Chloroplasts are organelles found in plant cells and eukaryotic algae which conduct photosynthesis. Chloroplasts are like mitochondria, but are found only in plants and protista. 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 infoldings, filling their inner spaces. Chloroplasts capture light energy from the sun to produce the free energy stored in ATP and NADPH 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. The inner membrane is now thought to correspond to the outer membrane of the ancestral cyanobacterium. Since they live in a highly-structured milieu, their 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 (such as the heterokonts and other protists such as Euglenozoa and Cercozoa), chloroplasts seem to have arisen through a secondary event of endosymbiosis, in which a eukaryotic cell engulfed a second eukaryotic cell containing chloroplasts, forming chloroplasts with three or four membrane layers. In some cases, such secondary endosymbionts have themselves been engulfed by still other eukaryotes, forming tertiary endosymbionts.

    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 encoded by genes contained in the cell nucleus, with the protein products trafficked to the chloroplast. Within the stroma are stacks of thylakoids, the sub-organelles 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 membrane of the thylakoid, and, as is also the case with mitochondria, involves the coupling of cross-membrane fluxes with biosynthesis.

    The photosynthetic proteins in the membrane bind chlorophyll, which is present with various accessory pigments. These give chloroplasts their green color. During autumn, the removal of chlorophyll from plant leaves exposes red and yellow pigments (such as xanthophyll) which were previously masked. Algal chloroplasts may be golden, brown, or red and show variation in the number of membranes and the presence of thylakoids.

    Pigments undergo electronic excitations driven by the absorption of sunlight — red and blue for chlorophyll. The green we see is the color not absorbed. The energy released by the electronically-excited pigments as they return to their ground state is the basis for the energy captured by photosynthesis to produce ATP and NADPH and the ultimate formation of sugars. Energy of the absorbed photons not used to produce chemical energy is eventually given off to the surroundings. Thus, chloroplasts are small heat engines operating between the hot light from the sun and the lower ambient molecular temperature. (Photovoltaic cells do likewise.)