Plastids are major double-membrane organelles found, among others, in the cells of plants. Plastids are the site of manufacture and storage of important chemical compounds used by the cell. Those plastids that contain chlorophyll can carry out photosynthesis. Plastids can also store products like starch and can synthesise fatty acids and terpenes, which can be used for producing energy and as raw material for the synthesis of other molecules.
All plastids are derived from proplastids, which are present in the meristematic regions of the plant. Undifferentiated plastids (proplastids) may develop into different variants depending upon which function they play in the cell. One such type of plastid is chloroplast where the photosynthesis primarily take place due to the presence of green pigment called chlorophyll. Other variants are as follows:
- Etioplasts (the predecessors of chloroplasts): Those chloroplasts that have not been exposed to light. They are usually found in flowering plants.
- Chromoplasts (coloured plastids): For pigment synthesis and storage. Pigments include carotenes and xanthophylls commonly found in flowers and fruits that gives them colours of yellow,orange and red.
- Gerontoplasts: Control the dismantling of the photosynthetic apparatus during plant senescence
- Leucoplasts (colourless plastids): Occur in parts of plants that are not exposed to light like roots and seeds. For monoterpene synthesis and leucoplasts sometimes differentiate into more specialized plastids such as :
- Amyloplasts: For starch storage and detecting gravity
- Elaioplasts: For storing fat
- Proteinoplasts: For storing and modifying protein
- Tannosomes: For synthesizing and producing tannins and polyphenols
Plastids possess a double-stranded DNA molecule that is circular, like that of prokaryotic cells. Plastid DNA exists as large protein-DNA complexes associated with the inner envelope membrane and called ‘plastid nucleoids‘. Each nucleoid particle may contain more than 10 copies of the plastid DNA. 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.