Photosynthesis: Study Notes

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Photosynthesis

Photosynthesis occurs in two steps.

Light Reaction

• This reaction occurs in the presence of sunlight and is therefore also called the photochemical reaction.
• The light reaction consists of two primary processes:

1. Electron Transport Chain

Steps and Details

• PSII Activation:
  • Light excites chlorophyll in Photosystem II (PSII), initiating the transfer of electrons to pheophytin, QA, QB, and then to the plastoquinone pool.
• Proton Pumping:
  • Energy derived from the electrons is used to pump protons into the thylakoid space, creating a proton gradient.
• PSI Activation:
  • Electrons continue to Photosystem I (PSI), where they are re-excited by light and passed to ferredoxin, which then transfers them to NADP⁺ to form NADPH.

Overall Electron Flow

• PSII → Plastoquinone → Cytochrome Complex → Plastocyanin → PSI → Ferredoxin → NADP⁺.

2. Photophosphorylation (Cyclic and Non-cyclic)

• Non-cyclic Photophosphorylation:
  • Electrons move through the electron transport chain, reducing NADP to NADPH and generating ATP without cycling back to the beginning.
• Cyclic Photophosphorylation:
  • Electrons cycle between PSI, ferredoxin, and the cytochrome complex.
  • Only ATP is produced in this process; no NADPH or oxygen is formed.

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