Photosynthesis - Complete NEET Biology Notes 2026
Master Photosynthesis for NEET 2026 with comprehensive notes on light reactions, Calvin cycle, C3, C4, and CAM pathways. NCERT-aligned content with diagrams and previous year questions.
Key Takeaways
- 1Photosynthesis occurs in chloroplasts - light reactions in thylakoids, Calvin cycle in stroma
- 2Light reactions produce ATP and NADPH; Calvin cycle uses them to fix CO2
- 3C4 plants have Kranz anatomy and avoid photorespiration
- 4CAM plants open stomata at night to conserve water
- 5Photorespiration reduces photosynthetic efficiency in C3 plants
Remember these points for your NEET preparation
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Photosynthesis - Complete NEET Biology Notes
Photosynthesis is one of the highest weightage chapters in NEET Biology, contributing 4-6 questions annually. Master this chapter for a strong score in Plant Physiology.
Overview of Photosynthesis
Definition: Process by which green plants convert light energy into chemical energy, synthesizing glucose from CO₂ and H₂O.
Overall Equation: 6CO₂ + 12H₂O + Light → C₆H₁₂O₆ + 6O₂ + 6H₂O
Where Does Photosynthesis Occur?
- Chloroplasts in mesophyll cells
- Light reactions: Thylakoid membranes
- Dark reactions: Stroma
Photosynthetic Pigments
Types of Pigments
| Pigment | Color | Absorption Peak | Function |
|---|---|---|---|
| Chlorophyll a | Blue-green | 430 nm, 662 nm | Primary pigment, reaction center |
| Chlorophyll b | Yellow-green | 453 nm, 642 nm | Accessory pigment |
| Carotenoids | Yellow-orange | 400-500 nm | Accessory, photoprotection |
| Xanthophylls | Yellow | 400-500 nm | Accessory pigment |
Key Points:
- Chlorophyll a is the only pigment that can directly participate in light reactions
- Accessory pigments absorb light and transfer energy to Chl a
- Carotenoids also protect against photooxidation
Absorption vs Action Spectrum
- Absorption spectrum: Wavelengths absorbed by pigments
- Action spectrum: Wavelengths effective for photosynthesis
- They closely match, proving pigments drive photosynthesis
Light Reactions (Photochemical Phase)
Occur in thylakoid membranes and require light.
Photosystems
| Photosystem | Reaction Center | Absorption Peak | Location |
|---|---|---|---|
| PS I | P700 | 700 nm | Stroma lamellae |
| PS II | P680 | 680 nm | Grana thylakoids |
NEET Tip: PS II was discovered after PS I but acts first in light reactions.
Steps of Light Reactions
1. Photoexcitation
- Light absorbed by antenna pigments
- Energy transferred to reaction center (P680 or P700)
- Electrons get excited to higher energy level
2. Electron Transport Chain (ETC)
Z-Scheme (Non-cyclic electron flow):
- PS II absorbs light → P680 excited → P680*
- Electron passes to primary acceptor (Pheophytin)
- Water splitting: 2H₂O → O₂ + 4H⁺ + 4e⁻ (photolysis)
- Electrons flow: Plastoquinone → Cytochrome b6f → Plastocyanin
- PS I absorbs light → P700 excited → P700*
- Electrons pass through ferredoxin
- NADP⁺ reductase: NADP⁺ + 2e⁻ + H⁺ → NADPH
Products of Non-cyclic:
- ATP (photophosphorylation)
- NADPH
- O₂ (from water splitting)
3. Cyclic Electron Flow
- Only PS I involved
- Electrons return to PS I via Cyt b6f
- Produces ATP only (no NADPH, no O₂)
- Occurs in stroma lamellae
Photophosphorylation
| Type | Electron Flow | Products | O₂ Evolution |
|---|---|---|---|
| Non-cyclic | PS II → PS I | ATP + NADPH | Yes |
| Cyclic | PS I → PS I | ATP only | No |
Chemiosmotic Theory (ATP Synthesis)
- Proton gradient created across thylakoid membrane
- H⁺ accumulates in thylakoid lumen (low pH)
- H⁺ flows through ATP synthase (CF₀-CF₁)
- ATP synthesized from ADP + Pi
Sources of H⁺ accumulation:
- Water splitting in lumen
- Plastoquinone pumping H⁺
- NADP⁺ reduction using stromal H⁺
Dark Reactions (Calvin Cycle)
Occur in stroma, don't require light directly but need ATP and NADPH from light reactions.
