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Thursday, September 30, 2010

Vacuoles

Thursday, September 30, 2010 - 0 Comments

They are membranous sacs that are part of cytomembrane system. Vacuoles occur in different shapes and sizes and have various functions. In animals like protozoa and sponges, contractile vacuoles are present that collect water from the cytoplasm and pump it outside to maintain organism’s interval environment. Other protozoa and sponges have vacuoles for storing food.

Centrioles and Microtubule Organizing Centres:

The specialized non membranous regions of cytoplasm near the nucleus are micro tubular organizing centres. These centres of dense material give rise to large number of microtubules with different functions in cytoskeleton. One such type of centre gives rise to centrioles that lie at right angles to each other. Centrioles are small hollow cylinders that occur in pairs in most animal and lower plant cells in distinctly staining region of the cytoplasm known as centrosome or centrosphere. They are located near the nuclear envelope and are double structures that lie at right angles to each other. Each centriole is about 300 – 500nm long and 200nm in diameter. Each centriole is composed of nine triplet microtubules that radiate from the centre like the spokes of a wheel. Adjacent triplets are attached to each other by fibrils. Centrioles are also present at the bases of cilia and flagella where they are known as basal bodies or kinetosomes. At the beginning of nuclear division, the centriole replicate and two new pairs migrate to opposite poles of the spindle, the structure on which the chromosomes become aligned. Centrioles help in the formation of poles during cell division mitosis and meiosis. Centrioles form the base of the cilia and flagella.

Friday, September 24, 2010

Cilia and Flagella (Short Note)

Friday, September 24, 2010 - 0 Comments

Cilia and flagella are elongated appendages on the surface of some cells by which the cells including many unicellular organisms move. In non motile cells, if cilia and flagella are present they move material over the cell’s surface. Flagella are 5 to 20 times as long as cilia and move some what differently. Cilia and flagella have similar structure and consist of membrane bound cylinders that enclose the matrix. In matrix is an axonene or axial filament which consists of nine pairs of microtubules arranged in circle around two central tubules. This is called 9+2 pattern of microtubules. Each microtubule pair, a doublet also has pairs of dynein arms made up of proteins, projecting toward neighbouring doublet and spokes extending toward the central pair of microtubules. Cilia and flagella move as a result of microtubule doublets shading along one another.
In the cytoplasm at the base of each cilium or flagellum lies a short of cylindrical basal body which is also made up of microtubules and is structurally identical to the centriole. The basal body controls the growth of microtubules in cilia and flagella. Microtubules in the basal body form 9+0 pattern: nine sets of three with none in the middle.
Flagella ad cilia have fundamental similarity of ultra structure. They are made up of two central fibres surrounded by nine double peripheral fibres arranged in a circle. This bundle of fibres is called axonene. Each peripheral fibre is composed of protein tubulin and consists of A and B microtubule. Each a microtubule has pairs of arms at regular intervals along its length. Arms are composed of another protein dynein that is capable of hydrolysing ATP because it is an enzyme ATP ASE. The central fibres are connected to a microtubule of peripheral fibres by radical spokes. These fibres arise from basal granule or kinetosome that is identical in structure to axonene but has 9+0 structure and is derived from a centriole.

Microtubules (Short Note)

Majority of eukaryotic cells contain unbranched hollow, cylindrical organelles called microtubules. They are very fine tubules made up of globular sub units of protein called tubulin. They may extend for several micrometers in length. At intervals cross bridges or arms sometimes project from their walls linked with other microtubules. They help in movement of organelles such as secretary vesicles. They are involved in the movement of chromosomes during division of the cell nucleus. Microtubules are involved in the over all shape changes that cells undergo during period of specialization.

Cytoskeleton (Short Note)


In most cells a flexible cellular framework known as cytoskeleton is present which is extended throughout the cytoplasm connecting various organelles and cellular components. The skeleton of the cell is present in the form of microtubules, intermediate filaments and microfilaments. Chemically cytoskeleton comprises of contractile proteins like tubulin, actin, myosin, troponin and tropomyosin.

Monday, September 20, 2010

Mitochondria or Chondriosomes as Power House of Cell

Monday, September 20, 2010 - 0 Comments

Mitochondrion is rod like organelle in the cytoplasm of all eukaryotic cells. The number of mitochondria in a cell is variable and ranges from one to ten thousand depending upon the cells’ function. It converts energy to a usable form, so the mitochondrion is considered to be the power house or power generator of the cell.
This shape of mitochondrion may be spherical, elongated or cylindrical in most of the animal and plant cells. The size varies depending upon the physiological conditions of the cells. Mitochondrion is double membrane bound organelle. Outer membrane is smooth while the inner membrane folds and doubles in on itself to form incomplete partitions called crystal. A small space known as inter-membranous space separates the outer membrane from the inner membrane. The crystal increases the surface area available for chemical reactions that trap usable energy for the cell. Inner membrane contains a gel like fluid the mitochondrial matrix where the inner membrane folding the crustae extends.

