Light-dependent reactions of photosynthesis occur in the thylakoid membranes of chloroplasts. The chloroplast contains grana, which are stacks of hollow disks. The membrane surrounding the grana is the thylakoid membrane. Like in the mitochondria, electron transfer reactions occur in internal membranes. Cyclic carbon fixation reactions, like the breakdown reactions in the citric acid cycle, occur in the stroma, the analog of the matrix in mitochondria.
Chlorophyll contains a hydrophyllic head group and a hydrophobic tail region.
A magnesium atom is held in the center of a cyclic, conjugated double bond porphyrin ring which is responsible for absorbing red light. Light is absorbed by antenna chlorophyll molecules, then transferred to the reaction center chlorophylls. Some 100's of antenna chlorophyll molecules transfer energy to a reaction center, with transfer times of about 10 -10 sec from the edge of the unit to the center. The chemiosmotic coupling is working here in a similar way to the mechanism of ATP generation used in mitochondria. The electron transport chain can be shown on an energy diagram.