Photosynthesis takes place in two steps: Light reaction and Dark reaction. Light reaction occurs in the grana of chloroplast. The chlorophyll absorbs the light energy. Hence the chlorophyll is called a photosystem or pigment system. Chlorophylls are of different types and they absorb different wavelengths of light. Accordingly chlorophylls exist two photosystems, namely photosystem I (PS I) and photosystem II (PS II).
Both photosystem (PS I and PS II) are affected by light with wavelengths shorter than 680nm (nanometer), while photosystem I is affected by light with wavelengths longer than 680nm.
Photosystem I includes the following pigments: Chlorophyll b, Chlorophyll -a 670, Chlorophyll -a 680, Chlorophyll -a 695, Chlorophyll -a 700 or P700, Carotenoids.
In photosystem I, P700 form of chlorophyll a is the active reaction centre.
Photosystem II includes the following pigments: Chlorophyll b, Chlorophyll - a 660, Chlorophyll -a 670, Chlorophyll -a 680 or P680, Chlorophyll -a 695, Chlorophyll - a 700 or P700, Phycobilins, Xanthophylls.
In photosystem II, P680 form of chlorophyll a is the active reaction centre.
Photosystem I and Photosystem II
Photosystem I (PS I)
Photosystem II (PS II)
|PS I is located at the outer surface of the grana thylakoid membrane.||PS II is located at the inner surface of the grana thylakoid membrane.|
|The photocentre is P700.||The photocentre is P680.|
|Pigments absorb longer wavelengths of light (>680nm).||Pigments absorb shorter wavelengths of light (<680nm).|
|Participates in cyclic as well as non cyclic photophosphorylation.||Participates only in non cyclic photophosphorylation.|
|It is not associated with photolysis of water.||It is associated with photolysis of water.|
|Main function is ATP synthesis.||Main functions are ATP synthesis and hydrolysis of water.|