Photosynthesis is a complex process that starts with light capture. Photosystems, packed with pigments like chlorophyll, absorb light energy. This energy excites electrons, kicking off a chain of reactions that produce ATP and NADPH, the energy currency of cells.

The Z-scheme shows how electrons flow through photosystems and electron transport chains. This process, called photophosphorylation, makes ATP. Water splitting, or photolysis, provides electrons and releases oxygen as a byproduct, fueling life on Earth.

Light Capture and Energy Conversion

Light capture by photosystem pigments

• Photosystems are protein complexes embedded in the thylakoid membrane that capture light energy for photosynthesis
  • Photosystem I (PSI) and Photosystem II (PSII) work together in a coordinated manner to drive the light-dependent reactions of photosynthesis
    • PSI contains P700, a specialized chlorophyll a molecule that serves as its reaction center
    • PSII contains P680, a specialized chlorophyll a molecule that serves as its reaction center
• Pigments in photosystems absorb specific wavelengths of light to harvest energy
  • Chlorophyll a serves as the primary pigment in photosystems
    • Absorbs mainly blue and red light while reflecting green light, giving plants their characteristic green color
  • Accessory pigments such as chlorophyll b and carotenoids (beta-carotene, lutein) enhance light absorption
    • Absorb light at different wavelengths than chlorophyll a and transfer the captured energy to chlorophyll a through resonance energy transfer
• Light energy excites electrons in chlorophyll a to a higher energy state, enabling their transfer to electron acceptors
  • Primary electron acceptor captures excited electrons from chlorophyll a, initiating the electron transport chain for energy production

Electron transport for energy production

• Electron transport chains (ETCs) transfer electrons from PSII to PSI, generating energy-rich compounds
  • Electrons from excited chlorophyll a in PSII are passed to the ETC, beginning the energy-producing process
  • Electrons are transferred through a series of redox reactions involving electron carriers
    • Cytochrome complex and plastoquinone are key electron carriers that facilitate the efficient transfer of electrons along the ETC
• Energy released from electron transport is used to pump protons (H+) into the thylakoid lumen, establishing a proton gradient
  • Creates a proton gradient across the thylakoid membrane, with a higher concentration of protons inside the lumen compared to the stroma
• ATP synthase harnesses the proton gradient to generate ATP through chemiosmosis
  • Protons flow down their concentration gradient through ATP synthase, driving the enzyme's rotational mechanism
  • Drives the synthesis of ATP from ADP and inorganic phosphate (Pi), providing energy for cellular processes
• Electrons from PSI are used to reduce NADP+ to NADPH, a key reducing agent in the Calvin cycle
  • Ferredoxin-NADP+ reductase catalyzes the reduction of NADP+ using electrons from PSI, ensuring a continuous supply of NADPH
  • NADPH serves as a reducing agent in the Calvin cycle, providing the necessary electrons for carbon fixation reactions

The Z-scheme and Photophosphorylation

• The Z-scheme describes the overall flow of electrons in the light reactions of photosynthesis
  • Illustrates the transfer of electrons from water through PSII, the electron transport chain, and PSI to NADP+
  • The name comes from the Z-shaped diagram representing the energy levels of electrons as they move through the process
• Photophosphorylation is the process of ATP production driven by light energy in photosynthesis
  • Non-cyclic photophosphorylation involves both PSII and PSI, producing both ATP and NADPH
  • Cyclic photophosphorylation involves only PSI and produces ATP without NADPH, allowing the cell to adjust the ATP:NADPH ratio as needed

Photolysis and Oxygen Generation

Photolysis and oxygen generation

• Photolysis is the light-driven splitting of water molecules, a crucial process in oxygenic photosynthesis
  • Occurs in the oxygen-evolving complex of PSII, which contains a cluster of manganese ions that catalyze water oxidation
• Light energy is used to oxidize water ($H_2O$) into protons (H+), electrons (e-), and molecular oxygen ($O_2$)
  • The overall reaction can be summarized as: $2H_2O \rightarrow 4H^+ + 4e^- + O_2$
• Protons (H+) released from water oxidation are released into the thylakoid lumen, contributing to the proton gradient for ATP synthesis
• Electrons (e-) from water replace those excited from chlorophyll a in PSII, ensuring a continuous flow of electrons
  • Maintains the flow of electrons through the ETC, enabling sustained energy production
• Oxygen ($O_2$) is released as a byproduct of photolysis, making photosynthesis the primary source of atmospheric oxygen
  • Oxygen is essential for aerobic respiration in most organisms (animals, plants, many microbes), supporting diverse life forms on Earth
  • Photosynthesis played a crucial role in the oxygenation of Earth's atmosphere over geological time, enabling the evolution of complex aerobic life