Photosystems or Pigment Systems

    Photosynthesis involve the participation of two distinct groups called photosystem I and      photosystem II. Each photosystem has a pigment protein complex composed of chlorophyll a, chl b, carotenoids and other components required for electron transport during light reaction. These pigment systems are as fllows:-

Photosystem I (PS I) :- It is a photosynthetic pigment system with some electron carriers is present on both nonappressed part of the grana thylakoids as well as on the stroma thylakoids. This system has more chlorophyll a molecules and chlorophyll b and carotenoids are less.  Photosystem I consists of a photocentre, light harvesting complex and some electron carriers. Photocentre contains molecules of chlorophyll a700 and P700 ( a special chlorophyll molecules that absorb visible light near 700 nm wavelength). PS I has a reducing agent A0, A1, Fe Sx, Fe SA and Fe SB ( iron sulphur protein), Fd ( ferrodoxin), cytochrome b6 - f complex and plastocyanin. PS I takes part in both cyclic and non cyclic photophosphorylation ( it is the synthesis if ATP in the presence of sunlight). PS I can carry cyclic photophosphorylation independently.photosys

Photosystem II :- It is a photosynthetic pigment system present in appressed part of the thylakoid. PS II contains a chl a, chl b and carotenoids. Chl a and b are in equal content. PS II complex contains a molecule of chl a680 or P680  ( a special chlorophyll a molecule that absorb visible light near 680 nm wavelength).  Carotenoids are higher as compared to PS I. PS II consists of a photocentre, oxygen evolving complex, light harvesting comples and some electron carriers. Oxygen evolving complex contains a Mn2+, Ca2+ and Cl ions. PS II is only participate in the non cyclic photophosphorylation.