The Light Reactions
The light reactions take place within the chloroplast, a specialized plant cell structure with several important parts. The stroma is the fluid-filled space where the Calvin cycle occurs, while grana are stacks of disc-shaped structures called thylakoids. The thylakoid membrane contains chlorophyll and proteins needed for light reactions, and the thylakoid space inside stores hydrogen ions.
Chlorophyll molecules are key to capturing light energy. Each contains a magnesium atom at its center that enables plants to convert light into energy. When sunlight hits chlorophyll, electrons become excited and pass through an electron transport chain, creating energy. The electrons lost from Photosystem II are replaced by splitting water molecules, which produces oxygen as a byproduct—this is why plants release oxygen!
During this process, the light reactions generate two critical products ATP and NADPH. ATP forms when hydrogen ions flow through ATP synthase proteins in the thylakoid membrane. Meanwhile, the energized electrons eventually combine with NADP+ to form NADPH. Both products will power the Calvin cycle, where actual sugar production occurs.
Did you know? Plants appear green because chlorophyll absorbs red and blue light while reflecting green wavelengths. The color you see is actually what the plant doesn't use!