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Triplet chlorophyll is a potent photosensitiser of molecular oxygen forming singlet oxygen which can cause oxidative damage to the pigments, lipids and proteins of the photosynthetic thylakoid membrane. To counter this problem, one photoprotective mechanism is so-called non-photochemical quenching (NPQ), which relies upon the conversion and ...
The excited, triplet state photosensitizer then reacts with a substrate molecule which is not molecular oxygen to both form a product and reform the photosensitizer. Type I photosensitized reactions result in the photosensitizer being quenched by a different chemical substrate than molecular oxygen.
The light-harvesting complex (or antenna complex; LH or LHC) is an array of protein and chlorophyll molecules embedded in the thylakoid membrane of plants and cyanobacteria, which transfer light energy to one chlorophyll a molecule at the reaction center of a photosystem. The antenna pigments are predominantly chlorophyll b, xanthophylls, and ...
The reaction center is made of two chlorophyll molecules and is therefore referred to as a dimer. [11] The dimer is thought to be composed of one chlorophyll a molecule and one chlorophyll a′ molecule. However, if P700 forms a complex with other antenna molecules, it can no longer be a dimer. [13]
The light harvesting complex in purple bacteria is multifunctional; at high light intensities, the light harvesting complex typically switches into a quenched state through a conformational change of the PPC, and at low light intensities, the light harvesting complex typically reverts to an unquenched state. [11]
The cofactors can be pigments (like chlorophyll, pheophytin, carotenoids), quinones, or iron-sulfur clusters. [7] Each photosystem has two main subunits: an antenna complex (a light harvesting complex or LHC) and a reaction center. The antenna complex is where light is captured, while the reaction center is where this light energy is ...
[9] [10] After absorbing a photon, the carotenoid transfers its excited electron to chlorophyll for use in photosynthesis. [9] Upon absorption of light, carotenoids transfer excitation energy to and from chlorophyll. The singlet-singlet energy transfer is a lower energy state transfer and is used during photosynthesis. [7]
Photosensitizers such as chlorophyll may convert triplet (3 O 2) to singlet oxygen: [6] Singlet oxygen is highly reactive with unsaturated organic compounds. Carotenoids, tocopherols, and plastoquinones contained in chloroplasts quench singlet oxygen and protect against its toxic effects.