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Chloroplastsaretheplaceforthemajorconversionofthesun'sradiationenergytochemicalenergy that is usable by organisms. Accordingly, they account for about 50% of the leaf protein, and the enzyme ribulose-1,5-bisphosphate carboxylase of chloroplast is by far the most abundant protein on the Earth.
Chloroplasts are not only key in photosynthesis, they are the place of the synthesis of fatty acids in plants and biosynthesis of amino acids, porphyrins, isoprenoids, and secondary metabolites that, sometimes, duplicate parallel biosynthetic pathways in cytosol. Only a small fraction of the involved enzymatic machinery is encoded in the chloroplast DNA. Nuclear DNA encodes most of the chloroplast proteins which, after being synthesised as precursors on cytosol ribosomes, are incorporated into the appropriate chloroplast substructures. Chloroplast evolutionarily derives from a primitive cyanobacteria that was engulfed by non-photosynthetic cells and, progressively, after losing most of its DNA, became the actual chloroplast that retains only a fraction of the original cyanobacterial genes. Most of the original enzyme machinery is now encoded in the cell nuclear DNA that controls the plastid divisions, among others.