The matrix contains ribosomes, circular DNA, enzymes and coenzymes in addition to other substances.
Mitochondria are self replicating organelle due to the presence of its own DNA and ribosomes. It replicates when a cell needs to produce more energy.
Mitochondria are seen to be in constant motion in living cells. Mitochondria are the centre of aerobic respiration. Interior of Mitochondrion contains fluid like organic matrix with a number of chemical compounds in it.
On the cristae are located enzymes and coenzymes by means of which carbohydrates (starch), fatty acids, lipids and amino acids (proteins) are metabolized to CO2 and H2O. Energy in the form of ATP is release in this process which is stored within mitochondria. Adenosine triphosphate (ATP) is energy rich compound and it provides energy to the cells of organs for various activities. Hence mitochondria are known as “Power house of cell” where energy is stored and released wherever and whenever required by a living body. Mitochondria have semiautonomous existence in the cell. They have their own DNA that directs production of some of their component proteins and they can divide in half and thus reproduce independently of cells normal cell division cycle.
Mitochondria are passed to an animal only by mother. Since mitochondria are present in eggs but not in the part of the sperm that enters the egg. Thus people can trace their mitochondria back to their mothers and grand mothers.
Functions: Mitochondrion is the centre of cells respiratory and metabolic activity where food is oxidized to carbon dioxide and water through kerb’s cycle and Electron transport chain. During these processes, production of energy in the form of ATP (adenosine triphosphate) also results. Therefore they are called “Power house of the cell”.

Lysosomes

Laso = dissolving + some = body, are membrane bound spherical organelles containing lysomosal enzymes, that are hydrolytic in nature. Important enzymes present in the lysosome are acid hydrolases, proteases, lipases and acid phosphatases. These enzymes are capable of digesting organic molecules like lipids, proteins nucleic acids and polysaccharides under acidic conditions. De dive first described their in 1949. Almost all cells contain lysosomes but their number if much increased in the cells involved in the process of phygocytosis except erythrocytes. There are two main types of lysosomes i.e. primary lysosomes and secondary lysosomes.

Formation of primary lysosomes: Enzymes present in the lysosomes are synthesized in rough endoplasmic reticulum and are then transported through the cytoplasm by transport vesicles into Golgi apparatus through Cis Golgi or forming face. These enzymes are further processed in Golgi apparatus and then budded off from Trans – Golgi or maturing face of Golgi apparatus in the form of primary lysosome.
Enzymes are sometimes synthesized by ribosomes, transported through endoplasmic reticulum into Golgi apparatus and from trans Golgi, they are punched of as vesicles, the primary lysosomes.
Secondary lysosomes: They are formed when primary lysosomes fuses with phagocytic vesicles, thus exposing the vesicles contents to lysosomal enzymes. These enzymes present in the primary lysosomes digest the food present and the soluble substances are diffused into the cytoplasm of the cell. Undigested material containing vacuole known as residual body is expelled out by exocytosis.
Phagosomes: When the primary lysosome fuses with a specialized white blood cell, the pliagocyte, an activated phaogsome or phagocytic vesicles formed. They fight against pathogen by engulfing them very rapidly than the ordinary phogocytes.
Autophagic vacuoles: During starvation or after the destruction of cell components especially the liver cells and cells destroyed during metamorphosis fuse with the primary lysosome to form autophagic vacuoles or cytolysosomes.
Function of lysosomes:

Functions are as under:
(1)        Phagocytosis: Any foreign object (pathogens) that gains entry into the cell is immediately engulfed by lysosome and is completely broken into simple digestible pieces. This process is called phygocytosis.
(2)        Intracellular digestion: They are involved in intracellular digestion since they have enzymes to digest the phagocytosed food particles present in food vacuoles.
(3)        Extra cellular digestion: They also help in extra cellular digestion by releasing enzymes.
(4)        Exo-cytosis: Sometimes enzymes of primary lysosome are released from the cell. This occurs during the replacement of cartilage by bone during development. Similarly the matrix of bone may be broken down during the remodelling of bone that can occur in response to injury, new stresses and so on.
(5)        Autophagy: (Self Eating): It is the process by which unwanted structures like damaged mitochondria etc within the cell are removed. Unwanted structures are first enclosed by single membrane, usually derived from smooth endoplasmic reticulum. Then this structure fuses with primary lysosome to form secondary lysosome called autophagic vacuole in which unwanted material is digested.
(6)        Autolysis: It is the self destruction of a cell by release of the contents of lysosomes within the cell. It is normal event in some differentiation processes and may occur throughout a tissue, as when a tadpole tail is reabsorbed during metamorphosis. It also occurs after cells die. Sometimes it occurs as a result of certain lysosomal diseases or after cell damage.
(7)        Recycling of important constituents: As a result of phago cytosis and digestion of different components, the lysosomes help in the recycling of important components of the cytoplasm.

