Final answer:
Electron excitation during photosynthesis occurs within the photosystems, which are special structures in the thylakoid membrane of chloroplasts.
Step-by-step explanation:
Excitation of electrons occurs in special structures of the thylakoid membrane called photosystems. Thylakoids are disc-shaped, membrane-bound structures inside a chloroplast where the light-dependent reactions of photosynthesis take place. Inside these thylakoids, chlorophyll and other pigments form an assembly known as a photosystem. These pigments capture light energy, which then excites the electrons within the chlorophyll molecules. A stack of thylakoids is called a granum, while the liquid-filled space surrounding the granum is known as the stroma. The thylakoid lumen is the aqueous space bound by the thylakoid membrane where protons accumulate during the light-driven electron transport.