Question

Differentiate between PS-I and PS-II.

Answer 1

Hint: In plants, photosynthesis takes place in two processes, one is a light-dependent reaction and another is a carbon-assimilation reaction. PS stands for Photosystem. In both the processes energy molecules are liberated.

Complete answer:
 The main differences between Photosystem I and II are:
> The outer surface of the thylakoid membrane is the location for PS-I and the outer surface is the location for PS-II towards the stroma. The center for PS-I is P700 and the center for PS-II is P680. The centers are named according to the wavelength of light they absorb. PS-I absorbs a longer wavelength of light of 700nm whereas PS-II absorbs 680nm which is a shorter wavelength of light.
> Photosystem I use light to convert NADP+ to NADPH2 in the presence of chlorophyll and other pigments. Photosystem II absorbs light and transfers electrons from water to plastoquinone. This dissociates the water molecules and produces hydrogen proton (H+) and oxygen (O2).Thus, the main function of Photosystem I is to synthesize NADPH whereas in Photosystem II hydrolysis of water takes place. Photosystem II synthesizes ATP molecules.
> Photosystem I involve both cyclic and non-cyclic phosphorylation. In Photosystem II, only non-cyclic phosphorylation is observed.
> The two subunits of Photosystem I are psaA and psaB. The two subunits of Photosystem II are D1 and D2.
> The process of photolysis occurs only in Photosystem II and not in Photosystem I. In the Photosystem-II molecular oxygen is evolved and photolysis of water takes place.

Note:
ATP and NADPH are energy molecules produced by PS-II and PS-I respectively.In Photosystem I process, molecular oxygen is not involved. Photosystem I and II produce energy that is used in the Calvin cycle.