechanism. 6. Confusing PEP carboxylase and RuBisCO — PEP carboxylase fixes CO₂ in mesophyll (C₄ first step); RuBisCO fixes CO₂ in bundle sheath (Calvin cycle). 7. Confusing the compensation point — C₃ plants have a higher CO₂ compensation point than C₄ because C₃ plants suffer photorespiration at low CO₂.
Exam Tips
- For VSA (1 mark): Know the definitions of RuBisCO, PEP carboxylase, chemiosmosis, and photorespiration. Know the products of cyclic vs non-cyclic photophosphorylation.
- For SA (2–3 marks): Compare C₃ vs C₄ plants, cyclic vs non-cyclic photophosphorylation, or light vs dark reactions.
- For LA (5 marks): Explain the Z-scheme with a diagram, describe the Calvin cycle with all three stages, or explain the C₄ pathway with Kranz anatomy.
- NCERT-tested lines: "RuBisCO is the most abundant protein in the world" and "The Calvin pathway occurs in all photosynthetic plants — C₄ plants too."
- Application question: "Why are C₄ plants more efficient at high temperatures?" — because PEP carboxylase concentrates CO₂ in bundle sheath, preventing RuBisCO from acting as oxygenase.
How to Use This Guide
This guide is structured for layered revision:
- First pass: Read Chapter Introduction and Key Definitions to orient yourself.
- Second pass: Study Detailed Concepts section by section, matching with your NCERT textbook.
- Third pass: Use the tables and Common Mistakes for quick comparison and self-testing.
- Final pass: Use Quick Revision and Exam-Day Recall for last-minute review.
Additional Detail: Light Reactions vs Dark Reactions
| Feature | Light Reactions | Dark Reactions (Calvin Cycle) |
|---|---|---|
| Location | Thylakoid membrane | Stroma |
| Direct light requirement | Yes | No (but depends on light reaction products) |
| Inputs | Light, H₂O, ADP, NADP⁺ | CO₂, ATP, NADPH |
| Outputs | O₂, ATP, NADPH | G3P (sugar precursor), ADP, NADP⁺ |
| Enzymes involved | Photosystems, Cyt b₆f, ATP synthase | RuBisCO, G3P dehydrogenase, etc. |
| Time of occurrence | Day only | Day (in nature) — requires light reaction products |
| Can occur in isolation? | Yes (if NADP⁺ and ADP available) | No — requires ATP and NADPH from light reactions |
Additional Detail: The Calvin Cycle Step by Step
Carboxylation (CO₂ fixation):
- RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyses the reaction.
- RuBP (5C) + CO₂ → unstable 6C intermediate → 2 molecules of 3-PGA (3-phosphoglycerate, 3C).
- This is the entry point of inorganic carbon into the biosphere.
- RuBisCO makes up 16% of chloroplast protein — it is indeed the most abundant protein on Earth.
Reduction (energy investment):
- 3-PGA is phosphorylated: 3-PGA + ATP → 1,3-bisphosphoglycerate + ADP
- 1,3-bisphosphoglycerate is reduced: 1,3-BPG + NADPH → G3P + NADP⁺ + Pi
- G3P (glyceraldehyde-3-phosphate) is the first sugar produced.
Regeneration (RuBP recovery):
- 5 of the 6 G3P molecules produced from 3 CO₂ are used to regenerate 3 RuBP.
- This requires 3 more ATP (one per RuBP regenerated).
- The regeneration pathway involves sugar phosphate rearrangements (similar to pentose phosphate pathway).
- 1 G3P is the net product — it exits the cycle for glucose/sucrose/starch synthesis.
Additional Detail: CAM Plants (Crassulacean Acid Metabolism)
- CAM plants (e.g. pineapple, cacti, orchids) also avoid photorespiration but differently from C4 plants.
- They open stomata at night to fix CO₂ into malic acid (stored in vacuoles).
- During the day, stomata close (reducing water loss), malic acid is decarboxylated to release CO₂ for the Calvin cycle.
- Unlike C4 plants (spatial separation of CO₂ fixation), CAM plants use temporal separation (night vs day).
- CAM plants are adapted to extremely arid conditions.
