Chloroplasts & Photosynthesis

Introduction to Photosynthesis

• Photosynthesis is a vital process that converts light energy into chemical energy stored in glucose.

Key Components

• Sunlight, chlorophyll, water (H₂O), and carbon dioxide (CO₂) are essential for photosynthesis.

Learning Objectives

1. Structure and Function
   • Compare and contrast mitochondria and chloroplasts.
2. Stages of Photosynthesis
   • Outline the two main stages: light reactions and light-independent reactions (Calvin Cycle).
3. Function of RuBisCO
   • Analyze RuBisCO's role in carbon fixation.

Structure of Chloroplasts

• Comparison to Mitochondria
  • Chloroplasts resemble mitochondria but have three compartments:
    • Outer membrane
    • Inner membrane
    • Thylakoid membrane (third layer) containing chlorophyll
• Size: Chloroplasts are approximately 2-10 μm in diameter.
• Key parts:
  • Stroma (fluid)
  • Thylakoids (stacked to form grana)

Light Absorption by Chlorophyll

• Chlorophyll absorbs mainly red (600-700 nm) and blue (400-500 nm) light.
• When absorbing light, chlorophyll becomes excited and transfers energy to high-energy electrons, enabling chemical-bond energy formation.

Photosynthesis and Cellular Respiration

• Photosynthesis Equation:
  • $CO2 + H2O + ext{sunlight} ightarrow O2 + ext{sugars}$
• Cellular Respiration Equation:
  • $ext{sugars} + O2 ightarrow CO2 + H2O + ext{energy}$
• Both processes are complementary; the products of one serve as the reactants for the other.

The Carbon Cycle

• The cycle involves the movement of CO₂ through the atmosphere and water via photosynthesis and cellular respiration by plants, algae, and some bacteria.
• Importance: Essential for sustaining food chains and ecosystems.

Stages of Photosynthesis

Stage 1: Light Reactions

• Location: Thylakoid membrane.
• Process:
  • Light energy is converted into chemical energy (ATP and NADPH).
  • Water (H₂O) is split to release electrons.
  • Outputs: ATP, NADPH, and O₂.

Stage 2: Light-Independent Reactions (Calvin Cycle)

• Location: Stroma.
• Process:
  • CO₂ is fixed into organic molecules using ATP and NADPH generated in the light reactions.
  • Enzyme involved: RuBisCO, which catalyzes the reaction of CO₂ with RuBP.
• Calvin Cycle Reaction:
  • $3 CO2 + 9 ATP + 6 NADPH ightarrow G3P$ (glyceraldehyde 3-phosphate)

The Role of RuBisCO

• RuBisCO (Ribulose-1,5-bisphosphate Carboxylase/Oxygenase) is the enzyme that catalyzes the first major step of carbon fixation in the Calvin cycle.
• It facilitates the reaction between carbon dioxide and ribulose bisphosphate to form 3-phosphoglycerate (3-PGA).

Complementary Nature of Photosynthesis and Cellular Respiration

• Energy transformation processes in plants:
  • Chloroplasts engage in photosynthesis, while mitochondria handle respiration.
  • Both organelles generate and utilize ATP as the primary energy currency of the cell.

Summary

1. Chloroplasts: Similar to mitochondria but contain a third compartment (thylakoid).
2. Photosynthesis: Converts light energy into ATP and NADPH, followed by carbon fixation to produce sugars.
3. Storage: Sugars from photosynthesis can be stored as starch or utilized for energy.

• Photosynthesis is crucial for life on Earth, providing the oxygen we breathe and the glucose that serves as energy for most living organisms.