Question

Summarize the function of the two photosystems in green plants and describe the flow of electrons.

Answer 1

In all photosynthetic systems, pigment molecules are bound to polypeptides forming pigment protein complexes called photosystem or pigment system. The photosystem have two components -

• Photochemical reaction center - It contain special Chl a molecule that carries out charge seperation and electron transport.
• Antenna complex - It cinsists Light harvesting complex. It captures light energy and feeds it to the reaction center.

Photosynthetic plants caontains two photosystem to carry out photosynthesis -

1. Photosystem I - In PSI, the photochemical reaction center is designated as P700 as it absorbs the wavelength at 700nm. It is found in stroma lamellae and at the edges of the grana lamellae (Non-appressed membrane).
2. Photosystem II - In PSII, the photochemical reaction center is designated as P680 as it absorbs the wavelength at 680nm. It is found in the grana lamellae (Appressed membrane)

Photosynthesis is a two stage process-

• Light reaction - It occurs in grana and requires light energy that is trapped by cholorophyll to make ATP (photophosphorylation) and NADPH which in turn are used in Dark reaction.
• Dark reaction - It is independent of light and occurs in stroma of chloroplasts.The product of light reaction ATP and NADPHare used to make carbohydrate.

The flow of electrons -

Non cyclic photophosphorylation begins with the absorption of a photon by PSII, causing an electron to move from P680 Chla to plastoquinone, QB and synthesis of molecular oxygen from water.

Transfer of electron -

P680-->Pheophytin-->Plastoquinone A-->plastoquinone B--> Cyt-b6f-->Plastocyanin-->P700-->Phylloquinone-->Fe-S-->Ferrodoxin-->NADP+ to form NADPH(In stroma)