Plastid : Types, Structure and Functions
Plastids are the cytoplasmic organelles which are associated with the special metabolic process of the plant cells. It is the second largest organelle of the cell which is bounded by a double unit membrane and may be colored or colorless. Mayer and Schimper first used the term plastid. Plastids are found in plant cells and some algae. It is primarily responsible for the manufacture and storage of some vital chemical substances which are used by the autotrophic eukaryotic cells. Plastids contain photosynthetic pigments for the production of glucose as energy by the process of photosynthesis with the help of sunlight and CO2.
Types of Plastid
Plastid can be divided into following types such as:
Chromoplasts: Chromoplasts (Gr. Chroma, color) are the photosynthetically active colored plastids of the plant cells. They produce pigments and store them which are mainly found in leaves, flowers, ripe fruits and even roots of the plants. The most common chromoplasts are as follows:
Chloroplast (Gr. Chloros, pale green) is found in green algae and higher plants. It is the chlorophyll-bearing plastid of the plant cells which take part in the photosynthesis. It contains the pigments like chlorophyll a, chlorophyll b, DNA and RNA.
Plant cells contain many chloroplasts which are evenly distributed throughout the cytoplasm. Generally, they are found concentrated near the cell wall or the nucleus. In higher plants, chloroplasts are generally spherical or disc-shaped. The chloroplasts of some algae are in the form of stellate plates or spiral bands. The size of the chloroplasts varies from 4µm-6µm in diameter with 1 µm-3 µm in thickness. The number of chloroplasts varies from cell to cell. Chlamydomonas (algae) often contains a single huge chloroplast while in higher plants there are usually 30-40 chloroplasts per cell. Generally, chloroplast develops from proplastids. Proplast is a small double membrane-bound structure seen in meristematic cells. In the presence of sunlight, proplasts develop into normal chloroplasts.
Structure of Chloroplasts
The chloroplasts contain the following structures:
(1) Unit membrane: Each chloroplast is bounded by two unit membranes namely outer and inner membrane. Each membrane is trilaminar with 50-60 Å thickness and made up of lipoprotein. Between the two membranes, space is present which is known as periplastidal space which is 100 to 300 Å in diameter.
(2) Stroma: It is a transparent, clear and homogeneous colloidal gel-like fluid present within the inner membrane. It is also known as matrix which contains the following substances:
(3) Thylakoids: The inner membranous structures of the chloroplasts are organized into flattened sacs, known as thylakoids. They are embedded in the aqueous matrix. In the higher plants, the thylakoids are arranged in stacks, like a pile of coins. Each thylakoid is 100-300 Å wide. The thylakoids make the site for the light-dependent reactions of photosynthesis. The inner chamber of thylakoids contains the following components:
(4) Grana: The stalked thylakoids constitute the grana. Granum (singular) is the functional unit of the chloroplast. The size of the ganaum may range from 0.3-2.7 µm. The number of thylakoids in a granum may vary from 50 to 100. Each chloroplast usually contains 40-60 grana in their matrix.
(5) Stroma Lamallae: The grana are interconnected by a network of tubules, known as stroma lamellae or intergranal frets.
(6) Quantosomes: The thylakoid is composed of smaller spherical bodies called quantosomes. Each quantosome consists of about 250-300 molecules of chlorophyll and few carotenoid particles which are capable of carrying on Hill reactions of photosynthesis.
Functions of Chloroplast
The chloroplast performs the following important functions:
(2) Phaeoplast: It is brown colored plastid which contains the pigment fucoxanthin. Fucoxanthin is a carotenoid pigment which absorbs light and transfers the energy to chlorophyll a. These types of pigments are found in diatoms, brown algae, and dinoflagellates, etc.
(3) Rhodoplast: It is red colored plastid which contains the pigment phycoerythrin and they are found in red algae.
4. Blue-green Chromoplast: This type of chromoplast contains following pigments like phycocyanin, phycoerythrin, chlorophyll a and carotenoids. They are found in the blue-green algae.
This type of plastid does not have any pigments. They store food materials like carbohydrates, lipid, and protein. They are found in the sex cells and in the region of the plant that does not receive light. They may be rod-like or spherical in shape and are of many types:
(1) Amyloplast: It is double-enveloped organelle which can perform various biological pathways. It synthesizes and stores starch through polymerization of glucose in the endosperm, tubers, and cotyledons. Sometimes amyloplast can turn into the chloroplast.
(2) Elaioplast: This is one type of non-pigmented leucoplast which stores the lipid, oils, and they occur in seeds.
(3) Proteinoplast: It is identified in the year of 1960. It is also known as proteoplasts, aleuronaplasts, and aleuroplasts. This type of plastid stores protein. They are found in seeds of many plants peanuts, pulses, nuts, etc.
Plastid At a Glance
Chlorophyll a and b
Higher green plants and green algae
Brown algae, Diatoms, Dinoflagellates
Absorbs light and transfer energy
Absorbs light and transfer energy
(4) Blue green chromoplast
Phycocyanin, phycoerythrin, chlorophyll a and carotenoids
Blue green algae
Food storage cell
Starch synthesis and store it
Some monocots and dicots seeds
Store oils and lipid
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