PHOTOSYNTHESIS IN HIGHER PLANTS
NCERT Masterclass & NEET Biology Study Module
1. Early Experiments in Photosynthesis
- Joseph Priestley (1770): Discovered oxygen (1774). Showed that plants restore to the air whatever breathing animals and burning candles remove (Bell jar, mouse, mint plant experiment).
- Jan Ingenhousz: Showed that sunlight is essential and only the green parts of the plants could release oxygen.
- Julius von Sachs (1854): Provided evidence for production of glucose when plants grow, stored usually as starch.
- T.W. Engelmann: Described the first Action Spectrum of photosynthesis using a prism, green alga (Cladophora), and aerobic bacteria. Found bacteria accumulated mainly in blue and red light regions.
- Cornelius van Niel: A microbiologist who demonstrated that photosynthesis is a light-dependent redox reaction. He proved that $O_2$ evolved comes from $H_2O$, not from $CO_2$ (using purple and green sulphur bacteria).
2. Site of Photosynthesis & Pigments
Photosynthesis takes place in chloroplasts. Grana (membranous system) is responsible for trapping light energy and synthesizing ATP and NADPH (Light Reaction). The Stroma is where enzymatic reactions synthesize sugar (Dark Reaction).
- Chlorophyll a: Chief pigment. Appears bright or blue green in chromatogram.
- Chlorophyll b: Yellow green.
- Xanthophylls: Yellow.
- Carotenoids: Yellow to yellow-orange.
3. The Light Reaction (Photochemical Phase)
Includes light absorption, water splitting, oxygen release, and formation of high-energy chemical intermediates (ATP and NADPH).
- Photosystems (PS): Two discrete light harvesting complexes (LHC). PS I has an absorption peak at 700 nm (P700), and PS II has a peak at 680 nm (P680).
- Photolysis of Water: Splitting of water is associated with PS II. It occurs on the inner side of the thylakoid membrane.
$2H_2O \rightarrow 4H^+ + O_2 + 4e^-$
4. Dark Reaction (Biosynthetic Phase)
Uses ATP and NADPH produced in the light reaction to synthesize sugars. Occurs in the stroma. Does not directly require light, but depends on the products of the light reaction.
The Calvin Cycle ($C_3$ Pathway)
Occurs in ALL photosynthetic plants. Takes place in 3 stages:
- Carboxylation: $CO_2$ combines with RuBP (5-Carbon) to form two molecules of 3-PGA. Enzyme: RuBisCO. (Most crucial step).
- Reduction: Utilization of 2 ATP and 2 NADPH per $CO_2$ fixed to form glucose.
- Regeneration: Regeneration of the $CO_2$ acceptor RuBP requires 1 ATP.
Total requirement for 1 Glucose: 6 turns of cycle $\rightarrow$ 18 ATP and 12 NADPH.
5. The $C_4$ Pathway (Hatch-Slack Pathway)
Plants adapted to dry tropical regions (Maize, Sorghum) have the $C_4$ pathway. They show a special leaf anatomy called Kranz Anatomy.
- Primary $CO_2$ Acceptor: Phosphoenol pyruvate (PEP, 3-Carbon) in mesophyll cells. Enzyme is PEPcase.
- First Stable Product: Oxaloacetic Acid (OAA, 4-Carbon).
- Bundle Sheath Cells: Thick walls, no intercellular spaces, lack grana but rich in RuBisCO. They perform the Calvin Cycle.
- ATP Requirement: C4 pathway requires 30 ATP and 12 NADPH for 1 glucose molecule.
- Advantage: Tolerates high temperatures, high light intensities, lacks photorespiration, leading to greater productivity.
6. Photorespiration & Factors Affecting Photosynthesis
Photorespiration: RuBisCO binds $O_2$ instead of $CO_2$. It forms 1 molecule of phosphoglycerate (3C) and 1 molecule of phosphoglycolate (2C). No ATP/NADPH is synthesized; rather ATP is consumed and $CO_2$ is released. Highly wasteful process. Prevented entirely in C4 plants.
Blackman's Law of Limiting Factors (1905): If a chemical process is affected by more than one factor, its rate will be determined by the factor nearest to its minimal value.
← Back to NEET Resource Hub🚀 NEET PHOTOSYNTHESIS MEGA QUIZ (100 MCQ)
Solve the 5 parts below to master Light/Dark Reactions, C3 vs C4, and Chemiosmosis.
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