10 Differences between Photosystem I and Photosystem II (PSI vs PSII)

September 2022 ยท 3 minute read
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. 
Difference between PS 1 and PS 2 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 and Photosystem II 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 

non cyclic photophosphorylation.		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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