The difference between photosystem I and photosystem II is primarily due to the difference in active reaction centre and photon absorption. P700 is the active reaction centre of PS-I, while P680 is the active reaction centre of PS-II. PS-I absorbs light of a longer wavelength (between 725-1035 nm), while PS-II absorbs light of a shorter wavelength (Chlorophylls are the light-harvesting pigments that constitute the photosystem and pigment system.

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A photosystem possesses an antenna complex (200-300 light-harvesting pigment molecules) and a reaction centre. Based on chlorophyll’s different absorption wavelength, two photosystems (PS I and II) generally exist. The auxiliary accessory pigments trap photons and hand them over to the specialized protein pigments of the reaction centre.

Content: Photosystem I Vs Photosystem II

Comparison Chart

PropertiesPhotosystem IPhotosystem II
MeaningPS-I is the photo centre that absorbs photons by the association of light harvesting pigments, reaction centre (P700) and other accessory pigments to produce NADPHPS-II is the photo centre that absorbs photons by the association of light harvesting pigments, reaction centre (P680) and other accessory pigments to produce ATP and oxygen via photolysis of water
LocationFound in the outer surface of the thylakoid membraneFound in the inner surface of the thylakoid membrane
Reaction centreP700 is an active reaction centreP680 is an active reaction centre
Photon absorptionThe light harvesting pigments of photosystem 1 absorb photons having wavelengths of 700 nm (P700)The light harvesting pigments of photosystem2 absorb photons having wavelengths of 680 nm (P680)
Involvement in photophosphorylationIt is involved in cyclic as well as non-cyclic photophosphorylationIt is only involved in non-cyclic photophosphorylation
Photolysis of waterPS-I do not carry out photolysis of waterPS-II results in the photolysis of water
SubunitsIt comprises psaA and psaB subunitsIt comprises D1 and D2 subunits
Protein pigmentsPhotosystem I includes chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, chlorophyll B, and carotenoids as the photo pigmentsPhotosystem II includes chlorophyll A-660, chlorophyll A-670, chlorophyll A-680, chlorophyll A-695, chlorophyll A-700, chlorophyll B, xanthophylls and phycobilins as the photo pigments
Chlorophyll to carotenoid ratio20-30 :13-7 :1
Oxygen productionDoes not occurOxygen production occurs
NADPH productionNADPH production occursDoes not occur
Type of reaction centrePS-I possesses iron sulphur or type-I RC PS-II possesses Q (Quinone) type or type-II RC
FunctionIt primarily produces NADPH It primarily produces ATP and causes water hydrolysis

Definition of Photosystem I

PS I refers to a photosystem that participates in a light reaction of photosynthesis. In contrast to PS II, PS I carries more chlorophyll-a content compared to chlorophyll b. Furthermore, PS I participates in cyclic phosphorylation and produces NADPH. Photosystem I has a reaction centre that possesses chlorophyll-a molecules. It is capable of absorbing light at a wavelength of 700 nm.

The light-harvesting or antenna complex of PS I absorbs photons and hand them over to its reaction centre containing chlorophyll-a. The reaction centre excites and releases high energy electrons. The high energy molecules enter the electron transport chain through electron carriers and release NADPH.

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Definition of Photosystem II

PS II refers to a light-dependent photosystem that participates in the photosynthetic light reactions. Opposite to PS I, it contains more chlorophyll b pigments compared with chlorophyll a. The PS II reaction centre contains chlorophyll-a that has an absorption peak of 680 nm (P680). Furthermore, the pigment molecules of PS II absorb photons and funnel them to the reaction centre or antenna complex.

The PS II reaction centre excites and releases high energy molecules. Consequently, the primary electron acceptor molecules pick high energy electrons and hand them over to the PS I through many carrier molecules. The excitation of electrons through electron carriers of low energy levels releases free energy that the cells harness to synthesize ATP from ADP via photophosphorylation.


Key Differences Between Photosystem I and Photosystem II

The light-harvesting pigments of photosystems I and II absorb photons with wavelengths of 700 nm (P700) and 680 nm (P680), respectively.A photosystem possesses two typical classes of reaction centres (type I and II RCs) responsible for photochemical reactions. PS-I has type-I RC or P-700, whereas PS-II has type-II RC or P-680.PS-I comprises psaA and psaB subunits and possesses iron sulphur or type-I RC, whereas PS-II includes D1 and D2 subunits and possesses Q (Quinone) or type-II reaction RC.

SimilaritiesBoth PS I and PS II are light-dependent photosystems that participate in the light reactions of photosynthesis.The light-harvesting pigments and the reaction centre (collectively called antenna complex) are two common elements in both the photosystems.Furthermore, photosystem I and II comprises light-harvesting pigments for photon absorption and an active reaction centre to excite electrons.The location of PS-I and PS-II is somewhat common in that they are found in the thylakoid membrane.

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Conclusion

Therefore, we can conclude that photosystems I and II play a fundamental role in trapping photons of selective wavelength and channelizing them to the active reaction centre. Plant cells harness the light energy harnessed to generate chemical potential energy like ATP and NADPH that is again used by the plant cells to synthesize glucose during the dark reaction of photosynthesis.