Plastids are microscopic organelles found in the cells of plants and some protozoans, such as Euglena and the cells of most photosynthetic microorganisms.
They are of three general types: leucocytes, chromoplasts and chloroplasts. Of these chloroplasts which contain chlorophyll (green pigments) are biologically the most important because they are the organelles responsible for photosynthesis.
Leucocytes are colourless and are the sites at which glucose is converted to starch and where lipids or proteins may be stored.
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Chromoplasts contain other pigments and are responsible for the bright colours of many kind of fruits, vegetables, flowers and leaves.
In plants, the chloroplasts are usually disc-shaped structure, 5-8 µm in diameter and about 1µm thick. Spherical, discoid or ovoid chloroplasts have also been reported.
Chloroplasts in their number, shape, size and distribution vary within the cell type and species, and often with the time of year and stage of development.
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The chloroplasts in a cell range from one-as in some algal cells-to as many as fifty in photosynthesizing cells of higher plants.
The plastids or chloroplasts like mitochondria are bounded by two unit membranes and have a system of internal membranes.
Each membrane of the chloroplast is trilaminar structure made up of lipoproteins and about 50 A thick.
The two unit membranes remain separated by periplastidial space which is about 100-300A wide in Nicotiana.
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The inner periplastidial space of the chloroplasts is filled with a watery, proteinaceous and transparent substance known as the matrix or stroma.
In the matrix the unit structures of the chloroplasts, the grana and intergrana connecting membranes are found embedded.
The grana are Iamellar or membranous, granular or chlorophyll bearing bodies. They vary in size from 0.3 to 1.7 µm in various species; in spinach chloroplasts they are about 0.6 µm in diameter.
In a given species grana may be present in choloroplast in some locations and absent from those in others; for example, in maize the mesophyll chloroplasts have grana but sheath parenchyma chloroplasts do not (Bishop, 1971).
Each granum is made up of 10-100 disc-like, super imposed stacks of membranous sacs called thylakoids or thylacoids (thylacoid- sac-like).
Each thylacoid remains separated from the stroma or matrix of the chloroplast by its unit membranes.
The grana of chloroplast are interconnected by a network of numerous tubules, the stromal lamellae or frets.
The stromal lamellae or frets are extensions of membranes of certain thylacoids into the matrix.
They are made up of many paracrystalline spheroid particles called quantasomes (Park, 1966). They are believed to be unit of photosynthesis, each consisting 250 molecules of chlorophyll.