What is plastids
Plastids are semiautonomous organelles having DNA,RNA,Ribosomes and double membrane envelope.These are largest cell organelles in plant cell.
(1).Haeckel(1865) discovered plastid,but the term was first time used by Schimper(1883).
(2).A well organised system of grana and stroma in plastid of normal barley plant was reported by de Von Wettstein.
(3).Park and Biggins(1964) gave the concept of quantasomes.
(4).The term chlorophyll was given by Pelletier and Caventou,and structural details were given by Willstatter and Stall
(5).The term thylakoid was given by Menke(1962).
(6).Fine structure was given by Mayer.
(7).Ribs and Plaut(1962) reported DNA in chloroplast and was called plastidome.
Types of Plastids:
According to Schimper,Plastids are of 3 types:
They are colourless plastids which generally occur near the nucleus in nongreen cells and possess internal lamellae.Grana and photosynthetic pigments are absent.They mainly store food materials and occur in the cells not exposed to sunlight.e.g.,seeds, underground stems,roots ,tubers,rhizomes etc.These are of three types:
Synthesize and store starch grains.e.g.,Potato tubers,wheat and rice grains.
2.Elaioplast (Lipidoplast, Oleoplast):
They store lipids and oils e.g.,castor endosperm,tube rose. etc.
It is also known as Proteinoplast.It store proteins e.g.,aleurone cells of maize grains.
Coloured plastids other than green are known as chromoplasts.These are present in petals and fruits.These also carry on photosynthesis.These may arise the chloroplasts due to replacement of chlorophyll by other pigments.
Green tomatoes and chillies turn red on ripening because of replacement of chlorophyll molecule in chloroplasts by the red pigment lycopene in tomato and capsanthin in chillies. Thus,chloroplasts are changed into chromatoplast.
All colour (except green) are produced by flavins,flavenoids and cyanin. Cyanin pigment is of two types one is anthocyanin(blue) and another is erythrocyanin (red).Anthocyanin(blue)and another is erythrocyanin(red). Anthocyanin are water soluble pigments and found in cell sap of vacoule.
Discovered by Sachs and named by Schimper. They are greenish plastids which possess photosynthetic pigments.Chloroplast are also known as factory for synthesis of sugers.
It is variable.Number of chloroplast is 1 in Spirogyra indica, 2 in Zygnema,16 in S.rectospora, upto 100 in mesophyll cells.The minimum number of one chloroplast per cell is found in Ulothrix and species of Chlamydomonas.
They have various shapes.
|Cup shaped||Chlamydomonas sp.|
|Collar or girdle shaped||Ulothrix|
|Spiral or ribbon shaped||Spirogyra|
It ranges from 3-10 micrometer in diamete. The discoid chloroplast of higher plants are 4-10 micrometer in length and 2-4 micrometer in breadth.Chloroplast of Spirogyra may reach a length of 1mm.Sciophytes(Shade plant) have larger chloroplast.
Proteins 50-60%; Lipids 25-30%; chlorophyll 5-10%; Carotenoids(carotenes and xanthophylls) 1-2%; DNA 0.5%; RNA 2-3%; Vitamins K and E; Quinines,Mg,Fe,Co,Mn,P, etc.in traces.