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Chloroplasts are organelles found in plant cells and eukaryotic algae which conduct photosynthesis. Chloroplasts are similar to mitochondria but are found only in plants. Both organelles are surrounded by a double membrane with an intermembrane space; both have their own DNA and are involved in energy metabolism; and both have reticulations, or many foldings, filling their inner spaces. Chloroplasts convert light energy from the sun into ATP through a process called photosynthesis.

Chloroplasts are one of the forms a plastid may take, and are generally considered to have originated as endosymbiotic cyanobacteria. In green plants chloroplasts are surrounded by two lipid bilayer membranes, now thought to correspond to the outer and inner membranes of the ancestral cyanobacterium. The genome is considerably reduced compared to that of free-living cyanobacteria, but the parts that are still present show clear similarities.

It is interesting to note that in some algae, chloroplasts seem to have arisen through a secondary event of endosymbiosis, where an eukaryotic cell joined with a second eukaryotic cell containing chloroplasts, forming chloroplasts with four membrane layers.

The fluid within the chloroplast is called the stroma, corresponding to the cytoplasm of the bacterium, and contains tiny circular DNA and ribosomes, though most of their proteins are synthesized by the cell nucleus. 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 or lumen. The photosynthesis reaction takes place on the surface of the thylakoid.

The photosynthetic proteins in the membrane bind chlorophyll, which is present with various accessory pigments. These give chloroplasts their green colour. Algal chloroplasts may be golden, brown, or red and show variation in the number of membranes and the presence of thylakoids.

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