Plastids- Leucoplasts & Chromoplasts | Biology NEET

STD-11     UNIT-3     CHA-8

CELL : STRUCTURE AND FUNCTIONS

 Plastids

• The term plastid was introduced by E. Haeckel in 1866.
• Plastids are semi-autonomous organelles having DNA and double membrane envelope which store or synthesize various types of organic compounds. 
• With the exception of some protistans, (e.g. Euglena, dinophyceae, diatoms) plastids are restricted to plants only, Plastids develop from colorless precursors called proplastids. 
• Proplastids have the ability to divide and differentiate into various types of plastids. 
• Depending upon their color, plastids are of three main types- leucoplasts, chromoplasts and chloroplasts (Schimper, 1883). 

Leucoplasts (Gk. Leucos- white, plastos, molded).

• They are colorless plastids which generally occur near the nucleus in nongreen cells and possess internal lamellae.
• Grana and photosynthetic pigments are absent.
• Leucoplasts have variable size and form,e.g.  rounded, oval, cylindrical, filamentous, etc.
• There are three types of special leucoplasts.
• Amyloplasts. They are the starch containing leucoplasts.
• An amyloplast is several times larger than the original size of leucoplast.
• It contains a simple or compound starch. 
• grain covered by a special protein sheath,eg, Potato tuber, Rice, Wheat.
• Elaioplasts (Lipidoplasts, Oleoplasts).
• The colorless plastids store fat, eg.Tube Rose.
• Aleuroplasts, Proteoplasts or Proteinoplasts.
• The plastids  Contains protein in the amorphous, crystalloid or crystallo globoid state (eg, aleurone cells of Maize grain, endosperm cells of Castor).

Chromoplasts (Gk. chroma - color, plastos, molded). 

• The plastids are yellow or reddish in color because of the presence of carotenoid pigments. 
• Chlorophylls are absent. 
• Chromoplasts are formed either from leucoplasts or chloroplasts. 
• Lamellae degenerate per tially or completely during chromoplast formation. 
• Change of color from green to reddish during the ripening of Tomato and Chilli is due to transformation of chloroplasts to chro moplasts. 
• The orange color of carrot roots is due to chromoplasts. 
• The pigments are often found in crystallised state so that the shape of the plastids can be like needles, spindles or irregular. 
• Chromoplasts provide color to many flowers for attracting pollinating insects.
• They provide bright red or orange color to fruits for attracting animals for dispersal.
• They are also the site of synthesis of membrane lipids. 

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