These are green plastids. They have green pigment the chlorophyll. Chloroplast takes part in photosynthesis. They are found in mesophyll cells of leaf and chlorenchyma cells in stem. Many algae have only single Chloroplast in their cells but other advanced groups of plants have 20-40 chloroplasts per cell. Their size and shape varies in different groups of plants. In algae they are stellete, ribbon-like, cup-like, reticulate, spiral, sheet like, disc or tablet like. In mosses, ferns and seed plants chloroplasts are flat, circular, ovate, elliptical.
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Their detailed structure has been revealed by electron microscope. Each chloroplast has two unit membranes made up of lipoprotein. Inside the membranes is the proteinaceous ground substance called stroma . Stroma contains a variety of particles, osmiophilic droplets, ribosomes, strands of DNA and RNA, dissolved salts, enzymes. Embedded in the stroma are many membranes, running parallel to each other throughout the length of the chloroplast. These are called lamellae, Each lamella is made up of two unit membranes. At some intervals the lamellae have 10-100 rounded flat sac like structures stacked one above the other. Such rounded flat structure is called thylakoid.
One stack of thylakoids is called granum. Different grana are connected with grana of other lamella with the help of tubular connections called stroma lamellae or frets. Each chloroplast contains 40-60 grana. Membrane of grana lamellae are made up of proteins and phospholipids and contains chlorophyll and carotenoids. On the inner wall of thylakoids are found small granules called quantasome. Park and Pon (1961) and later Park and Biggins (1964) reported quantasome. In the past, quantasome was considered as basic photosynthetic unit, but it is not a functioning photosynthetic unit. Each quantasome has 230 chlorophyll molecules.
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Chlorophyll molecule has a complex porphyrin ring (head) to which is attached long hydrophobic phytyl (C28 H39) chain (tail). The porphyrin ring is composed principally of four pyrrole nuclei linked together bearing side chains. The metal constituent of the molecule, magnesium, is incorporated in this ring. The hydrocarbon phytyl group upon hydrolysis gives rise to the alcohol phytol which is an isoprenoid derivative.