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.
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.
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.
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.
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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All living organisms are composed of cells and cell products. Each eukaryotic cell contains several cytoplasmic organelles. Among the organelles, endoplasmic reticulum performs as intracellular transport system for various substances and also helps to exchange the materials between nucleus and cytoplasm. Prokaryotic cells and RBC (red blood cell) do not have any endoplasmic reticulum.
Endoplasmic reticulum is the interconnected system of membrane-bounded tubules and vesicles which form irregular reticulum or network in the cytoplasmic matrix. All nucleated animal and plant cells contain endoplasmic reticulum. This membranous system extends throughout the cytoplasm from nuclear membrane to plasma membrane. According to some scientists, endoplasmic reticulum is originated by the invagination of nuclear membrane but some suggested that they grow through the expansion of the pre-existing membrane.
In the light magnifying microscope, it would appear that a net in the cytoplasm consequently named as endoplasmic reticulum. The name endoplasmic reticulum was authored in 1953 by Keith R. Porter yet it had first seen by Porter, Claude and Fulum in 1945 after observing it under electron microscope of liver cells.
Morphologically, endoplasmic reticulum may happen in the three structures:
Cisternae: They are long, flattened, unbranched membrane-bound tubule with a diameter of 40-50 µm. They arranged parallel to each other to form lamellae. They are coated with ribosome.
Vesicles: They are oval or rounded membrane-bound isolated vacuolar structure with a diameter of 25-500 µm.
Tubules: They are long and branched structures with a diameter of 40-90 µm which form the reticular system in the cytoplasm along with cisternae and vesicles. Like the unit membrane, it is made up of lipo-protein.
There are two types of Endoplasmic reticulum such as Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER). Rough endoplasmic reticulum is also known as granular endoplasmic reticulum because it is attached with ribosomes while smooth endoplasmic reticulum is also known as agranular endoplasmic reticulum because they do not have attached ribosome with them.
Chemically, it is made up of the following components:
Endoplasmic Reticulum (ER) performs the following functions:
Golgi body is a flattened, membrane-bounded, parallely arranged sacs and other vesicles usually located near the nucleus in the cytoplasmic matrix of almost all eukaryotic cells. It is also known as Golgy complex, Golgi apparatus, Golgiosome, Lipochondria and in the plant cell, it is also called Dictyosome. Camillo Golgi first observed it in the nerve cell of barn owl in 1898.
Occurrence: They are generally present in the cytoplasm near the nucleus but in the glandular cells, they are located between the nucleus and apex of the cell.
Structure of the Golgi body: Its shape varies from cell to cell. It is large in nerve cell and small in muscle cells. The nerve cell, liver cell and most of the plant cells contain multiple Golgi bodies. The Golgi body forms from plasma membrane, nuclear envelop and endoplasmic reticulum. Generally, Golgi body consists of three membranous components such as:
Cisternae: They are parallely arranged sacs like structure which are covered by smoothed-surfaced single membrane and piled one upon the other to form stacks. The number of sacs in the stack ranges from 3-20 in number which depends on the cell type. They are usually equally spaced in the stack.
Small vesicles: The vesicles are small droplet-like sacs and originate from the cisternae by building or “pinching off” process.
Large vacuoles: They are clear and large sac-like structures. They contain electron-dense materials.
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The word mitochondria come from two Greek words, mito meaning thread and chondrion meaning granule. It has double membrane enclosed rounded or rod-like or filamentous bodies which generate chemical energy in the form of ATP. It is scattered throughout the cytoplasm in most of the cells.
Scientist Kolliker (1850) first observed mitochondria in the striated muscle. Flemming (1882) named it as fila. Rechard Altmann (1897) referred to the mitochondria as Bioplasts. Carl Benda (1897) first called these organelles as mitochondria.
