The reaction center is a part of the photosynthetic apparatus in chloroplasts where the photochemical process occurs. Reaction centers are a specific pair of chlorophyll molecules in a photosystem that collect light energy absorbed by other chlorophyll molecules in a photosystem and pass it along to an electron acceptor, such as an electron transport chain. The remaining chlorophyll molecules are referred to as antenna molecules of the light-harvesting complexes because they absorb the photons and pass the energy by resonance transfer to the reaction centers. The process occurs on the order of 10^-10 seconds.