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Chloroplasts are organelles found in plants and eukaryotic algae which conduct photosynthesis. These are one of the forms a plastid may take, and are generally considered to have originated as endosymbiotic cyanobacteria. In green plants they are surrounded by two lipid bilayer membraness, which correspond to the host cell and ancestral bacterium, though algal chloroplasts show notable variations from this pattern. The fluid within the chloroplast is called the stroma, corresponding to the cytoplasm of the bacterium, and contains tiny circular DNA and ribosomes. The genome is considerably reduced compared to that of free-living cyanobacteria, but the parts that are still present show clear similarities.

Within the stroma are stacks of thylakoids, the sub-organelle where photosynthesis actually takes place. A stack of thylakoids is called a granum. A thylakoid looks like a flattened disk, and inside is an empty area called the thylakoid space. The photosynthesis reaction takes place on the surface of the thylakoid.

The structure and biochemistry of chloroplasts are also similar to those of cyanobacteria. Carbon dioxide is converted into carbohydrates via the Calvin cycle, which takes place in the stroma. Thylakoids, tiny membrane-enclose sacs, are present, and in most forms are arranged into stacks called grana. The thylakoid membranes are the site of photosynthesis, converting the energy of light into ATP and oxidizing water to oxygen. The photosynthetic proteins in the membrane bind chlorophyll, which is present with various accessory pigments. These give chloroplasts their colour, which may be green, golden, brown, or red.