Calvin Cycle (C3 Pathway)
Location: Stroma of chloroplast
Key Enzyme: RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase)
Three Stages
Stage 1: Carbon Fixation
- CO₂ + RuBP (5C) → 2 × 3-PGA (3C)
- Enzyme: RuBisCO
- First stable product: 3-PGA (hence C3 plants)
Stage 2: Reduction
- 3-PGA → G3P (Glyceraldehyde-3-phosphate)
- Uses ATP and NADPH
- G3P is the actual sugar product
Stage 3: Regeneration of RuBP
- 5 G3P molecules → 3 RuBP molecules
- Uses ATP
- Allows cycle to continue
Summary of Calvin Cycle (per CO₂ fixed)
| Input | Amount |
|---|---|
| CO₂ | 1 molecule |
| ATP | 3 molecules |
| NADPH | 2 molecules |
To make 1 glucose (6 carbons): 6 CO₂, 18 ATP, 12 NADPH
C4 Pathway (Hatch-Slack Pathway)
Why C4?
- C3 plants face photorespiration (RuBisCO fixes O₂ instead of CO₂)
- Photorespiration wastes energy and releases CO₂
- C4 plants evolved to minimize photorespiration
Kranz Anatomy
C4 plants have special leaf anatomy:
| Cell Type | Location | Chloroplasts | Function |
|---|---|---|---|
| Mesophyll | Outer | Granal | Initial CO₂ fixation |
| Bundle sheath | Around vascular bundles | Agranal | Calvin cycle |
C4 Pathway Steps
In Mesophyll Cells:
- CO₂ + PEP (3C) → OAA (4C)
- Enzyme: PEP carboxylase (no oxygenase activity)
- OAA → Malate or Aspartate (4C)
- 4C acid moves to bundle sheath
In Bundle Sheath Cells: 5. 4C acid decarboxylated → CO₂ + Pyruvate 6. CO₂ enters Calvin cycle (fixed by RuBisCO) 7. Pyruvate returns to mesophyll
C3 vs C4 Plants
| Feature | C3 Plants | C4 Plants |
|---|---|---|
| First product | 3-PGA (3C) | OAA (4C) |
| CO₂ acceptor | RuBP | PEP |
| Primary enzyme | RuBisCO | PEP carboxylase |
| Kranz anatomy | Absent | Present |
| Photorespiration | High | Negligible |
| CO₂ compensation point | 50 ppm | 10 ppm |
| Optimum temp | 20-25°C | 30-40°C |
| Examples | Wheat, Rice, Potato | Maize, Sugarcane, Sorghum |
CAM Pathway
CAM: Crassulacean Acid Metabolism
Adaptation
- Found in succulents and desert plants
- Temporal separation of CO₂ fixation and Calvin cycle
- Conserves water by opening stomata at night
CAM Mechanism
Night (Stomata Open):
- CO₂ fixed by PEP carboxylase
- OAA → Malic acid
- Malic acid stored in vacuole
Day (Stomata Closed): 4. Malic acid decarboxylated → CO₂ + Pyruvate 5. CO₂ used in Calvin cycle 6. Pyruvate regenerates PEP
Examples: Cactus, Pineapple, Agave, Bryophyllum
C4 vs CAM
| Feature | C4 | CAM |
|---|---|---|
| Separation | Spatial (cells) | Temporal (day/night) |
| Stomata | Open during day | Open at night |
| Water efficiency | High | Very high |
| Productivity | High | Low |
Factors Affecting Photosynthesis
Blackman's Law of Limiting Factors
Rate of photosynthesis is limited by the factor present in minimum quantity.
