Photosynthesis in Higher Plants
NCERT Line-by-Line Breakdown for NEET 2026
Unit 4: Plant PhysiologyPhotosynthesis is the physico-chemical process by which green plants use light energy to drive the synthesis of organic compounds. It is the basis of life on earth because it produces Food and releases Oxygen. In this chapter, we explore the machinery of the Chloroplast, the Light Reaction, and the Dark Reaction (C3 & C4 pathways).
1. Site of Photosynthesis & Pigments
A. The Chloroplast
Photosynthesis takes place in the green leaves, specifically in the Mesophyll cells.
- Grana (Membrane System): Site of Light Reaction (Traps light energy, synthesizes ATP & NADPH).
- Stroma (Fluid Matrix): Site of Dark Reaction (Enzymatic reduction of CO2 into sugar).
B. Pigments
Separated by Paper Chromatography. They absorb light at specific wavelengths.
| Pigment | Color on Chromatogram | Role |
|---|---|---|
| Chlorophyll a | Bright or Blue-green | Primary Pigment (Reaction Center). |
| Chlorophyll b | Yellow-green | Accessory pigment. |
| Xanthophylls | Yellow | Accessory pigment. |
| Carotenoids | Yellow to Yellow-orange | Protects Chl-a from Photo-oxidation. |
2. Light Reaction (Photochemical Phase)
Includes light absorption, water splitting, oxygen release, and formation of high-energy intermediates (ATP & NADPH).
A. The Electron Transport
Z-Scheme: Electrons move from PS II → Uphill → Electron Acceptor → Downhill (ETS) → PS I → Uphill → NADP+.
• Absorbs 680 nm (P680).
• Located on inner side of thylakoid membrane.
• Associated with Splitting of Water (Photolysis) → Releases O2.
• Absorbs 700 nm (P700).
• Located on both non-appressed part of grana and stroma lamellae.
• Reduces NADP+ to NADPH.
B. Cyclic vs Non-Cyclic Photophosphorylation
| Feature | Non-Cyclic | Cyclic |
|---|---|---|
| Photosystems | Both PS I and PS II. | Only PS I. |
| Products | ATP + NADPH + O2. | Only ATP. |
| Location | Grana lamellae. | Stroma lamellae. |
3. Dark Reaction (Biosynthetic Phase)
Independent of direct light but dependent on the products of light reaction (ATP & NADPH).
A. C3 Cycle (Calvin Cycle)
Universal cycle (occurs in all photosynthetic plants).
- Primary Acceptor: RuBP (5-carbon).
- First Stable Product: 3-PGA (3-carbon).
- Key Enzyme: RuBisCO (Most abundant enzyme).
- Calculation: For 1 Glucose molecule, 6 turns are required.
Input: 6 CO2, 18 ATP, 12 NADPH.
B. C4 Cycle (Hatch & Slack Pathway)
Found in plants adapted to dry tropical regions (e.g., Maize, Sorghum).
- Primary Acceptor: PEP (3-carbon) in Mesophyll.
- First Stable Product: OAA (4-carbon).
- Enzymes: PEPcase (Mesophyll) & RuBisCO (Bundle Sheath).
- Advantage: Can tolerate high temp, high light intensity, and avoids Photorespiration.
4. Photorespiration & Factors
A. Photorespiration (C2 Cycle)
A wasteful process in C3 plants where RuBisCO binds with O2 instead of CO2.
Result: Release of CO2 with utilisation of ATP. No Sugar or ATP is produced. Occurs in Chloroplast, Peroxisome, and Mitochondria.
B. Factors Affecting Photosynthesis
Blackman’s Law of Limiting Factors (1905): “If a chemical process is affected by more than one factor, then its rate will be determined by the factor which is nearest to its minimal value.”
- Light: Linear relationship at low intensity. At high intensity, saturation occurs (10% of full sunlight).
- CO2 Concentration: The major limiting factor. C3 plants saturate at 450 µl/L, C4 at 360 µl/L.
- Temperature: C4 plants have higher temperature optimum (30-40°C) than C3 plants (20-25°C).
📝 Rapid Fire MCQs
Q1. The first stable product of CO2 fixation in Sorghum is:
- A) Phosphoglyceric acid (PGA)
- B) Oxaloacetic acid (OAA)
- C) Pyruvic acid
- D) Succinic acid
Click to check Answer
Answer: B) Oxaloacetic acid (OAA) (Sorghum is a C4 plant).
Q2. In chloroplasts, protons accumulate in the:
- A) Stroma
- B) Intermembrane space
- C) Lumen of thylakoids
- D) Matrix
Click to check Answer
Answer: C) Lumen of thylakoids.