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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.
Remember these points for your NEET preparation
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.
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?
| 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:
Occur in thylakoid membranes and require light.
| 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.
Z-Scheme (Non-cyclic electron flow):
Products of Non-cyclic:
| Type | Electron Flow | Products | O₂ Evolution |
|---|---|---|---|
| Non-cyclic | PS II → PS I | ATP + NADPH | Yes |
| Cyclic | PS I → PS I | ATP only | No |
Sources of H⁺ accumulation:
Occur in stroma, don't require light directly but need ATP and NADPH from light reactions.
Location: Stroma of chloroplast
Key Enzyme: RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase)
| Input | Amount |
|---|---|
| CO₂ | 1 molecule |
| ATP | 3 molecules |
| NADPH | 2 molecules |
To make 1 glucose (6 carbons): 6 CO₂, 18 ATP, 12 NADPH
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 |
In Mesophyll Cells:
In Bundle Sheath Cells: 5. 4C acid decarboxylated → CO₂ + Pyruvate 6. CO₂ enters Calvin cycle (fixed by RuBisCO) 7. Pyruvate returns to mesophyll
| 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: Crassulacean Acid Metabolism
Night (Stomata Open):
Day (Stomata Closed): 4. Malic acid decarboxylated → CO₂ + Pyruvate 5. CO₂ used in Calvin cycle 6. Pyruvate regenerates PEP
Examples: Cactus, Pineapple, Agave, Bryophyllum
| Feature | C4 | CAM |
|---|---|---|
| Separation | Spatial (cells) | Temporal (day/night) |
| Stomata | Open during day | Open at night |
| Water efficiency | High | Very high |
| Productivity | High | Low |
Rate of photosynthesis is limited by the factor present in minimum quantity.
| 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 |
Chloroplast → Peroxisome → Mitochondria → Chloroplast
Q1 (NEET 2023): The first stable product of C3 cycle is:
Q2 (NEET 2022): Kranz anatomy is found in:
Q3 (NEET 2021): During cyclic photophosphorylation:
Q4 (NEET 2020): The enzyme responsible for CO₂ fixation in C4 plants is:
Q5 (NEET 2019): In CAM plants, stomata are:
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.
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