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Photosynthesis in Higher Plants – Understanding Light and Dark Reactions, Pigments, C4 Pathway, and Factors Affecting Photosynthesis – Class 11 Biology


📘 Photosynthesis in Higher Plants – Class 11 NCERT Detailed Explanation


🔶 Introduction



Photosynthesis is the fundamental process by which green plants, algae, and certain bacteria convert light energy into chemical energy. In higher plants, this process takes place in the chloroplasts, using sunlight, carbon dioxide, and water to produce glucose and release oxygen.



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🌱 What is Photosynthesis?


Photosynthesis is defined as the process by which autotrophic organisms convert light energy (usually from the Sun) into chemical energy stored in glucose. The overall simplified chemical equation is:


6CO_2 + 6H_2O \xrightarrow{light} C_6H_{12}O_6 + 6O_2



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🔍 Where Does Photosynthesis Occur?


Photosynthesis mainly occurs in the mesophyll cells of leaves, specifically within chloroplasts, which contain the green pigment chlorophyll.



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📚 Historical Background


Joseph Priestley (1774): Showed plants release oxygen.


Jan Ingenhousz: Found sunlight is essential for oxygen release.


Julius von Sachs: Proved that glucose is formed and stored as starch.


T.W. Engelmann: Demonstrated action spectrum of photosynthesis using algae and aerobic bacteria.


Cornelius van Niel: Proposed that water is the source of oxygen in photosynthesis.




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🔬 Pigments Involved in Photosynthesis


Chlorophyll a (main pigment)


Chlorophyll b


Xanthophylls


Carotenoids



Only chlorophyll a directly participates in light reactions, but all other pigments act as accessory pigments.



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🌈 Absorption Spectrum vs Action Spectrum


Absorption spectrum: Wavelengths absorbed by different pigments.


Action spectrum: Effectiveness of different wavelengths in photosynthesis.



T.W. Engelmann's experiment demonstrated that red and blue lights are most effective.



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⚡ Light Reaction (Photochemical Phase)


Takes place in the thylakoid membranes and includes:


1. Photon absorption by chlorophyll



2. Excitation of electrons



3. Photolysis of water



4. Formation of ATP and NADPH




Light reactions include two photosystems:


PS I (P700)


PS II (P680)



🔁 Cyclic and Non-Cyclic Photophosphorylation


Cyclic: Only ATP is formed (involves PSI only).


Non-cyclic: Both ATP and NADPH are formed; oxygen is released.




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🌳 Dark Reaction (Biosynthetic Phase)


Occurs in the stroma of chloroplasts. It does not require light directly but uses ATP and NADPH from the light reaction.


Calvin Cycle (C3 Cycle) – Steps:


1. Carboxylation – CO₂ is fixed by RuBisCO enzyme.



2. Reduction – Formation of glyceraldehyde-3-phosphate (G3P).



3. Regeneration – Regeneration of RuBP (5-carbon compound).





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🌿 C4 Pathway (Hatch and Slack Pathway)


Found in plants like maize and sugarcane. CO₂ is first fixed into a 4-carbon compound (oxaloacetic acid) in mesophyll cells, then transferred to bundle sheath cells for the Calvin cycle.


Advantages:


Reduces photorespiration


More efficient in high light and temperature




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🌬️ Photorespiration (Wasteful Process)


Occurs when RuBisCO binds with O₂ instead of CO₂. No ATP or NADPH is produced, and it reduces photosynthetic efficiency. More common in C3 plants.



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🔍 Factors Affecting Photosynthesis


1. Light intensity



2. Carbon dioxide concentration



3. Temperature



4. Water availability




The Blackman’s law of limiting factors states that the slowest factor limits the overall rate of photosynthesis.



📌 Conclusion


Photosynthesis in higher plants is a highly organized and regulated process involving complex reactions. Understanding this process not only reveals the survival stra

tegy of plants but also highlights its importance in sustaining life on Earth.


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