Notes on the meaning, structure and functions of Plastids



Plastids are large cytoplasmic organelles, enclosed by a double membrance. They are mostly found in plants with exception of some lower groups like fungi. Few animals, like protozoan flagellates, also contain these. Besides metabolism, they impart colouration to plant tissues. On the basis of their colouration, plastids are of three ma­jor types chloroplasts, leucoplasts and chromoplasts.

Chloroplasts are the green plastids found in all green coloured eucaryotic cells. They contain abundant photosynthetic pigments the chloro­phyll. Organised chloroplasts are lacking in prokaryotic cells like blue-green algae and bacteria. Chloroplasts may be ribbon like, cuplike, lens-shaped, oval, spherical, discoid, polygonal or reticulate. Higher plants possess mostly discoid chloroplasts. Its diam­eter varies from 4-8u and is about 2u in thickness. A leaf parenchyma cell may possess 20-40 chloroplasts.

Each chloroplast is covered by two unit membranes, each about 40-60A thick. The space between the two membranes is called periplastidlal space, which measures 100-300A apart. The membranes are smooth, but the inner membrane is less permeable than the outer and is rich in carrier proteins. The internal space of the plastid is filled with a colourless proteinaceous matrix called stoma. The stroma contains starch grains, li­pid droplets, a small circular double helical DNA, free ribosomes and sev­eral enzymes. Presence of genetic material ensures its own genetic system; but it still depends on the cellular DNA.

Hence, it is referred to as semiautonomous organelles. The dark reaction of photosynthesis takes place in stroma. Additional mem­branous structures, called thylakoids, are found to be present in the stroma. About 20-50 thylakoids are stacked one above the other form a granum. Their number varies from 4-100 in a chloroplast. They constitute the site for light reaction of photosynthesis. Two adjacent grana are joined with one another by one or more lamellae called stroma lamellae or fret channel. The main function of chloroplast is photosynthesis.

The lamellae in chloroplast are composed of bimolecular layers of lipo­pigment sandwiched between two pro­tein layers. In addition to phospho­lipids, the lipopigment layers contain chlorophyll and carotenoid pigments. The chlorophyll pigments are so ar­ranged that a group of 230-300 mol­ecules become distinct in the lamellae and are called the quantasomes. Mor­phologically these are the smallest functional units of photosynthesis.

Leucoplasts are colourless plastids found mostly in the embryonic and sexual cells. They are also found in meristematic tissues and in regions of plants which do not receive light. Leucoplasts are either oval, spherical or filamentous and occur in large num­bers in cells of fruits, seeds, tubers and rhizomes. They serve mainly for the storage of materials like starch, oil and protein. Amyloplasts of potato store starch, elaioplasts of monocotyledons store oil and aleuronoplasts of castor seeds store proteins.

Chromoplasts are coloured plastids found in flowers and fruits. They contain fat-soluble yellow, orange and red pigments chiefly carotenoids. Leucopene of tomato, prolycopene of berries and capsanthene of red pepper are taken as examples. Chromoplasts are often found in fungi and bacteria.