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Light-Dependent Reactions

Introduction to Light-Dependent Reactions

  • Light-dependent reactions are the first stage of photosynthesis, occurring in the thylakoid membranes of chloroplasts.

Overview of Photosynthesis

  • The overall equation for photosynthesis:
    • 6CO2 + 6H2O + \text{light energy} \rightarrow C6H{12}O6 + 6O2
  • Photosynthesis processes CO2 and H2O incrementally:
    • CO2 + H2O + \text{light energy} \rightarrow CH2O + O2
    • Where CH_2O represents the general formula for a sugar.

Electromagnetic Energy

  • Sunlight is a form of electromagnetic energy with visible light ranging from:
    • 380 to 750 nm in wavelength.

Wavelengths and Energy

  • Shorter wavelengths (e.g., violet light 380 nm) have higher energy than longer wavelengths (e.g., red light 750 nm).

Pigments in Photosynthesis

  • Pigments absorb light, notably:
    • Chlorophyll (a and b): absorbs violet, blue, and red light, reflecting green.
    • Pigments determine an action spectrum for photosynthesis, indicating light wavelengths that maximize photosynthetic activity.

Light Reactions Summary

  • The light reactions can be divided into three parts:
    1. Photoexcitation: Absorption of light by chlorophyll, exciting electrons.
    2. Electron Transport: Excited electrons are transferred through an electron transport chain, leading to a proton gradient.
    3. Chemiosmosis: Movement of protons through ATP synthase, producing ATP.

Detail on Photoexcitation

  • When a photon strikes chlorophyll, electrons become excited and move from a lower to a higher energy state.
  • The outcomes of excitation include:
    • Emission of fluorescence or thermal energy.
    • Energy transfer to neighboring molecules.
    • Transfer to an electron-accepting molecule in the electron transport chain.

Photosystems

  • The light-dependent reactions feature two main photosystems:
    • Photosystem II (PS II): Dominant pigment P680, absorbs light at 680 nm.
    • Photosystem I (PS I): Dominant pigment P700, absorbs light at 700 nm.
  • Photosystems consist of:
    • Reaction-center complex: contains chlorophyll a and a primary electron acceptor.
    • Light-harvesting complex: Various pigments transferring energy.

Electron Transport Chain (ETC)

  • Electrons from PS II are transferred through a series of proteins including plastoquinone (PQ), cytochrome b6-f complex, and plastocyanin (Pc).
  • The flow of electrons generates a proton motive force used to produce ATP through chemiosmosis.

Photosystem II Process

  1. Light energizes an electron in P680.
  2. Excited electron transferred to a primary electron acceptor.
  3. Water is split, providing electrons to replenish P680 and releasing oxygen as a byproduct
    • H2O ightarrow 2H^+ + 2e^- + \frac{1}{2} O2.

Production of NADPH

  • In PS I, electrons are re-energized and transferred to ferredoxin (Fd).
  • Ferredoxin passes electrons to NADP+ reductase, producing NADPH:
    • NADP^+ + 2H^+ + 2e^- \rightarrow NADPH + H^+.
  • NADPH provides high-energy electrons for the Calvin cycle.

ATP Production

  • The electrons' movement through the electron transport chain creates a proton gradient, driving ATP synthesis via chemiosmosis.
  • ATP formation is termed noncyclic photophosphorylation, highlighting its dependence on light energy.

Cyclic Electron Flow

  • In cases where ATP is needed without reducing NADP+, photosystem I can function independently, allowing cyclic electron flow:
    • Electrons are recycled back to PQ, leading to ATP production without generating NADPH or consuming water.
    • This serves to satisfy ATP demand in various chloroplast reactions.