Photosynthesis in Higher Plants

Photosynthesis in Higher Plants

• Photosynthesis is an enzyme-regulated anabolic process that manufactures organic compounds from carbon dioxide and water using sunlight as energy.

Historical Perspective

• Joseph Priestley (1770): Showed plants take in CO₂ and release O₂.
• Jan Ingenhousz (1779): Release of O₂ happens only in the presence of sunlight and from the green parts of plants.
• Theodore de Saussure (1804): Water is essential for photosynthesis.
• Julius Von Sachs (1854): Green parts produce glucose, stored as starch.
• T.W. Engelmann (1888): Demonstrated the effect of different wavelengths of light on photosynthesis, plotting the first action spectrum.
• C.B. Van Niel (1931): Defined photosynthesis as a light-dependent reaction where hydrogen reduces CO₂ to form sugar, providing a simplified chemical equation.
• Hill (1937): Oxygen evolution occurs during the light reaction.
• Calvin (1954-55): Traced the pathway of carbon fixation.
• Hatch and Slack (1965): Discovered the C4 pathway of CO₂ fixation.

Site for Photosynthesis

• Photosynthesis primarily occurs in the leaves' mesophyll cells within chloroplasts.
• Chloroplasts are the sites of photosynthesis within the leaf.
• Thylakoids in chloroplasts have pigments to capture solar energy and initiate photosynthesis.
• The grana (membrane system) traps light energy and produces ATP and NADPH (dark reaction in the stroma).

Pigments Involved in Photosynthesis

• Chlorophyll a: The primary pigment, acts as a reaction center, capturing and converting light energy to chemical energy. (blue-green in chromatographs).
• Chlorophyll b: (yellow-green)
• Xanthophylls: (yellow)

Light Reaction

• Light absorption, water splitting, O₂ evolution, and the formation of high-energy compounds (ATP and NADPH) are part of the photochemical phase (light reaction).
• Light-harvesting complexes (LHCs) capture light energy.
• Photosystem II (PSII) absorbs light at 680 nm (P680).
• Photosystem I (PSI) absorbs light at 700 nm (P700).
• Electron transport chain (Z-scheme); electrons move from PSII to PSI, generating ATP and reducing NADP⁺ to NADPH.
• Photolysis of water produces O₂, H⁺, and electrons.

The Electron Transport

• Electrons from chlorophyll 'a' in PSII are excited and transferred to electron acceptors.
• The electron moves downhill in terms of redox potential through the electron transport chain.
• The electron eventually reduces NADP⁺ to NADPH + H⁺.

Photolysis of Water

• Water molecules are split to produce O₂, H⁺ ions, and electrons in the light.
• These electrons replenish those lost from chlorophyll during the light-dependent reactions.

Non-Cyclic Photophosphorylation

• Involves both photosystems (PSII and PSI) in series.
• The electron transport chain links the two photosystems.

Cyclic Photophosphorylation

• Involves only PSI, with electrons circulating within the photosystem.
• Generates only ATP; no NADPH is formed.

Chemiosmotic Hypothesis

• Explains ATP synthesis due to proton gradient across thylakoid membrane.
• Protons are transported across the thylakoid membrane, producing a proton gradient. Facilitating ATP generation.
• ATPase enzyme uses this gradient to produce ATP.

Biosynthetic Phase (C3 Cycle)

• Uses ATP and NADPH from the light-dependent reactions to fix CO₂ into organic molecules.
• The Calvin cycle is a cyclical series of reactions that fixes carbon dioxide into glucose.
• Involves three stages: carboxylation, reduction, and regeneration.

C4 Pathway

• Plants in dry tropical regions use this pathway for efficient CO₂ fixation.
• Initial CO₂ fixation involves oxaloacetic acid (OAA) in mesophyll cells.
• OAA is converted to malic acid (or aspartate) and transported to bundle sheath cells.
• CO₂ is released in bundle sheath cells, stimulating the Calvin cycle.

Photorespiration

• An alternative pathway where RuBisCo also reacts with oxygen instead of carbon dioxide.
• Can reduce photosynthetic efficiency in some plants.

Law of Limiting Factors

• If a chemical process is affected by multiple factors, its rate is limited by the factor nearest to its minimum value.

Explore the fascinating process of photosynthesis in higher plants, a vital mechanism for life on Earth. Delve into the historical discoveries that shaped our understanding, from Priestley's findings on oxygen release to Calvin's work on carbon fixation. Test your knowledge of the key figures and their contributions in this essential biological process.