The first part of photosynthesis is a light dependent reaction meaning it requires light energy from the sun to occur. This part of photosynthesis happens in the chloroplast which is a cellular organelle found only in photosynthetic organisms. Light from the sun is converted into ATP energy in the process of photosynthesis. Photosystem I and photosystem II are the two parts of the light reaction phase of photosynthesis. Photosystem II occurs before photosystem I.
Photosystem II: electrons are removed from water molecules, oxygen is produced and light from the sun excites chlorophyll electrons which are sent to the electron transport chain.
Photosystem I: takes in electrons from photosystem II, light excites chlorophyll electrons again, ATP is produced and electrons go through the electron transport chain for a second time to produce NADPH