The light reactions of photosynthesis involve several protein complexes. The complexes are embedded within the chloroplast's thylakoid membrane. The first complex uses the energy from photons of light to excite electrons. After two electrons have been excited, they are tranferred to a mobile carrier protein (magenta). The carrier also picks up two hydrogen ions, also known as protons. The tranferred electrons are replaced by the splitting of water molecules. After two water molecules have been split, one molecule of O2 is made. The mobile carrier transports the electrons to the second complex (yellow) and releases the two protons across the membrane. As the two electrons are transferred through the second complex, it pumps two more hydrogen ions across the membrane. The electrons are then tranferred to a second mobile carrier (dark blue). The second carrier transfers the electrons to the third major complex. The third complex uses photons to re-energize the electrons, after which they are transferred by another mobile carrier (purple) to the fourth complex (pink). The fourth complex combines the two electrons, one proton, and a molecule of NADP+ to create a molecule of NADPH. ATP synthesis occurs at a fifth complex. It is powered by the hydrogen ion gradient created by the protons that were pumped across the membrane.