Photo-Systems – The Light Traps
The orderly arrangement of photosynthetic pigments on thylakoid membrane is called photo-system.
Hill and Fay Benda11 (1960) proposed that photo-chemical phase of photosynthesis involves two events, one that tended to oxidize the cytochrome and the other that tended to reduce it. It is now known that oxidation is brought by far-red light of wavelengths longer than 680 nm and the reduction by red light of 680 nm. Another, chlorophyll a molecule was discovered by Kok with absorption maximum at 703 nm. This was called P700. P700 is present in much smaller amounts than chl. a 673 and chl. a 683.
The above mentioned experimental evidences and electron microscope studies led to conclusion that two separate groups of pigments exist in chloroplasts:
One that absorbs long red wavelengths and called photo-systems I (PS I).
While, the second absorbs wavelengths shorter than 680 nm and is called as photo-system II (PS II).
The photo-systems are linked together by an electron transport chain.
Composition of Photosystems
The photo-system I contains chlorophyll a, small amount of chlorophyll b and some beta-carotenes (about 50 molecules) attached to proteins by non-covalent bonds. One of the chlorophyll a molecule absorbs light near 700 nm as well as at shorter wavelengths. This molecule was named as P700 and declared Reaction Centre for PS I. The light energy absorbed by other chlorophyll a molecules and accessory pigments is transferred to this molecule. Two iron-containing proteins called Fe-S proteins are also present in PS I.
Photo-system II tow consists of about 200 molecules of chlorophyll a and b and about 50 carotenoids. Like PS I, a chlorophyll a molecule is able to absorb wavelength less than 680 nm, called P680 which accepts excited electrons of P680. PS II contains one or more Mn-containing proteins called the manganese protein. These proteins contain four Mn2+ ions and a chloride ion bridges two Mn2+ ions together.
Location of Photo-systems
The photo-systems are spatially separated. PS II is located predominantly in the membranes of grana thylakoids (stacked regions of grana lamellae), whereas the PS I is found almost exclusively in the membranes of stroma thylakoids and at the edges of the grana thylakoids.
Antenna Systems or Light Harvesting Complex
Besides photo-systems, two other major green bands can be separated from chloroplasts by electrophoresis. Each contains both chlorophyll a and chlorophyll b and xanthophyll’s but little number of beta-carotenes. These pigments act as light absorbing antenna systems and deliver energy to their respective reaction centers. These antenna systems are called light harvesting complexes and each photosystem has its own antenna system. The antenna system associates with PS I is called light harvesting complex I (LHC I), whereas the one associated with PS II is known as light harvesting complex II (LHC II).