(Biology Note) – Photosynthesis (Detailed)

 


🌿 Biology Notes – Photosynthesis (Detailed)

📌 1. Introduction

Photosynthesis is one of the most essential biological processes on Earth. It is the method by which green plants, algae, and certain bacteria use sunlight to synthesize food from carbon dioxide and water. This process not only provides energy for the plant itself but also forms the foundation of the food chain and produces the oxygen we breathe.

🌞 2. Definition

Photosynthesis is the process through which autotrophic organisms (like green plants) convert light energy, usually from the sun, into chemical energy in the form of glucose (a type of sugar). This process takes place primarily in the chloroplasts of plant cells.

⚛️ 3. General Equation of Photosynthesis


6CO_2 + 6H_2O + \text{light energy} \xrightarrow{\text{chlorophyll}} C_6H_{12}O_6 + 6O_2
  • CO₂ (carbon dioxide) comes from the atmosphere.
  • H₂O (water) is absorbed by the roots from the soil.
  • Chlorophyll absorbs sunlight.
  • C₆H₁₂O₆ (glucose) is the sugar produced.
  • O₂ (oxygen) is released as a by-product.

🧬 4. Structure of Chloroplast

  • Chloroplast is a double-membrane organelle found mainly in the mesophyll cells of leaves.
  • Inside the chloroplast:
    • Thylakoids: Flattened sacs where light-dependent reactions occur.
    • Grana: Stacks of thylakoids.
    • Stroma: Fluid-filled space where the Calvin cycle (light-independent reactions) takes place.
    • Chlorophyll: Green pigment embedded in thylakoid membranes.

🔬 5. Stages of Photosynthesis

A. Light-dependent Reactions (Occurs in the Thylakoid Membranes)

  • Requires sunlight.
  • Water molecules are split (photolysis), releasing:
    • Oxygen (O₂) as a by-product.
    • Hydrogen ions (H⁺) and electrons (e⁻) used to form:
      • ATP (Adenosine Triphosphate)
      • NADPH (Reduced Nicotinamide Adenine Dinucleotide Phosphate)

B. Light-independent Reactions (Calvin Cycle – Occurs in the Stroma)

  • Does not require light directly.
  • Uses ATP and NADPH (from light-dependent reactions) and CO₂ to synthesize glucose.
  • Main enzyme involved: RuBisCO
  • Final product: Glucose (C₆H₁₂O₆)

🌱 6. Factors Affecting Photosynthesis

  1. Light Intensity
    • Higher intensity increases the rate up to a certain point.
  2. Carbon Dioxide Concentration
    • More CO₂ increases the rate until the plant reaches saturation.
  3. Temperature
    • Enzymes involved work best at an optimal temperature (~25–35°C).
  4. Water Availability
    • Essential for photolysis. Lack of water can limit the rate.
  5. Chlorophyll Content
    • More chlorophyll = more light absorbed = faster photosynthesis.

📈 7. Significance of Photosynthesis

  • Primary Source of Food: All food chains begin with plants.
  • Oxygen Production: Essential for the respiration of most living organisms.
  • Carbon Dioxide Balance: Helps regulate the CO₂ levels in the atmosphere.
  • Fuels and Resources: Plants store solar energy in the form of biomass, which can later be used as biofuels.

🔄 8. Comparison with Cellular Respiration

Feature Photosynthesis Cellular Respiration
Occurs in Chloroplasts Mitochondria
Reactants CO₂, H₂O, sunlight Glucose, O₂
Products Glucose, O₂ CO₂, H₂O, energy (ATP)
Energy Conversion Light → Chemical Chemical → Usable Energy (ATP)
Type of Organism Autotrophs (plants, algae, bacteria) All living organisms

🧠 9. Fun Facts

  • Only about 1–2% of sunlight is actually used in photosynthesis.
  • Earth’s oxygen is largely produced by phytoplankton in oceans.
  • Some plants (like cacti) use modified photosynthesis called CAM photosynthesis to survive in dry environments.

📝 10. Conclusion

Photosynthesis is not just a plant process—it is the cornerstone of life on Earth. Through this elegant process, light energy from the sun is stored in the bonds of organic molecules, driving the entire biosphere. Understanding photosynthesis helps us appreciate how delicate and interconnected life is, from tiny algae to towering trees and even to ourselves.


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