Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A photosynthetic reaction center is a complex of three types of protein that is the site where molecular excitations originating from sunlight are transformed into a series of electron-transfer reactions. The reaction center proteins bind functional co-factors, chromophores or pigments such as chlorophyll and pheophytin molecules. These absorb light, promoting an electron to a higher energy level within a pigment. The free energy created is used to reduce a chain of electron acceptors which have subsequently lowered redox-potentials, and is critical for the production of chemical energy during photosynthesis.