Photosynthesis - Complete NEET Biology Notes 2026

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

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

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):

1. PS II absorbs light → P680 excited → P680*
2. Electron passes to primary acceptor (Pheophytin)
3. Water splitting: 2H₂O → O₂ + 4H⁺ + 4e⁻ (photolysis)
4. Electrons flow: Plastoquinone → Cytochrome b6f → Plastocyanin
5. PS I absorbs light → P700 excited → P700*
6. Electrons pass through ferredoxin
7. 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

Chemiosmotic Theory (ATP Synthesis)

1. Proton gradient created across thylakoid membrane
2. H⁺ accumulates in thylakoid lumen (low pH)
3. H⁺ flows through ATP synthase (CF₀-CF₁)
4. 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)

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:

C4 Pathway Steps

In Mesophyll Cells:

1. CO₂ + PEP (3C) → OAA (4C)
2. Enzyme: PEP carboxylase (no oxygenase activity)
3. OAA → Malate or Aspartate (4C)
4. 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

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):

1. CO₂ fixed by PEP carboxylase
2. OAA → Malic acid
3. 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

Factors Affecting Photosynthesis

Blackman's Law of Limiting Factors

Rate of photosynthesis is limited by the factor present in minimum quantity.

Key Factors

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

1. Initial CO₂ fixation by PEP carboxylase (no oxygenase activity)
2. High CO₂ concentration in bundle sheath cells
3. 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

1. Photosynthesis equation: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6O₂ + 6H₂O
2. Light reactions location: Thylakoid membrane
3. Calvin cycle location: Stroma
4. First stable product C3: 3-PGA
5. First stable product C4: OAA
6. CO₂ acceptor in C3: RuBP
7. CO₂ acceptor in C4: PEP
8. Most abundant enzyme: RuBisCO
9. Cyclic phosphorylation: ATP only, PS I only
10. Non-cyclic phosphorylation: ATP + NADPH + O₂
11. ATP needed per glucose: 18 ATP
12. 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.

Not sure which photosynthesis questions repeat every year?

Light reactions and the Calvin cycle have a fairly predictable set of question types that NEET returns to across papers. Message us on WhatsApp at +91 88264 44334 and we'll send the light-reaction and Calvin-cycle question types that have actually shown up in recent NEET papers.

Related reading: Photosynthesis Complete NEET Guide · NEET Plant Physiology Weightage Analysis · Class 11 Biology Coaching