Photosynthesis powers life on Earth by converting light into chemical energy. Light reactions, occurring in thylakoid membranes, involve two photosystems working together to capture light and drive electron transport.

These processes create ATP and NADPH, fueling the Calvin cycle. The Z-scheme illustrates how electrons flow from water to NADP+, highlighting the intricate dance of energy conversion in plants.

Photosynthesis: Light Reactions and Photosystems

Photosystems in light capture

• Photosystems are protein complexes embedded in thylakoid membranes that capture light energy and convert it into chemical energy usable by the cell
  • Contain pigments such as chlorophyll a (primary pigment), chlorophyll b, and carotenoids (accessory pigments) that absorb specific wavelengths of light
  • Pigments are organized into light-harvesting complexes (LHCs) that funnel absorbed energy to the reaction center
• Two types of photosystems work together to drive light-dependent reactions: Photosystem I (PSI) and Photosystem II (PSII)
  • PSI has a reaction center called P700, which absorbs light at a wavelength of 700 nm
  • PSII has a reaction center called P680, which absorbs light at a wavelength of 680 nm
• When a pigment molecule absorbs a photon, it becomes excited and releases an energized electron
  • This electron is passed to an electron acceptor in the reaction center, initiating electron transport (plastoquinone in PSII, ferredoxin in PSI)

Electron transport chain function

• The electron transport chain (ETC) consists of a series of protein complexes and electron carriers located in the thylakoid membrane that shuttle electrons from PSII to PSI
  • Includes plastoquinone, cytochrome b6f complex, plastocyanin, and ferredoxin
• Electrons from excited PSII are passed to plastoquinone, then to cytochrome b6f complex, plastocyanin, and finally to PSI
  • As electrons move through ETC, energy is released and used to pump protons (H+) from stroma into thylakoid lumen, creating a proton gradient
• Proton gradient powers ATP synthesis via chemiosmosis
  • ATP synthase enzyme uses the proton gradient to catalyze formation of ATP from ADP and inorganic phosphate (Pi)
• Electrons from PSI are ultimately used to reduce NADP+ to NADPH via ferredoxin-NADP+ reductase
  • NADPH serves as a reducing agent to power the Calvin cycle and convert CO2 into organic compounds (glucose)

Photosystem I vs Photosystem II

• Photosystem II (PSII):
  1. Absorbs light with a wavelength of 680 nm at its reaction center (P680)
  2. Splits water molecules (H2O) into protons (H+) and oxygen (O2) through photolysis, releasing electrons
  3. Feeds electrons into the ETC via plastoquinone
• Photosystem I (PSI):
  1. Absorbs light with a wavelength of 700 nm at its reaction center (P700)
  2. Receives electrons from the ETC via plastocyanin
  3. Uses energized electrons to reduce NADP+ to NADPH via ferredoxin and ferredoxin-NADP+ reductase
• Both photosystems work in tandem, with PSII providing electrons to ETC and PSI using those electrons to generate NADPH

Z-scheme in linear electron flow

• The Z-scheme represents linear electron flow during light reactions, depicting the redox potential changes of electron carriers
  • Electrons follow a Z-shaped path as they are transferred from H2O to NADP+, hence the name
• Electrons from PSII are passed to ETC and eventually to PSI, allowing production of both ATP and NADPH in a linear fashion
  • This linear flow maintains the necessary balance between ATP and NADPH production for the Calvin cycle
• The Z-scheme highlights the key steps:
  1. Water photolysis at PSII, releasing electrons and O2
  2. Electron transport through ETC (plastoquinone, cytochrome b6f, plastocyanin)
  3. Light excitation of PSI and reduction of NADP+ to NADPH
  4. Proton gradient formation and ATP synthesis via chemiosmosis
• The Z-scheme emphasizes water's role as the original electron donor and oxygen as a byproduct of the light reactions