4. Cell organelles: Plastids

     In  the cytoplasm there  are  a number of  small rounded or ellipsoid bodies called plastids. Plastids are found in Plants.  They  have special pigments called Chlorophyll. They have the capacity to  synthesise substances like starch and protein. 

A.   CHLOROPLASTS:

    Plant  chloroplasts are large sac like organelles about  5 to 10µm long having three membrane namely inner outer and thylakoid membrane. Thylakoid membrane forms a  network of flattened disc called thylakoids which are frequently arranged in stacks called grana. They are inter connected  by a tubular structure called stroma lamellae. A thylakoid has a flattened  disc shape structure inside it has  empty space called  thyllacoidel  space or lumen. DNA  and several types or RNA have been discovered  in association with chloroplasts. In the presence of sunlight  the chloroplasts produce carbohydrates from carbon-di-oxide  from the atmosphere.

B.   CHROMOPLAST:

    In the petals of many flowers which are coloured yellow or red. Here there are plastids have the capacity for developing and retaining pigments called Xanthophyll and carotin. They are irregular in  shape and are called chromoplasts.

3. LEUCOPLASTS:

     In  underground plant parts like the stem and roots are found certain  colourless plastids called leucoplasts. They have the capacity for forming starch from soluble sugers. These three plastids are inter connected and capable of getting converted into another. The  chloroplasts get converted into leucoplasts  in the absence of light. Leucoplasts become chloroplast when exposed to sunlight.