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 latter including diatoms, dinoflagellates, and euglenids, and other members of Protista, 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.