Chloroplast

In photosynthesis-capable organisms classified as Eukarya, the one of the three domains of Earth's living systems the cells of whose organisms contain a nucleus, the photosynthesis-initiating molecular apparatus resides in the cells' cytoplasm, in organelles called chloroplasts, tiny, somewhat football-shaped, bacteria-sized structures, a few micrometers in size, up to several hundred in number in cells with high photosynthetic rates, each chloroplast a separate compartmented structure whose boundary consists of two membranes, the interior of the inner membrane of which contains a semiliquid matrix, called stroma, suspending a system of membranes, called thylakoids, whose membranes embed molecules of chlorophyll and other pigments that absorb energy from sunlight, the initiating step of the physico-chemical process of photosynthesis.    

The domain Eukarya includes photosynthesis-capable organisms in two of its four kingdoms, namely Plantae (the plant kingdom) and Protista (the protist kingdom), the former including photosynthesis-capable algae, mostly single-celled or colonizing members of the plant kingdom, and the green plants we see all around us, the Protista including diatoms, dinoflagellates, and euglenids, and other members of the protist kingdom, a mixed group of mostly single-celled organisms that do not classify under the Eukarya's kingdoms of plants, fungi (Fungi) and animals (Animalia). Biologists refer to organisms in Eukarya as eukaryotes.