WK5.13: Photosynthesis

Overview of Photosynthesis

  • Photosynthesis is an essential biochemical process that converts light energy into chemical energy stored in glucose.
  • Consists of two main stages: Light Reactions and the Calvin Cycle (Dark Reactions).

Important Concepts

Metabolism

  • Metabolism involves the transformation of food into energy.
  • Heterotrophs consume organic molecules from other organisms (e.g., animals, plants).
  • Autotrophs produce organic molecules from inorganic sources (e.g., plants using CO2).

Autotrophs and Photoautotrophs

  • Most plants, algae, and certain bacteria are photoautotrophs that use light for energy.
  • Photoautotrophs obtain energy from sunlight to convert carbon dioxide and water into glucose and oxygen.

Photosynthesis Reactions

General Photosynthesis Equation

  • 6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂
Components involved:
  • Light: Source of energy.
  • Water (H₂O): Provides electrons and protons.
  • Carbon Dioxide (CO₂): Carbon source for glucose.
  • Oxygen (O₂): By-product released into the atmosphere.

Structure of Chloroplast

  • Chloroplasts are the organelles where photosynthesis takes place.
    • Comprised of outer and inner membranes with an intermembrane space.
    • Stroma: Dense fluid where the Calvin cycle occurs.
    • Thylakoids: Membranous sacs where light reactions occur.
    • Grana: Stacks of thylakoids.
  • Chlorophyll: Green pigment located in the thylakoid membranes that captures light energy.

Light Reactions

  • Occur in the thylakoid membranes and capture light energy to produce ATP and NADPH.
  • Involved Photolysis of water to produce electrons, protons, and oxygen.
Steps in Light Reactions:
  1. Photon absorption by chlorophyll excites electrons.
  2. Electrons are transferred to the Electron Transport Chain (ETC).
  3. Water molecules split (photolysis) - releasing oxygen and providing electrons for the chlorophyll.
  4. Production of ATP via chemiosmosis (proton gradient through ATP synthase).
  5. Formation of NADPH from NADP+ by using electrons from the ETC.

Calvin Cycle (Light-Independent Reactions)

  • Occurs in the stroma.
  • Fixes CO₂ through a series of reactions, ultimately synthesizing glucose.
Steps in the Calvin Cycle:
  1. Carbon Fixation: CO₂ is attached to ribulose bisphosphate (RuBP) via the enzyme RuBisCO to form a 6-carbon intermediate that splits into two 3-phosphoglycerate (3-PGA) molecules.
  2. Reduction Phase: 3-PGA is converted into glyceraldehyde-3-phosphate (G3P) using ATP and NADPH from light reactions.
  3. Regeneration of RuBP: Some G3P molecules are used to regenerate RuBP, enabling the cycle to continue.
  4. Production of glucose: The remaining G3P can be used to form glucose, succumbing to glycolysis and cellular respiration.

Importance of Photosynthesis

  • Transforms solar energy into chemical energy in the form of glucose, serving as an essential energy source for most life forms.
  • Released oxygen is vital for aerobic respiration in plants and animals.
  • Stomata play a crucial role in gas exchange, regulating the intake of CO₂ and release of O₂ and water vapor.

Conclusion

  • Understanding photosynthesis is critical in biology, as it explains how autotrophic organisms produce energy and sustain life on Earth.
  • Review will continue with related concepts in metabolism for a comprehensive understanding.