Photosystems:

The thylakoid membrane consists of both photosystem I (PS I) and photosystem II (PS II). Inside the thylokoid membrane it is called thilokoid lumen and outside is called chloroplast stroma.

PS I optimally absorbs photons of a wavelength of 700 nm, and because of the wavelength it is denoted as P700. While, PS II optimally absorbs photons of a wavelength of 680 nm and hence, known as P680. PS II uses light energy to oxidize two molecules of water into one molecule of molecular oxygen. The 4 electrons removed from the water molecules are transferred through plasto quinine and cytochrome bf to PS I ultimately reduce 2NADP+ to 2NADPH. This electron transport process generates a proton gradient across the thylakoid membrane. This proton motive force is then used to drive the synthesis of ATP from ADP.

The overall reaction;

Dark reaction means the reaction that does not need the presence of light mandatorily .

Herein, the water oxidation for the generation of molecular oxygen will be discusses.

Production of O₂ in plants:

Production of O₂ by the oxidation of H₂O in green plants takes place in PS II.

Figure 7.7. Simplest representation of P680.

P680 contains Mg²⁺ coordinated to a prophyrin unit. It absorbs light of 680 nm wavelength and goes to its excited state (P680*). It transfer an electron to pheophytin α and becomes P680⁺ which is a strong oxidizing agent. It oxidizes water to oxygen where water molecule is bound to a tetranuclear Mn₄ cluster.