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.
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 (PS I vs PS II)
Photosystem I (PS I)
|
Photosystem II (PS II)
|
PS
I is located at the outer surface of the grana thylakoid membrane (non appressed
granal regions and stroma lamella).
|
PS
II is located at the inner surface of the grana thylakoid membrane (appressed
granal region).
|
The
photocentre or reaction centre is P700.
|
The
photocentre is P680.
|
PS
I has an iron-sulphur (FeS) type reaction centre (or type I)
|
PS
II has a quinone type reaction centre (also known as Q-Type or type II)
|
The
core complex of PSI is composed by a smaller number of proteins (~15
subunits) than PSII
|
The
core of PSII is a multi-subunit complex composed of about ~25-30 subunits.
|
Pigments
absorb longer wavelengths of light (>680nm).
|
Pigments
absorb shorter wavelengths of light (<680nm).
|
Rich
in chlorophyll a than chlorophyll-b
|
Rich
in chlorophyll b than chlorophyll-a
|
Participates
in cyclic as well as
|
Participates
only in non-cyclic photophosphorylation.
|
It
is not associated with photolysis of water.
|
It
is associated with photolysis of water.
|
PS
I generates a strong reducing agent
(reducing NADP+ to NADPH) and a weak oxidant
|
PS
II is the strong oxidant (capable of
oxidizing H2O) and a weak reductant
|
Main
function is ATP synthesis in cyclic photophosphosphorylation and NADPH
synthesis in non-cyclic photosynthesis.
|
Main
functions are ATP synthesis and hydrolysis of water.
|
Reference: Caffarri, S., Tibiletti, T., C
Jennings, R., & Santabarbara, S. (2014). A comparison between plant
photosystem I and photosystem II architecture and functioning. Current Protein
and Peptide Science, 15(4), 296-331.
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