Introductory Crop Physiology (1+1)

Photosystems (Two pigments systems)

Photosystems (Two pigment systems)

The discovery of red drop and the Emerson’s enhancement effect led the scientists to suggest that photosynthesis is driven by two photochemical processes.
These processes are associated with two groups of photosynthetic pigments called as pigment system I and pigment system II.
Wavelength of light shorter than 680 nm affect both the pigments systems while wavelength longer than 680 nm affect only pigment system I.
• In green plants, pigment system I contains chlorophyll a, b and carotene.
• In this pigment system, a very small amount of chlorophyll a absorbing light at 700 nm, known as P700 however constitutes the reaction centre of photosystem I.


• The pigment system II contains chlorophyll b and some forms of chlorophyll a (such as chlorophyll a 662, chlorophyll a 677 and chlorophyll a 679) and xanthophylls.
  
• A very small amount of special form of chlorophyll called P680 constitute the reaction centre of pigment system II.
  
• Carotenoids are present in both the pigment systems


• The two pigment systems I and II are interconnected by a protein complex called cytochrome b6–f complex.
  
• The other intermediate components of electron transport chain viz., plastoquinone (PQ) and plastocyanin (PC) act as mobile electron carriers between the complex and either of the two pigment systems.
  
• The light energy absorbed by other pigment is ultimately trapped by P700 and P680 forms of chlorophyll a which alone take part in further photochemical reaction.


• Pigment system I (PSI) complex consists of 200 chlorophylls, 50 carotenoids and a molecule of chlorophyll a absorbing light at 700 nm(P700) and this constitute the reaction centre of photosystem I.
  
• Pigment system II (PSII) complex consists of 200 chlorophylls, 50 carotenoids and a mole of chlorophyll a absorbing light at 680 nm, called P 680 at the centre.
  
• This constitutes the reaction centre of pigment system II.


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