plastides

Plastids, historically called plastides, are a family of membrane-bound organelles found in the cells of plants and algae. They derive from proplastids in meristematic tissues and can differentiate into several forms depending on developmental and environmental cues. Most plastids contain a double envelope and, in chloroplasts, a system of internal thylakoid membranes arranged into grana. Plastids contain their own circular genome and ribosomes and import most of their proteins from the nucleus.

Chloroplasts perform photosynthesis and synthesize fatty acids and amino acids; they contain chlorophyll and carotenoids. Chromoplasts synthesize and store pigments other than chlorophyll, giving fruits and flowers their colors. Leucoplasts are colorless and include amyloplasts (starch storage), elaioplasts (lipid storage), and proteinoplasts (protein storage).

Other roles include plastid biosynthesis of isoprenoids, fatty acids, heme, and some amino acids, as well as the synthesis of pigments, defensive compounds, and signaling molecules. They can also participate in nitrogen and sulfur metabolism in some contexts.

Plastid genomes are typically small, circular DNAs of about 120–170 kilobases, encoding around 100–120 genes, mainly for photosynthesis and gene expression. Most plastid proteins are nucleus-encoded and imported via transit peptides. Plastids usually replicate by division and are inherited maternally in many land plants, though patterns vary.

Plastids originated from ancient cyanobacterial endosymbionts, as evidenced by their double membranes, 70S ribosomes, and gene remnants. The plastid genome and the import of nuclear-encoded proteins reflect significant integration with the host cell.