What are plastids?
Plastids are organelles characteristic of plant cells—they are clearly differentiated protoplasmic bits of specialised structure and function. Various types of plastids can be distinguished; they differ from each other in size, number, shape and pigmentation. Generally plastids are round, small, granular, discoid etc.; rod-like plastids are also of frequent occurrence.
Plastids are found in which cell?
Plastids are often coloured. In algae large plastids occur. With the possible exception of the very lowest groups of plants, plastids are present almost in all living plant cells and possibly in every cell at its early development; later they become confined to certain types of cells meant for specialised functions such as photosynthesis, colour show and storage. Lower plants may lack plastids or may contain one or two in each cell, but each protoplast of higher plants may contain many plastids.
Plastids are protoplasmic in nature ; though of gel consistency, they have semi-permeable double-layered limiting membrane, called peristromium. The main body of the plastid consists of a colourless cytoplasmic matrix called stroma, internal lamellar organisation may be present within the stroma.
(a) Formation of Plastids
Plastids never develop de novo, they are always derived from pre-existing very small bodies (4μ-6μ in diameter) termed as proplastids or plastid primordia, which are present in egg cells and in the cells of apical meristems. Plastids of all types are finally formed from the proplastids by division or constriction.
Leucoplasts and chromoplasts may originate by the arrest at particular stages of the normal sequence of the development of a chloroplast from a proplastid. If formation of a proplastid into a chloroplast is interrupted due to lack of light an elaioplast is formed.
(b) Types of Plastids
On the basis of the presence or absence of pigments, plastids are principally classified into two types, such as pigmented and non-pigmented. i.e. colourless plastids are known as leucoplasts. Pigmented i.e. coloured plastids consist of (1) chloroplasts, which are green in colour and in which the pigment chlorophyll predominates and (ii) chromoplasts, which are usually yellow, orange or red and in which pigment carotene predominates.
(i) LEUCOPLASTS OR LEUCOPLASTIDS
What is Leucoplast?
Leucoplasts are colourless i.e. non-pigmented plastids. They occur in mature cells that are not exposed to light, e.g. in the pith of many stems or in underground organs (stems, roots etc.). Leucoplasts are of varied and irregular shape; generally, they are elongated, spheroidal, rod-like or fusiform in shape.
Leucoplasts often appear as small masses of protoplasm which are variable and unstable in form; they are usually concentrated around the nucleus. When leucoplasts are exposed to sunlight, they are converted into chloroplasts or chromoplasts. They can change structure readily and are therefore highly plastic.
Types and functions of leucoplasts — Mainly two types of leucoplasts are found e.g. (i) amyloplasts and (ii) elaioplasts. Amyloplasts are starch-storing leucoplasts whereas elaioplasts are fat or oil-storing leucoplasts.
Amyloplasts are found usually in storage organs e.g. in potato tubers, corms and in other deep-seated tissues. They can develop into chloroplasts on exposure to light. Leucoplasts that are concerned to the formation of proteins are termed aleurone-plasts or proteinoplasts. Elaioplasts are mainly found in the cells of monocotyledonous plants and liverworts.
(ii) CHROMOPLASTS OR CHROMOPLASTIDS
What is Chromoplast?
Non-green plastids ranging in colour from yellow tones through orange to yellowish-red are known as chromoplasts. All such colours are due to the presence of carotenoid pigments viz orange-red carotenes and yellow xanthophylls. The pigments are present in the chromoplasts in various forms such as diffused, granular or crystalline.
Chromoplasts have various shapes which are usually irregular: they may be more or less round, elongated or angular and many of them are also lobed. It is presumed that it is the crystalline form that gives the various angular shapes to the chromoplasts as seen in carrot root. In many plant organs chromoplasts are capable of reverse differentiation into chloroplasts such as in carrot roots.
Chromoplasts are concerned with the formation of colours of fruits, flowers and sometimes roots (e.g. carrot).
Origin — The origin of chromoplast is directly from rudiments of colourless plastids (leucoplasts) or proplastids or from chloroplastids as a result of change.. as seen in green fruits after ripening.
Chromoplast Function — The function of this plastid is obscure. Perhaps it is helpful in imparting beautiful colours of flowers which is concerned in attracting insects and birds for bringing about cross-pollination. The attractive colours of fruits also help in the dispersal of fruits and seeds by animals.
(iii) CHLOROPLASTS OR CHLOROPLASTIDS
What is Chloroplast?
Chloroplasts are found in the cytoplasm of the cells in all green plants exposed to light and they are specially abundant in cells actively engaged in photosynthesis. In higher plants chloroplasts are mostly uniform in size and have the form of flattened discs, plates, ellipsoids or ovoids.
Among the higher plant, the chloroplasts are approximately 1μ thick and 3-4μ in diameter, but larger and smaller ones also exist; depending on the particular tissue as well as the plant, the number of chloroplasts in higher plants is always more than one per cell.
In some lower plant groups like algae they are of extreme form, large ribbon-like, reticulate or horseshoe-like. Chloroplasts of some plants (algae, bryophyta) contain one or more refractive protein bodies, called pyrenoids—during photosynthesis, starch grains are formed around each pyrenoid.
Each chloroplast is bound by a double-layered membrane and contains a relatively homogeneous matrix called stroma. Within stroma the photosynthetic pigments, chlorophylls and various carotenoids, are concentrated in distinct groups of about 0.3μ to 0.5μ diameter structures called grana. Each granum appears as a column stacked upon one another. Each granum disc is also made up of a pair of membrane lamellae or thylakoids (grana lamellae).
The grana are connected with each other by paired membrane structures that also extend out into the stroma (stroma lamellae) of the chloroplast. Grana are considered to be protein structure, and to carry chlorophyll intermixed with carotenoids and phospholipids. The stroma is composed of non-pigmented lipoproteins and associated enzymes.
The green colouring matter of the chloroplast is not a single pigment but consists of four pigments viz. chlorophyll a (C55H72MgN4O5), chlorophyll b (C55H70MgN4O6) orange-red carotene (C40H56) and yellow xanthophyll (C40H56O2)—the latter two pigments are called carotenoid pigments. In addition to chlorophyll and other pigments, chloroplasts also contain protein and RNA, at least in certain species, some DNA (Cutter, 1978).
Origin — Chloroplasts are produced in cells by replication of proplastids. According to Morrison (1967), chloroplastids originate from the plasma membrane of a primordial cell in the same manner as the mitochondria.
According to Cutter (1978), “during the formation of chloroplasts from proplastid, flattened vesicles bud off from the inner membrane of the proplastid. These increase in number and form collapsed double-membrane lamellae, which are aggregated in rows in some areas to give rise to the grana and become green.”
Chloroplast Function — The function of chloroplasts is very important; these green plastids are mainly concerned with energy reactions in the cell, with the elaboration of chlorophyll and carbohydrate synthesis i.e. photosynthesis. Nowadays, a close relationship between protein metabolism in the leaf and the photosynthetic activities of the chloroplasts has been suggested by Rhodes and Yemm (1963).