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PSI and PSII are two types of photosystems that are involved in the light reaction of photosynthesis. They differ in several aspects, such as:

• Location: PSI is located at the outer surface of the grana thylakoid membrane (non-appressed granal regions and stroma lamella), while PSII is located at the inner surface of the grana thylakoid membrane (appressed granal region).
• Reaction center: PSI has a P700 chlorophyll a molecule as its reaction center, which absorbs light best at 700 nm wavelength, while PSII has a P680 chlorophyll a molecule as its reaction center, which absorbs light best at 680 nm wavelength.
• Reaction type: PSI has an iron-sulfur (FeS) type reaction center (or type I), while PSII has a quinone type reaction center (or type II).
• Function: PSI generates a strong reducing agent (reducing NADP+ to NADPH) and a weak oxidant, while PSII is a strong oxidant (capable of oxidizing H2O) and a weak reductant⁴. PSI participates in both cyclic and non-cyclic photophosphorylation, while PSII participates only in non-cyclic photophosphorylation.
• Pigments: PSI is rich in chlorophyll a than chlorophyll b, while PSII is rich in chlorophyll b than chlorophyll a PSI also has carotenoids as accessory pigments, while PSII has phycobilins and xanthophylls as accessory pigments..

(1) Differentiate between PS – I and PS – II. – Toppr. https://www.toppr.com/ask/question/differentiate-between-psi-and-psii/.
(2) Photosystem – Wikipedia. https://en.wikipedia.org/wiki/Photosystem.
(3) What are the differences between PSI and PSII? – Wise-Answer. https://wise-answer.com/what-are-the-differences-between-psi-and-psii/.
(4) 10 Differences between Photosystem I and Photosystem II (PSI vs PSII). https://www.majordifferences.com/2013/12/difference-between-photosystem-i-and.html.