Light Reaction – in a nutshell Occurs in the thylakoid membrane of the chloroplast. Is a series of steps that takes light, moves electrons along the membrane, and eventually makes the energy molecules – NADPH and ATP.
Step 1 Sunlight (photons) hit a special protein on the membrane called “Photosystem 2” that contains chlorophyll a. Also here - water is split into Hydrogen, electrons, and Oxygen (a waste product). Electrons become “excited”, causing them to leave photosystem 2 and move along an “electron transport chain”.
Step 2 As electrons move along the chain, hydrogen ions build up inside the thylakoid. The electrons reach another protein containing chlorophyll a called “Photosystem 1”.
Step 3 Electrons again are “excited” and move along to another electron transport chain. These electrons reach the side of the membrane that is next to the stroma (fluid of the chloroplast). Here they combine with H and NADP (the final electron acceptor) to make NADPH (an energy molecule).
Making ATP The hydrogen ions that have built up inside the thylakoid now can move through an enzyme called “ATP synthase”. The movement of these hydrogen ions through the ATP synthase provides the energy needed to convert ADP to ATP.
Final products of the light reaction 1. NADPH (energy molecule) 2. ATP (energy molecule) 3. oxygen (waste product) The energy molecules can then be used in the Calvin cycle (dark reaction)