Photosynthesis (Ps)

Photosynthesis Overview

• Essential process for life on Earth
• Energy source for growth and metabolism of photoautotrophs

Summary of the Photosynthetic Reaction

• Photosynthesis converts light energy into chemical energy.
• Overall reaction:
  • $6CO<em>2 + 6H</em>2O + ext{light energy} <br /> ightarrow C<em>6H</em>{12}O<em>6 + 6O</em>2$
• Products include:
  • Glucose (C6H12O6), a carbohydrate
  • Oxygen (O2)

Fates of Glucose Produced by Photosynthesis

1. Cellular Respiration
   • Glucose can be converted into ATP (30 ATP molecules generated per glucose molecule).
2. Structural Carbohydrates
   • Forms cellulose for cell walls or starch for energy storage.
3. Precursor for Other Biomolecules
   • Can form carbon skeletons for:
     • Amino acids
     • Fats

Location of Photosynthesis in Plants

• Chloroplasts
  • Found primarily in leaves and green parts of stems, flowers.
• Stomata
  • Small openings in leaves for gas exchange:
  • CO2 enters
  • O2 exits
• Mesophyll Cells
  • Contain chloroplasts:
  • Palisade Mesophyll
    • Densely packed for maximum light absorption (more chloroplasts).
  • Spongy Mesophyll
    • Loosely arranged for gas exchange (fewer chloroplasts).
• Guard Cells
  • Control the opening and closing of stomata.

Structure of Chloroplasts

• Double-membrane organelle.
• Stroma
  • Fluid-filled space inside the chloroplast.
• Thylakoids
  • Flattened membrane sacs stacked in grana.
  • Lumen
  • Hollow center of thylakoids.
• Granum
  • Stack of thylakoids involved in light reactions.

Photosynthesis Process

Stages of Photosynthesis

1. Light Reactions (Light-Dependent Reactions)
   • Occur in thylakoid membranes.
   • Convert sunlight into chemical energy (ATP and NADPH).
   • Inputs:
     • Sunlight
     • Water (H2O)
   • Outputs:
     • ATP
     • NADPH
     • Oxygen (O2)
2. Calvin Cycle (Light-Independent Reactions)
   • Occurs in the stroma.
   • Uses ATP and NADPH from light reactions to fix carbon:
     • CO2 is converted into glucose.

Detailed Mechanism of Light Reactions

• Sunlight (Electromagnetic energy):
  • Composed of photons.
  • Wavelength range for photosynthesis is between 380 nm to 750 nm (visible light).
• Photosynthetic Pigments
  • Absorb specific wavelengths:
  • Chlorophyll a
    • Main pigment, absorbs blue-green light.
  • Chlorophyll b
    • Accessory pigment, absorbs yellow-green light.
  • Carotenoids
    • Accessory pigments, provide photoprotection, absorbs orange/yellow light.
• Spectrophotometry
  • Used to determine the absorption spectra of pigments.

Electron Excitation in Chlorophyll

• Ground State
  • Stable state of chlorophyll.
• Excited State
  • Unstable state upon absorption of light energy leads to:
  • Fluorescence (light and heat).
• Photosystems
  • Organized structures composed of chlorophyll:
  • PS II (Photosystem II)
    • Lower energy level.
  • PS I (Photosystem I)
    • Slightly higher energy level, both contribute to light reactions.

Electron Transport Chain and Energy Production

• Linear Electron Flow
  • Occurs in sequence between PS II and PS I, leading to:
  • Oxidation of water (producing O2).
  • Reduction of NADP+ to NADPH.
  • Generation of ATP via chemiosmosis (proton motive force).
• Cyclic Electron Flow
  • Involves only PS I, serves to pump protons for additional ATP synthesis without producing NADPH.

Summary of Light Reactions Outputs

• Inputs:
  • Sunlight, H2O
• Outputs:
  • ATP, NADPH, O2

Calvin Cycle Details

• Inputs
  • 6 CO2
  • ATP (energy source)
  • NADPH (reducing power)
• Processes
  1. Carbon Fixation
  • CO2 fixed by combining with RuBP (Ribulose bisphosphate) to form a 6C intermediate.
  1. Reduction Phase
  • 3C molecules are formed through reduction using ATP and NADPH.
  1. Regeneration of RuBP
  • Requires ATP for conversion back to RuBP to allow cyclic continuity.
• Outputs
  • 1 glucose molecule requires:
  • 6 CO2
  • ~18 ATP
  • ~12 NADPH

Implications of Photosynthesis

• Vital for energy flow in ecosystems and the carbon cycle.
• Supports life forms dependent on oxygen.