It is not found in the prokaryotic cell and matured circulated RBC. Generally they are evenly distributed in the cytoplasm. It is also found in the base of the proximal convoluted tubules of nephron. The number of mitochondria present in the cell depends on its activities. Plant cell contains less number of mitochondria than animal cell. A normal liver cell may contain 1000-1600 mitochondria while some oocytes may contain more than 300000 mitochondria. On the other hand, Microasterias algae contain only one mitochondrion
Mitochondria have generally spherical or rod-shaped or filamentous structures. It is about 3.0-7.0 µm in length and 0.2-2.0 µm in diameter. It usually consists of two unit membranes, two chambers, mitochondrial matrix and mitochondrial particles. Every mitochondria is bounded by the double membranes, the outer membrane and inner membrane. Each membrane is made up of lipo-protein and it is about 60 Angstrom thick. The outer membrane is smoothed and covers the mitochondria but the inner membrane remains folded inward at various points to form a number of incomplete partitions which are known as the cristae or mitochondrialis cristae. The two membranes remain 60-80 Angstroms apart from each other. The space between the two membranes is filled up with fluid.
Chambers: It has two chambers, outer and inner chambers. The outer chamber is the space between the outer and inner membranes which is filled with watery fluid. The inner chamber is covered by the inner membrane. The inner chamber contains mitochondrial matrix.
Mitochondrial matrix: The inner chamber is filled with a relatively dense proteinaceous material usually called mitochondrial matrix. This matrix contains dense granules, ribosomes and mitochondrial DNA. The enzyme of the Krebs cycle is located in the matrix.
Mitochondrial particles: The cristae that project into the matrix are usually incomplete septa or ridge. The cristae are covered by mushroom-like particles of 85 Angstrom. These particles are known as F1-Particles or Fernandez-Moran subunits or elementary particles. Each particle consists of three parts; head, stem or stalk and base. The particles are spaced about 100 Angstrom interval on the cristae.
The cell is the structural and functional unit of life, which is also known as "building blocks of life." The science which deals with the study of cells is called Cytology or cell biology. Robert Hooke first discovered the cells in 1665. Matthias Jakob Schleiden and Theodor Schwann first developed a cell theory in 1839. Based on cell theory, all organisms are made from one or more cells. The word "cell" is derived from Latin words, cella, which means "small room."
The cell contains cytoplasm with proteins and nucleic acids, which is bounded by a membrane. Under a microscope, most plant and animal cells are visible, and their dimensions range from 1-100 µm. The number of cells varies from species to species. The human contains about 4×1013 cells. The smallest known cell is a tiny single-celled bacterium, known as Mycoplasma, which is 0.2 μm in diameter.
There are various definitions of the cell which are given below:
The number of cell varies in the living organisms. The unicellular organisms like bacteria, amoeba, diatoms, Euglena etc contain single cell in their body but most of the plants and animals are multi-cellular organisms which contain many cell in their body. The number of the cell is never fixed for any multi-cellular organism.
The shape of the cells is highly variable. Generally, the animal cell is spherical in shape, but it may be elongated, cylindrical, oval, rounded, triangular, cuboidal, polygonal or irregular in different plants and animals. The shape of the cell remains correlated with its functions. The external and internal environment of the organism may also cause shape variations in the cell due to internal or mechanical stress or pressure and surface tension. The shape of the cell may vary from organ to organ, plant to plant and animal to animal.
The size of the cells varies from cell to cell. Most of the eukaryotic cells are microscopic in size but they are bigger than the bacteria. Generally, size of the cell varies from 1-175000 µm. Among the living organisms, the smallest cell is mycoplasma bacteria which are 0.1-0.25 µm in diameter while the biggest cell is ostrich egg (170 ×135 mm). The longest cell is the neuron cell which is about a meter or more in length.
Generally, the cell is of two types:
This type cell is lack of nuclear envelop and well defined cytoplasmic organelles such as endoplasmic reticulum(ER), Golgi body, Mitochondria, centriole etc. Example of eukaryotic cells: Bacteria, blue green algae etc.
The eukaryotic cell has the nucleus with a definite nuclear membrane. This type of cell also contains cytoplasmic organelles like endoplasmic reticulum, Golgi bodies, mitochondria, lysosomes, etc.
