As in the overview of photosynthesis, the light-dependent reactions utilise light energy to convert it into more usable chemical energy.
So naturally, it starts with light. This is the brief sequence of events:
1. The electrons present in the chlorophyll of the plant’s chloroplasts are brought to a higher energy level (they enjoy dancing more) by light energy. This takes place on the thylakoid membrane, and more specifically in a conglomerate of proteins/enzymes dedicated to this reaction, called photosystem II. It’s known as photoionisation.
2. To maintain a fresh supply of dancing electrons, light also splits (photolysis) the H2O into… electrons, protons and… wait for it. Wait for it. Wait… Oxygen! So that’s how the oxygen by-product is…