Most eukaryotic photosynthetic cells contain organelles called chloroplasts. The inner membrane is arranged as stacks of vesicles. Each vesicle is a thylakoid and the stack is a granum. Photosynthetic pigments are located on the thylakoid membrane in clusters called antenna assemblies. Light energy is focused onto a particular pigment molecule whose electrons are excited as the energy is absorbed. A high-energy electron escapes, but is replaced by one from water. The role of water in photosynthesis is electron donor. The water molecule is split in the presence of light in a process called photolysis. Oxygen is released as a byproduct. The high-energy electron moves through an electron transport chain which functions, like the one found in respiration, to produce ATP. The electron eventually joins a second antenna assembly, where it replaces an escaped electron which has entered another electron transport chain. The energy released in this second electron transport chain aids in the formation of NADPH. ATP and NADPH from these light reactions provide the energy for the next stage of photosynthesis, the light-independent reactions.