SATHEE: Notes From Toppers

Photosynthesis

1. Light Reactions:

Absorption of light energy by chlorophyll a and accessory pigments located in the thylakoid membranes of chloroplasts.
Photosystem I: Composed of P700 reaction center, light-harvesting complex, and electron transport chain. Absorbs light energy and transfers electrons to ferredoxin.
Photosystem II: Contains P680 reaction center, light-harvesting complex, and electron transport chain. Splits water molecules, releasing oxygen and generating high-energy electrons.
Electron Transport Chain: Series of electron carriers embedded in the thylakoid membrane that facilitate the transfer of electrons from photosystems to NADP+, leading to the generation of ATP through chemiosmosis.

2. Calvin Cycle (Dark Reactions):

Carbon fixation: CO2 molecules combine with ribulose-1,5-bisphosphate (RuBP) to form two molecules of 3-phosphoglycerate (3-PGA) through the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco).
Reduction: 3-PGA molecules are reduced to glyceraldehyde-3-phosphate (G3P) using ATP and NADPH generated in the light reactions.
Regeneration of RuBP: Some G3P molecules are used to regenerate RuBP, the primary CO2 acceptor, to continue the cycle.

3. Factors Affecting Photosynthesis:

Light intensity: Directly affects the rate of photosynthesis up to a certain point, beyond which it becomes limiting.
Carbon dioxide concentration: Higher CO2 levels enhance the rate of photosynthesis until saturation is reached.
Temperature: Optimal temperature for photosynthesis is between 20-25°C. Extreme temperatures can inhibit the process.
Water availability: Adequate water supply is crucial for photosynthesis as it is involved in various reactions and maintains turgidity of the chloroplast.
Pigment concentration: The amount and type of pigments, particularly chlorophyll, influence the efficiency of light absorption and photosynthesis.
Chloroplast structure: The organization and arrangement of chloroplasts within mesophyll cells optimize light capture and maximize photosynthetic efficiency.

4. Importance of Photosynthesis:

Conversion of light energy into chemical energy: Photosynthesis converts light energy from the sun into chemical energy stored in the form of glucose and other organic compounds.
Production of oxygen: Releases oxygen as a byproduct of water splitting during the light reactions, contributing to the Earth’s atmosphere.
Carbon sequestration: Removes carbon dioxide from the atmosphere, playing a crucial role in regulating the global carbon cycle and mitigating climate change.
Source of food and energy: Provides the foundation of food chains and supplies the energy required for almost all life forms on Earth.

5. Photorespiration and its Significance:

Photorespiration: A process that competes with photosynthesis in the consumption of RuBP. Oxygen reacts with RuBP instead of CO2, resulting in the release of CO2 and consumption of oxygen.
Factors affecting photorespiration: High temperature, low CO2 concentration, and high oxygen concentration favor photorespiration.
Significance: Although it reduces carbon fixation efficiency, photorespiration acts as a protective mechanism by dissipating excess energy, preventing photodamage, and maintaining redox balance in chloroplasts.

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