NCERT Reading Method
1. Read the chapter once to understand the full flow: early experiments → pigments → light reactions → Calvin cycle → C4 pathway → photorespiration → factors. 2. Mark all definitions (RuBisCO, Z-scheme, chemiosmosis, Kranz anatomy, photorespiration, PEP carboxylase) and every formula or yield (ATP/NADPH per glucose in Calvin cycle). 3. Pay close attention to the distinction between PS I and PS II, and between cyclic and non-cyclic photophosphorylation. 4. Solve in-text and exercise questions without copying — especially C3 vs C4 comparisons and the Calvin cycle. 5. Compare your answers with NCERT language and correct missing keywords. 6. Reattempt wrong questions after one day.
Answer-Writing Tips
- Use NCERT keywords in definitions — "RuBisCO is the most abundant protein in the world" and "The Calvin pathway occurs in all photosynthetic plants."
- Draw the Z-scheme diagram for any question on light reactions — it is the most important diagram in this chapter.
- For comparison questions (C3 vs C4, cyclic vs non-cyclic), always use a table format.
- In 5-mark answers on the Calvin cycle, describe all three stages (carboxylation, reduction, regeneration) and mention ATP/NADPH usage.
- For photorespiration questions, mention the three organelles involved (chloroplast, peroxisome, mitochondria) and emphasise that it produces no ATP or sugar.
- Mention Blackman's law of limiting factors when discussing factors affecting photosynthesis.
Practice Checklist
- Solve all NCERT exercise questions from this chapter — especially C3 vs C4 and Calvin cycle questions.
- Prepare five one-mark facts: RuBisCO full name, PS II reaction centre (P680), Calvin cycle discoverer, C4 first stable product (oxaloacetate), photorespiration conditions.
- Write one three-mark answer on "Differentiate between cyclic and non-cyclic photophosphorylation."
- Write one five-mark answer on "Describe the Calvin cycle with a diagram" or "Explain the Z-scheme of electron transport."
- Draw the C4 pathway with mesophyll and bundle sheath cells from memory.
- Update your mistake notebook before moving to the next chapter.
Quick Revision Plan
| Time | Task |
|---|---|
| 15 minutes | Read marked NCERT lines — focus on Z-scheme, Calvin cycle steps, and C4 pathway |
| 20 minutes | Revise this guide, redrawing the Z-scheme and Calvin cycle diagrams |
| 25 minutes | Solve textbook or sample questions — especially comparison tables and pathway descriptions |
| 10 minutes | Correct mistakes and note weak areas |
Quick Revision
| Topic | One-Line Recall |
|---|---|
| Early experiments | Priestley (O₂), Ingenhousz (light needed), Sachs (starch), Engelmann (red/blue most effective) |
| Pigments | Chl a = reaction centre; Chl b, carotenoids, xanthophylls = accessory |
| PS II vs PS I | PS II (P680, splits water, first); PS I (P700, reduces NADP⁺, second) |
| Z-scheme | PS II → PQ → Cyt b₆f → PC → PS I → Fd → NADP⁺; non-cyclic flow |
| Photophosphorylation | Non-cyclic: ATP + NADPH + O₂; Cyclic: ATP only (PS I alone) |
| Chemiosmosis | Proton gradient across thylakoid → ATP synthase → ATP |
| Calvin cycle | Carboxylation → Reduction → Regeneration; 3 CO₂ = 1 G3P; uses 18 ATP + 12 NADPH per glucose |
| C₄ pathway | PEP carboxylase (mesophyll) → oxaloacetate → malate → bundle sheath → CO₂ → Calvin cycle |
| Kranz anatomy | Bundle sheath surrounds vein; mesophyll surrounds bundle sheath |
| Photorespiration | RuBisCO + O₂ → PGA + phosphoglycolate; wasteful; involves chloroplast–peroxisome–mitochondria |
| Limiting factors | Blackman's law; light, CO₂, temperature, water — lowest factor limits rate |
| O₂ source | Water (photolysis by PS II), NOT from CO₂ |
Exam-Day Recall
Before the exam, revise Photosynthesis using your marked NCERT pages, the Z-scheme diagram, and the C3-vs-C4 comparison table. If you can trace the Z-scheme electron flow, describe all three Calvin cycle stages with ATP/NADPH counts, and explain why C4 plants avoid photorespiration aloud in five minutes, your basics are ready.