Photosystem I

Photosystem I is a pigment–protein complex in oxygenic photosynthesis that absorbs light and transfers high-energy electrons to ferredoxin, enabling NADPH production.

P700: The special chlorophyll a pair at Photosystem I’s reaction center, with a light-induced absorption change near 700 nanometers. Its photoexcitation initiates the charge separation that powers Photosystem I.
Thylakoid membrane: The internal membrane system of chloroplasts and cyanobacteria where photosynthetic light reactions occur. Photosystem I is embedded in this membrane, which organizes its electron-transfer partners.
Calvin cycle: A set of reactions that uses carbon dioxide, ATP, and NADPH to produce carbohydrate precursors in photosynthetic organisms. NADPH generated downstream of Photosystem I supplies reducing power for carbon fixation.
Anoxygenic photosynthesis: Photosynthesis that uses electron donors other than water and does not release oxygen. Its reaction centers show alternatives to the two-photosystem oxygenic arrangement containing Photosystem I.
Plastocyanin: A copper-containing protein that carries electrons through the thylakoid lumen to Photosystem I. It replenishes electrons lost when P700 is photoexcited.
Antenna complex: An array of pigment-binding proteins that absorbs light and transfers excitation energy to a reaction center. Its pigments broaden the light-harvesting capacity of Photosystem I.
NADPH: A reduced nicotinamide adenine dinucleotide phosphate molecule that donates electrons in biosynthetic reactions. Photosystem I electron transfer helps generate this reducing agent for cellular biosynthesis.
Bacterial reaction center: A light-driven protein complex in photosynthetic bacteria that separates charge across a membrane. Its simpler single-reaction-center architecture contrasts with oxygenic Photosystem I’s multisubunit complex.
Ferredoxin: A small iron–sulfur protein that accepts electrons from Photosystem I and transfers them to enzymes. It is the immediate electron acceptor named in Photosystem I’s defining function.
Iron–sulfur protein: A protein containing iron–sulfur clusters that mediate electron transfer in biological systems. Photosystem I uses iron–sulfur centers to pass electrons toward ferredoxin.

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