1. Photosystems: 

a. Two kinds of photosystems are present in thylakoid membranes of chloroplasts. 

b. Each has its own set of light harvesting chlorophyll and carotenoid molecules. 

c. Chlorophyll and accessory pigments help to capture light energy over large area and pass it on to the photocenters. 

d. Thus, a photon absorbed anywhere in the harvesting zone of P-680 center can pass its energy to the P680 molecule. 

e. The cluster of pigments which transfer their energy to P680 absorb at or below 680nm. 

f. Together with P-680 they form Photosystem – II. 

g. Likewise, P-700 forms Photosystem – I along with pigment molecule which absorbs light at or below 700 nm.

2. Photosystem II: 

a. Photolysis of water and release of oxygen takes place in this system. 

b. In this process, when PS-II absorbs light, electrons are released and chlorophyll molecule is oxidized. 

c. Electrons emitted by P680 (PS-II) are ultimately trapped by P700 (PS -1). 

d. Oxygen is the byproduct by the photosynthesizing plants. 

e. Protons accumulate inside the thylakoid resulting in a Proton gradient. 

f. When the protons diffuse across the thylakoid membrane into stroma against the H+ gradient, energy, is released. 

g. This is used to produce ATP. 

3. Photosystem 1: 

a. Upon absorption of light quanta by PS-I (P700) reaction center emits energy rich electrons. 

b. These flow down a chain of electron carriers to NADP along with the proton generated by splitting of water. 

c. This result in the formation of NADPH. 

d. Hydrogen attached to NADPH is used for reduction of CO₂ in dark reaction also called as reducing power of the cell.