Plastid, Plastid

Plastids are the site of manufacture and storage of important chemical compounds used by the cell. Plastids often contain pigments used in photosynthesis, and the types of pigments present can change or determine the cell's colour.

Depending on their morphology and function, plastids have the ability to differentiate, or redifferentiate, between these and other forms. All plastids are derived from proplastids (formerly "eoplasts", eo -: dawn, early), which are present in the meristematic regions of the plant. Proplastids and young chloroplasts commonly divide, but more mature chloroplasts also have this capacity.

The plastome contains about 100 genes encoding ribosomal and transfer ribonucleic acids (rRNAs and tRNAs) as well as proteins involved in photosynthesis and plastid gene transcription and translation. However, these proteins only represent a small fraction of the total protein set-up necessary to build and maintain the structure and function of a particular type of plastid.

The proplastid contains a single nucleoid located in the centre of the plastid. The developing plastid has many nucleoids, localized at the periphery of the plastid, bound to the inner envelope membrane. During the development of proplastids to chloroplasts, and when plastids convert from one type to another, nucleoids change in morphology, size and location within the organelle. The remodelling of nucleoids is believed to occur by modifications to the composition and abundance of nucleoid proteins.

Their function differ from the leukoplasts in plants. Etioplast, amyloplast and chromoplast are plant-specific and do not occur in algae. Algal plastids may also differ from plant plastids in that they contain pyrenoids.

The plastids differ by their pigmentation, but also in ultrastructure. The chloroplasts e.g. have lost all phycobilisomes, the light harvesting complexes found in cyanobacteria, red algae and glaucophytes, but - only in plants and in closely related green algae - contain stroma and grana thylakoids. The glaucocystophycean plastid - in contrast to the chloroplasts and the rhodoplasts - is still surrounded by the remains of the cyanobacterial cell wall. All these primary plastids are surrounded by two membranes.

In some cases these plastids may be reduced in their metabolic and/or photosynthetic capacity.

Source: Wikipedia > Plastid