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Photosynthesis: A Comprehensive Overview
Definition and Core Concept
- Definition: The process by which green plants and other organisms synthesize their own food (glucose).
- Energy Conversion: It converts light energy (solar energy) into chemical energy stored in the bonds of sugar molecules.
Chemical Reaction
- Overall Balanced Equation:
6CO₂ + 12H₂O --(Light Energy / Chlorophyll)--> C₆H₁₂O₆ + 6H₂O + 6O₂ - Reactants (Inputs): Carbon Dioxide (CO₂) and Water (H₂O). Sunlight is the energy source.
- Note: Oxygen (O₂) is a product, not a reactant.
- Products (Outputs):
- Main Product: Glucose (C₆H₁₂O₆).
- By-product: Oxygen (O₂), which is released into the atmosphere.
- Source of Liberated Oxygen: The oxygen gas released comes exclusively from the photolysis (splitting) of water molecules, not from carbon dioxide.
Site and Organisms
- Cellular Site: Occurs within organelles called chloroplasts.
- Primary Organ: The leaf is the main organ for photosynthesis.
- Lamina: The broad, flat part provides a large surface area to absorb sunlight and CO₂.
- Stomata: Tiny pores that allow for gas exchange (CO₂ enters, O₂ exits).
- Key Organisms:
- Global Oxygen Production: The majority of Earth’s oxygen is produced by algae in the oceans.
- Efficient Converter: Chlorella (a single-celled green alga) has maximum efficiency in converting solar energy to chemical energy.
Light Requirements and Effectiveness
- Light Requirement: Occurs only in the presence of visible light (wavelengths between 3800 Å and 7600 Å).
- Light Sources:
- Primarily sunlight, but it can also occur under artificial light.
- Some marine algae can utilize moonlight.
- Light Absorption by Chlorophyll:
- Absorbed Light: Strongly absorbs blue, red, and violet light.
- Reflected Light: Green light is not absorbed effectively and is reflected, making plants appear green.
- Most Effective Light for Photosynthesis:
- Red light is the most effective.
- Blue light is the next most effective.
- Evidence: Demonstrated by Engelmann’s experiment (1888), which showed maximum oxygen production (indicated by aerobic bacteria accumulation) under red light.
- Exception: Some bacteria (e.g., green sulphur bacteria) can perform photosynthesis using invisible infrared light.
Process Details and Energy
- Molecular Requirements for Glucose: To synthesize one molecule of glucose (C₆H₁₂O₆), the process requires:
- 6 molecules of CO₂
- 18 molecules of ATP
- 12 molecules of NADPH
- Site of Energy Conversion: The conversion of light energy into chemical energy (as ATP and NADPH) occurs in the chloroplasts during the light-dependent reactions.
- Role of Plant Parts:
- Leaves: Primary site due to chlorophyll content and adaptations for light/CO₂ absorption.
- Roots: Do not perform photosynthesis (lack chlorophyll). Their role is absorption and anchoring.
Environmental Impact and Daily Cycle
- Role in the Carbon Cycle: Photosynthesis removes CO₂ from the atmosphere.
- Contrasts with: Respiration, decay, and volcanic action, which release CO₂.
- Gas Exchange at Night:
- Photosynthesis stops at night due to the absence of light.
- Plants only respire, consuming O₂ and releasing CO₂.
- This is why it is considered harmful to sleep under a tree at night, as oxygen levels decrease and CO₂ levels increase locally.
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