Antenna complex

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The antenna complex is an array of chlorophyll molecules embedded in the thylakoid membrane which transfer energy to a pair of chlorophyll a molecules at the reaction center of a photosystem.

The antenna pigments are predominantly chlorophyll a, chlorophyll b and carotenoids. Their absorption spectra are non-overlapping in order to broaden the range of light that can be absorbed in photosynthesis. The carotenoids have another role as an antioxidant to prevent photo-oxidative damage of chlorophyll molecules. Each antenna complex has between 250 and 400 pigment molecules and the energy they absorb is shuttled by resonance energy transfer to a specialized chlorophyll a at the reaction center of each photosystem .

For photosystem II, when either of the two chorophyll a molecules at the reaction center absorb energy, an electron is excited and transferred to an electron-acceptor molecule, phaeophytin, leaving the chlorophyll a in an oxidized state. The oxidised chlorophyll a replaces the electrons by photolysis that involves the reduction of water molecules to oxygen, protons and electrons.