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.
Chlorophyll contains a magnesium atom, held in the center of a ring of atoms.
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 on tenth of a billionth of a second from the edge of the unit to the center. This process is called chemiosmosis. The Calvin cycle combines three carbon dioxide molecules into one molecule of three carbon glyceraldehyde 3-phosphate. Some plants, particularly those with tropical origins, have a mechanism for storing carbon dioxide by combining it with a three carbon molecule to form a four carbon molecule.