Plastids are double membraned organelles which are found in plant cells only. They are usually spherical or discoldal in shape and their average size is 4 to 6 um. A plastid shows two distinct regions-grana and stroma Grana are stacks of membrane-bound, flattened, discold sacs containing chlorophyll molecules. These molecules are responsible for the production of food by the process of photosynthesis.
They are, therefore, called Kitchen of the cell. They are the main functional units of the chloroplast. The homogenous matrix in which grana are embedded is known as stroma. A variety of photosynthetic enzymesw and starch grains are present in the stroma. The stroma is colourless, whereas the grana contain the pigments. Plastids are living and multiply by division of the pre-existing plastids called proplastids.
Plastids are of three types:
- Leucoplasts are colorless plastids. They store the food of the plant body in the form of starch, protein and lipids. They occur most commonly in the storage cells of roots and underground stems.
- Chloroplasts are green plastids because of the presence of chlorophyll. Chloroplasts occur abundantly in green leaves, and also to some extent in green parts of the shoot.
- Chromoplasts are variously colored plastids. They are mostly present tin flowers and fruits.
One form of plastid can change into another. For example, leucoplasts can change into chloroplasts when the former are exposed to light for a long period.
- By trapping solar energy, green plastids manufacture food through photosynthesis
- Chromoplasts provide colored to various flowering parts.
- Leucoplasts help in storage of protein, starch and oil.