Photosynthesis hence basically include two steps

A. Utilizing and storing the energy of the sun (therefore photo)

B. Synthesis of organic molecules

The former step of the utilization of sunlight energy to produce energy-storing compounds like ATP and NADPH, and then using them to carry out synthesis.

The light reaction involves a light Harvesting complex, which itself includes:

• Photo systems
• Accessory pigments

Photosystem I was discovered earlier, and thus named PS I.

Absorption peak observed at 700 nm. (Far red radiations)

P700 when comes in contact with radiation whose wavelength is less than 700 nm it gets activated (as lower wavelength have higher energy).

It forms P700+ and loses one electron due to photoemission.

This electrons enters a transport ⛓️ chain and continuously synthesize out energy 🔋 storing compounds.

Let's directly jump to point here

• Absorption maxima - 680nm (Red Radiations)
• Discovered later
• Forms P680+ on losing one electron on account of photoemission.
• P680+ is the most powerful oxidizing agent that can oxidize water to Oxygen. And hence maintains its supply of electron from water.

When electron doesn't return to it's origin.

• 680nm light is available.
• P680 stimulated by it and Electron is emitted by it.
• P680+ is formed and excited electron is accepted by Phaeophytin (primary electron acceptor)
• Phaeophytin transfers it to Plastoquinone. That also intakes an H+ ion from the stroma as it is Hydrogen carrier. And PQH2 reduced to plastoquinone.
• This hydrogen along with electron transferred to cytochrome b6f.
• This proton is actively dumped into the lumen of thylakoid by cytochrome b6f.
• And electron is transferred to plastocyanin (Cu2+ present).
• And ultimately supplies electron to PSI

In PSI electrons are supplied by PSII. From P700 electron is emitted which is accepted by the IronSulphur complex.

Then transferred to ferrodoxin. And ultimately then transferred to the NADP reductase. Which also removes H+ ion from the stroma and produce NADP.

PSI functions at low energy (high wavelength). It supplies its electron to FeS.

Then it goes to Plastquinone and forms reduced Plastoquinone.

Then electron shifted to Cytochrome b6f and and releases H+ ion inside the lumen.

Finally, electron reaches plastocyanin and then back to PSI.