light-dependent reactions

Pathways of Photosynthesis

  • Solar Energy

    • Most abundant source of energy on Earth.

    • A single day of solar energy can supply human energy needs for 55 years.

    • Research is ongoing to capture and use sunlight efficiently.

    • Organisms have harnessed this energy pathway for over 2 billion years.

Photosynthesis in Eukaryotes and Prokaryotes

  • Photosynthesis occurs in chloroplasts of specialized cells in eukaryotes.

  • In prokaryotes, the process occurs in the cytosol and along the cell membrane.

Light-Dependent Reactions

  • Photosystems

    • Photosystem II and Photosystem I are the key light-capturing complexes.

    • The electron carriers alternate between oxidation and reduction, similar to mitochondrial systems.

Photosystem II

  • Early photosynthetic organisms used water as a source of hydrogen and electrons.

  • The splitting of water (photolysis) occurs here, producing oxygen.

    • Oxygen accumulation led to aerobic life's evolution.

  • P680:

    • Light energy absorbed by antenna complex excites P680, leading to P680*.

    • The excited electron is transferred to the primary acceptor, resulting in a positive P680*.

    • P680* can oxidize water molecules to replenish lost electrons.

Linear Electron Transport and ATP Synthesis

  1. Oxidation of P680:

    • Excitation of P680 results in transfer to the primary acceptor.

  2. Plastoquinone Oxidation:

    • Electrons pass to plastoquinone, moving protons into the lumen, increasing proton concentration.

  3. Electron Transfer:

    • Electrons transfer to plastocyanin, which shuttles them to photosystem I.

  4. Oxidation of P700:

    • Light absorption in photosystem I excites P700, transferring electrons to the primary acceptor.

  5. Electron Transfer to NADP+:

    • Electrons passed to ferredoxin which reduces NADP+ to NADPH.

Formation of NADPH

  • A second electron and a proton from the stroma reduce NADP, creating NADPH.

  • NADPH carries high-energy electrons, establishing a proton gradient.

Chemiosmotic Synthesis of ATP

  • Similar to cellular respiration, a proton gradient is established across the thylakoid membrane.

  1. Protons Movement:

    • Protons enter lumen via plastoquinone's redox reactions.

  2. Protons from Water Splitting:

    • Water splitting increases proton concentration.

  3. Reduction of NADPH:

    • Protons removed when NADPH is formed, balancing concentrations.

  • ATP synthase allows protons to flow back into the stroma, generating ATP via chemiosmosis.

The Role of Light Energy

  • Electron flow through the transport chain mirrors spontaneous downhill movement in respiration.

  • Photosystems I and II work to boost electron energy levels.

  • Z Scheme:

    • Describes the energy pathway of electrons in photosynthesis.

Linear Electron Transport - A Balance Sheet

  • 2H2O → 4H+ + 4e- + O2 explains photon absorption for O2 production.

    • Eight photons of light are required to produce one O2 molecule (four per photosystem).

Cyclic Electron Transport

  • Photosystem I functions independently in cyclic electron transport.

  • Reduced ferredoxin donates electrons back to plastoquinone, continuously moving protons.

  • Produces ATP without oxidizing water or reducing NADP+.

  • Important for the Calvin cycle, as it provides ATP needed for CO2 reduction.