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Plastids are a group of organelles found in plant cells and eukaryotic algae. They are characterized by a double membrane and are responsible for various functions, including photosynthesis and food storage. The most well-known type of plastid is the chloroplast, which contains chlorophyll and is the site of photosynthesis. Other types of plastids include chromoplasts, which are responsible for the colors of flowers and fruits, and leucoplasts, which are involved in the storage of starch, oils, and proteins.

Plastids are believed to have originated from free-living cyanobacteria that were engulfed by an ancestral eukaryotic cell in a process called endosymbiosis. This theory is supported by the fact that plastids have their own circular DNA, similar to bacterial DNA, and replicate independently of the nuclear DNA. They also possess their own ribosomes, which are also similar in size and structure to bacterial ribosomes.

The development and differentiation of plastids from proplastids, which are small, undifferentiated plastids found in meristematic tissues, are influenced by environmental factors and the developmental stage of the cell. For example, exposure to light is necessary for the development of chloroplasts from proplastids. The internal structure of plastids is complex, with a system of membranes called thylakoids, which are arranged in stacks called grana within the chloroplast stroma. This intricate structure is crucial for the efficient capture of light energy during photosynthesis.