Anabolic Pathways and Photosynthesis

Introduction to Anabolic Pathways

• Focus on ANABOLISM, building biomolecules, shifts from CATABOLISM which extracts energy from biomolecules.

Photosynthesis Overview

• Basic Equation:

  • $CO<em>2 + H</em>2O ⇌ CH<em>2O + O</em>2$ (Photosynthesis combines CO2 and water to form sugars and releases oxygen).

• Importance of Sunlight:

  • The ultimate energy source for all life on Earth. Required for complex structure formation, which increases entropic cost.

Key Concepts in Photosynthesis

• Light Reactions:

  • Energy from sunlight triggers electron flow and oxygen evolution independent of CO2.

  • Key experiments that established photosynthesis involve:

  • Hill Reaction: Light causes O2 evolution from water.

  • Ochoa Reaction: Light causes reduction of NADP.(Independent of CO2).

• Energy Conversion:

  • Light energy is converted to ATP through Photophosphorylation.

  • Main reactions:

  • $2H<em>2O ⇌ O</em>2 + 4e^- + 4H^+$

  • $4e^- + 4H^+ + CO<em>2 ⇌ CH</em>2O + H_2O$

  • These half-reactions are thermodynamically unfavorable.

The Electromagnetic Spectrum in Photosynthesis

• Light and Energy:

  • Energy of light governed by:

  • Planck’s constant $h = 6.626 imes 10^{-34} J\bullet s$

  • Speed of light $c = 3 imes 10^{8} m/s$

  • For one mole of photons, energy is calculated by:

  • $E = 2.84 imes 10^{-19} J/photon imes 6.022 imes 10^{23} photons/einstein$

    • Resulting in $E = 171 kJ/einstein$

  • Sufficient energy in light photons to produce multiple ATP.

Role of Pigments

• Chlorophyll Absorption:

  • Different chlorophylls absorb light at different energies (Chl a vs. Chl b).

• Charge Separation:

  • Upon light absorption, chlorophyll excites electrons, leading to charge separation in the reaction center (P680).

The Photosystems

• Photosystem II (P680):

  • Primary function is the oxidation of water and evolution of O2.

• Photosystem I (P700):

  • Produces NADPH, the strongest known biological reductant.

Electron Transport Chain in Photosynthesis

• Electron Flow:

  • Electrons are transferred from water through the plastoquinone to cytochrome b6f complex, and then to plastocyanin before reaching Photosystem I (PS I).

• Proton Motive Force:

  • Protons are translocated during electron transport, generating a gradient for ATP synthesis.

NADPH Production

• NADP+ Reduction:

  • Reduced to NADPH through the action of NADP+ reductase, using electrons from ferredoxin (Fd).

  • Overall reaction: $2NADP^+ + 2H<em>2O ⇌ O</em>2 + 2NADPH + 4H^+$

Summary of Photosynthesis Process

• Overall Reaction Summary:

  • Incorporates light energy to convert CO2 and water into glucose and oxygen, establishing the foundation of life through biosynthesis.

• Red-Drop Experiment:

  • Demonstrated the presence of two distinct photosystems.