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Chloroplasts are organelles in plant and eukaryotic algae cells. In green plants and red algae, they are surrounded by two lipid bilayer membranes. The outer membrane originates from the plant cell, whereas the inner membrane is from the chloroplast, which supposedly is the remainder of blue-green algae incorporated by the host cell by endosymbiosis.

Chloroplasts were probably incorporated several times. Most blue-green algae contain only chlorophyll A, as do the chloroplasts of red algae, but a few forms (prochlorophyta) also have chlorophyll B, and these correspond to the chloroplasts of green plants and euglenids. Other eukaryotic algae contain chlorophyll C, although no corresponding prokaryote is known. In most of these forms the chloroplasts are bound by extra membranes, suggesting they were not incorporated directly. In stramenopiles and cryptomonads, where there are four membranes, they are considered to be a reduced form of a eukaryotic endosymbiont that was itself carrying chloroplasts, and in cryptomonads there is still a reduced nucleomorph between the second and third membranes that probably represents the remains of the symbiont's nucleus. In colored euglenids and dinoflagellates, where there are three membraes, the chloroplasts were probably retained from some ingested alga (both groups have colorless members that feed by phagocytosis).

The fluid within the membrane-enclosed space is called stroma, and used to be the cytoplasm of the endosymbiotic algae. It contains tiny circular DNA and ribosomes similar to prokaryotes. The Calvin cycle, a chemical pathway to synthesize carbohydrates from carbon dioxide universal to all photosyntetic lifeforms, takes place in the stroma.

Thylakoids are tiny membrane-enclosed sacs floating in the stroma. They are stacked in groups called grana. The thylakoid membranes are the site of photosynthesis, converting light into energy stored in ATP. The photosynthetic proteins in the membrane bind chlorophyll, which gives the thylakoids and, as they come in large numbers, the plant cells their green color.