The eukaryotic cell is the true cell which has the following characteristic features:
The eukaryotic cells have different shapes, sizes and physiology but all the cells are typically composed of:
Cell-covering has two parts such as plasma membrane and cell wall. Most of the cells are enclosed by a thin porous semi permeable membrane which is known as plasma membrane. The plasma membrane may be modified to form villi, cilia, flagella, cavities, and other special structures. The cell wall is present only in the plant cell. It is present outside of the cell membrane which is a thick semirigid, laminated, non-living cellulose covering.
Plasma membrane is a permeable membrane by which extracellular substances entered into the cell while the cell wall provides protection and support to the plasma membrane and cytoplasm.
The substance which occurs between the plasma membrane and nuclear membrane is called cytoplasm. It is made up of the matrix and the organelles. Matrix is an amorphous, translucent, homologous colloidal liquid which is known as hyaloplasm or cytoplasmic matrix.
Cytoplasmic matrix contains glycolytic enzymes and structural materials such as sugars, amino acids, water, vitamins, nucleotides etc. They carry out the instructions sent from the nucleus. They also provide sites for cellular activities.
The organelles are the membrane-bound living structures of a cell which are situated within the cytoplasm. Generally, they perform various important biosynthetic and metabolic activities such as transportation, support, storage, reproduction, respiration etc. The eukaryotic cell contains the following organelles:
It is very important organelle of the cell and is covered by a single smooth membrane of lipoprotein. It consists of cisternae, vesicles and vacuoles.
It plays an important role for the transportation of materials within the cell. It forms secretary vesicles and lysosomes. They also form cell wall of the plat cell and plasma membrane.
They are inter-connectin tubules and vesicles which are bounded by a single unit membrane. The membranes may be rough and smooth due to presence or absence of ribosomes.
They maintain intracellular circulatory system. They act as synthetic and storage organs. They provide mechanical support to the cell by making cytoplasmic frame work.
It is found only in the animal cell and it is a tiny spheroid particles which consist of hydrolytic enzyme. They are enclosed within the single lipoprotein membrane.
They mainly take part in the intracellular digestion of food materials within the cell by the process of pinocytosis and phagocytosis.
They are minute spherical non-membranous structures which consist of RNA and protein. They have two structural unequal sized subunits. The smaller subunit is called 40s subunit and a large subunit is called 60s subunit. They are located freely in the cytoplasm or remain attached to the surface of endoplasmic reticulum, nucleus etc.
They provide sites for protein synthesis.
It is filamentous or granular hollow type structure which is bounded by a double lipoprotein membrane. The inner granulated membrane is convoluted to form cristae and divides the mitochondria into two chambers, the outer chamber and matrix filled inner chambers.
They are called power house of the body because they produce energy as ATP through Kreb`s cycle, electron transport chain, bet-oxidation of fatty acids etc.
It is a disc shaped chlorophyll containing organelle which is bounded by double membrane. It is only present in the plant cell. Within the inner membrane, stroma or matrix is present. Stroma contains small cylindrical structures called grana. Granum is a flattened vesicles which contains small structures or quantosomes.
They act as a storage for starch, pigments for photosynthesis. They help in the biosynthesis of food stuffs by the process of photosysnthesis.
It is present only animal cell. It contains dense cytoplasm which is placed near the nucleus of the cell.
It forms the spindle during the cell division and help in the movement of chromosomes.
Nucleus is the brain of the cell which is generally rounded in shape and is placed at the center of the cell. The typical nucleus consists of the following components:
Nuclear membrane: It is also known as nuclear envelope. It is a bilayer membrane which is made of lipids and the genetic material in eukaryotic cells. It encloses the nucleus where nucleo-cytoplasmic interchange takes place.
Nucleoplasm: It is clear like water substance which is present in the space between the nuclear membrane and nucleolus. It contains ribose sugar, phosphorus, protein, nuclic acid and nucleotides.
Nucleolus: It is a dense spherical body covered by single membrane which is present in the nucleoplasm. It mainly consists of nucleoprotein.
Chromosomes: In the nucleoplasm, a thread tike elongated structure is present which is known as chromosome. They appear only during the cell division.
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