Plastids: definition, types, structure &function
what are the types of plastids explain? What are the function of plastids?
In this article we will discuss about plastids in the cells:(1) definition,(2) types, (3) composition and structure of chloroplastid, (4) function.
- Composition and structure of chloroplastid.
Plastids constitute the most distinguishing and important feature of plant cells. They are the largest cytoplasmic organelle, hence visible under ordinary light microscope. Plastids contain their own genetic material and behave as self-replicating units. There is considerable variation among plastids with regard to their shape, size, colour and function. On the basis of colour pigments, plastids are of the following 3 types.
It was A.V Leeuwenhoek who first. observed chloroplasts in the leaves of plants. These are the most abundant and biologically most important among the plastids. Chloroplasts appear in many shapes and sizes. They may be disc-shaped or lens shaped or ribbon-shaped. In a cross section, a typical chloroplast is convex but appears circular when viewed at the surface. The growing tips or the meristematic tissues possess proplastids which develop into chloroplasts. The average size of chloroplast ranges from 4 u to 6 μ. Mostly they are located near the nucleus. In higher plants, the number of plastids varies from 20 to 40 per cell.
Chloroplasts are composed of chlorophyll (6 10 %), proteins (45-50 %), phospholipids (20-25 %), carotenoids(1-2 %), DNA (3.5 %),RNA (4.5%), ribosomes, cytochromes, vitamins (K, E ), iron, manganese, copper, zinc etc.
Chloroplasts are enclosed by an envelop made up of two membranes. The outer membrane is smooth and controls the exchange of materials between the cytoplasmic matrix and the organelle. The inner membrane is folded into a large number of parallel membranous sheets called lamellae. The lamellae lie embedded in the fluid-filled matrix, called stroma.The lamellae are arranged one over the other to form a system of fluid-filled sacs called thylakoids. The thylakoids are neatly stacked together like a pile of coins to form a composite body called grana. The number of grana varies from 40 to 60 per chloroplast. The thylakoids contain photosynthetic units called quantosomes. The quoutosomes are made up of chlorophyll pigment a, pigment b, carotenoid and xanthophyll. The function of chlorophyll is to trap light energy for the synthesis of NADPH and ATPase which are necessary for reactions during carbon dioxide assimilation.
These plastids manufacture and store carotene and xanthophyll which are responsible for orange, ped and yellow colour of plant parts. It’s has following types:
The phaeoplast (Gr., phaeo-dark or brown; plast-living) contains the pigment fucoxanthin which absorbs the light. The phaeoplasts occur in the diatoms, dinoflagellates and brown algae.
The rhodoplast (Gr., rhode= red; plast=living) contains the pig ment phaeoerythrin which absorbs the light. The rhodoplasts occur in the red algae.
These plastids are colourless and serve as storage organelles.Leucoplasts are found in abundance in seeds, fruits, rhizomes and cotyledons.Depending upon the type of materials stored, they are of the following types:
The amyloplasts are those leucoplasts which synthesize and store the starch.
The elaioplasts stores the lipids (oils) and occur in seeds of monocotyledons and dicotyledons. They also includes sterol-rich sterinochloroplast.
The proteinoplasts are the protein storing plastids.
- Plastids are semi-autonomous, self-replicating bodies. They contain their own genetic material and protein-synthetic machinery.
- Carotenes are essential in the synthesis of Vit. A in animal tissues.
- Chloroplasts are important for life as they produce and provide most of the food energy to the heterotrophs.
- Leucoplasts serve as storage sites for carbohydrates.
- Chloroplasts are associated with cytoplasmic inheritance.
- The transmission of plastids directly to daughter cells produce genetic continuity.