Key Factors
| Factor | Effect |
|---|---|
| Light intensity | Rate increases, then plateaus |
| CO₂ concentration | Rate increases up to 0.05% |
| Temperature | Optimum varies; enzymes denature at high temp |
| Water | Affects stomatal opening, electron donor |
Light Saturation Point
- Beyond this intensity, rate doesn't increase
- C4 plants have higher saturation point than C3
CO₂ Compensation Point
- CO₂ concentration where photosynthesis = respiration
- Lower in C4 plants (more efficient CO₂ fixation)
Photorespiration
What is Photorespiration?
- RuBisCO fixes O₂ instead of CO₂
- Produces phosphoglycolate (2C) instead of 3-PGA
- Wastes ATP and releases CO₂
- No ATP or NADPH produced
Pathway
Chloroplast → Peroxisome → Mitochondria → Chloroplast
Why C4 Plants Don't Photorespire
- Initial CO₂ fixation by PEP carboxylase (no oxygenase activity)
- High CO₂ concentration in bundle sheath cells
- RuBisCO operates efficiently
Previous Year NEET Questions
Q1 (NEET 2023): The first stable product of C3 cycle is:
- (a) OAA
- (b) 3-PGA ✓
- (c) RuBP
- (d) G3P
Q2 (NEET 2022): Kranz anatomy is found in:
- (a) C3 plants
- (b) C4 plants ✓
- (c) CAM plants
- (d) All plants
Q3 (NEET 2021): During cyclic photophosphorylation:
- (a) Only PS II is involved
- (b) Only PS I is involved ✓
- (c) Both PS I and PS II are involved
- (d) Neither is involved
Q4 (NEET 2020): The enzyme responsible for CO₂ fixation in C4 plants is:
- (a) RuBisCO
- (b) PEP carboxylase ✓
- (c) Carbonic anhydrase
- (d) Malic enzyme
Q5 (NEET 2019): In CAM plants, stomata are:
- (a) Open during day
- (b) Open during night ✓
- (c) Always open
- (d) Always closed
Quick Revision Points
- Photosynthesis equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
- Light reactions location: Thylakoid membrane
- Calvin cycle location: Stroma
- First stable product C3: 3-PGA
- First stable product C4: OAA
- CO₂ acceptor in C3: RuBP
- CO₂ acceptor in C4: PEP
- Most abundant enzyme: RuBisCO
- Cyclic phosphorylation: ATP only, PS I only
- Non-cyclic phosphorylation: ATP + NADPH + O₂
- ATP needed per glucose: 18 ATP
- NADPH needed per glucose: 12 NADPH
FAQs
Q: Why is PS II named so even though it acts first? A: Photosystems were named based on their order of discovery, not their order of function. PS I was discovered first, hence the name. Functionally, PS II acts before PS I in the Z-scheme.
Q: Why don't C4 plants photorespire? A: C4 plants use PEP carboxylase for initial CO₂ fixation, which has no oxygenase activity. Additionally, they concentrate CO₂ in bundle sheath cells, ensuring RuBisCO preferentially fixes CO₂.
Q: What is the advantage of CAM over C4? A: CAM plants have the highest water use efficiency because they keep stomata closed during hot daytime. This is critical for desert plants. However, C4 plants have higher productivity.
Q: Why is RuBisCO considered inefficient? A: RuBisCO has low catalytic rate (only 3 CO₂ fixed per second) and cannot distinguish well between CO₂ and O₂, leading to photorespiration in C3 plants.
Q: What happens to photosynthesis at very high temperatures? A: High temperatures denature enzymes, increase photorespiration (O₂ solubility decreases less than CO₂), and cause stomatal closure leading to CO₂ limitation.