Cell : The Unit of Life

Plastids are small bodies found free in the cytoplasm of most plant cells. Plastids are of three types, chloroplast, chromoplast and leucolast. Each type of plastid shows specific position or occurrence in a plant body, i.e., plastids are region specific and occur in various plant parts according to their function. Leucoplasts are colourless plastids which generally occur near the nucleus in nongreen cells and possess internal lamellae. Grana and photosynthetic pigments are absent. Amyloplasts are starch containing leucoplasts, elaioplasts (Lipidoplasts) are the colourless plastids which store fat and aleuroplasts are protein containing plastids. Chromoplasts are yellow or reddish in colour because of the presence of carotenoid pigments. They provide colour to many flowers for attracting pollinating insects, provide bright red or orange colour to fruits for attracting animals for dispersal and are the site of synthesis of membrane lipids. Chloroplasts art- greenish plastids which possessphotosyntheticpigments, chlorophylls and carotenoids, and take part in the synthesis of food from inorganic raw materials in the presence of radiation energy. They occur in major number in the photosynthetic mesophyll cells of leaves and green stem. Apart from being region specific plastids may show species specificity e.g., chloroplast present in euglenoids have three envelopes whereas those1 present in higher plants are covered by two envelopes.
Moreover, distribution pattern of plants in a region also affect type and positioning of plastids e.g., plants occuring in deep layers of a water body have abundance of red pegment phycoerythrin whereas plants occuring in illuminated shallow water contain more chlorophylls.