Thursday, September 16, 2010

Golgi Apparatus

Thursday, September 16, 2010 - 0 Comments

Camillo Golgi in 1898 while studying the eukaryotic cells observed a system of tightly packed smooth surfaced vesicles lying near the nucleus. This structure was named as Golgi apparatus, Golgi body, and Golgi complex in animals and in plants as dictyosome. Golgi complex is an organelle with diverse shape and number. In most of animals’ cells there is present only one Golgi apparatus but are abundantly found in cells that secrete chemical substances like pancreatic cells which secrete digestive enzymes and neve cells that produce neurotransmitters. In certain plant cells the number may be in hundreds. Golgi apparatus is formed flattered sacs or cisternae but some tubules and vesicles may also participate in the formation of Golgi complex. Number of fluid filled flattered sacs may range from 3 – 7 in most of animals out lower organisms have up to 30 flattered cells. These flattered cells are arranged in a concentric fashion, the convex sacs lie closer to the nuclear membrane and are termed as cis Golgi or forming face. Farthest concave sacs are named as trans-Golgi or maturing face. Proteins or material enter Golgi body through forming face and after modification are released from maturing face.
Golgi body is chemically made up of lipoprotein and a number of other organic molecules that are present or transported through it. Golgi complex is continuous with endoplasmic reticulum canals on one side and to secretary vesicles leading the cell membrane on the other.

Mechanism of secretion of Golgi complex: Following six steps of secretion are involved in pancreas and other zymogen secreting glands.
(1)        Ribosome stage: It explains synthesis of protein molecules protein molecules by ribosomes attached with ribosomal endoplasmic reticulum.
(2)        Cistarnae stage: This stage involves the flow of protein (forward form ribosomes) through ER tubes called cisternae towards dictyosomes.
(3)        Intracellular transport: Secreted proteins are pinched off as transitional vesicles and tubules from ER and they flow in the cytoplasm towards dictyosomes where they fuse to form large condensing vacuole at the forming face of Golgi.
(4)        Concentration of Secretion: By process of concentration the condensing vacuole is converted into zymogen granules.
(5)        Intracellular stage: Zymogen granules are now changed by Golgi into secretary granules and are stored in the cell and are released in response to proper stimulus (a hormone or neurotransmitter) that acts on the cell.
(6)        Exocytosis: The discharge of secretary granules is effected by exocytosis.
Function of Golgi bodies:
(1)        Secretion: It secrets many secretary granules like lysosomes, peroxisomes.
(2)        Exocytosis: Proteins packed in secretary vesicles are released into cytoplasm close to the plasma membrane when the vesicles reach the plasma membrane, they fuse with it and release their contents to the outside of cell by exocytosis.
(3)        Storage of proteins: Proteins synthesized by Ribosomes are sealed off in little packets called transfer vesicles which pass from the endoplasmic reticulum to Golgi apparatus and fuse with it. In Golgi apparatus proteins are concentrated and chemically modified and can be used with cell or can be exported out of the cell.
(4)        Formation of Glycolipid and Glycoprotein: Carbohydrates, lipids and proteins synthesized by endoplasmic reticulum are modified as glycolipid and glycoprotein in within Golgi complex.
(5)        Cell wall formation: Golgi bodies are also involved in the formation of new plant cell wall.

Endoplasmic Reticulum of the Cell

Endoplasmic reticulum (ER) is a complex, membrane bound labyrinth of flattered sheets, sacs and tubules that branches and spreads throughout the cytoplasm. The number and shape of endoplasmic reticulum may vary from one cell type to another, also in different physiological and developmental stages of the same cell type. The enclosed shapes called cisternae contain certain chemical substances that may vary from cell to cell. Endoplasmic reticulum has communication with the exterior, with the nuclear envelope as well as to the Golgi apparatus. Endoplasmic reticulum is continuous from the nuclear envelope to the plasma membrane in the form of a series of channels that help various material to circulate throughout the cytoplasm. It is also a storage unit for enzymes and other proteins and a point of attachment for ribosomes.
Types of endoplasmic reticulum: On the basis of appearances, endoplasmic reticulum is classified as:
(1) Rough or granular endoplasmic reticulum.
(2) Smooth or agranular endoplasmic reticulum.

(1)        Rough or granular endoplasmic reticulum:
This is the type of endoplasmic reticulum that bears on its cytoplasmic face large number of small granular structures, the ribosomes. Due to the presence of these ribosomes endoplasmic reticulum gives rough appearance so named as rough or granular endoplasmic reticulum. Ribosomes present on ER help in protein synthesis that is then transported to different parts of the cell including Golgi bodies through cisternae.









(2)        Smooth or agranular endoplasmic reticulum:
This is the type of endoplasmic reticulum without ribosomes. They have their own enzyme system and perform certain important functions.
Most cells contain both types of endoplasmic reticulum although relative proportion varies among cells.










Functions of Endoplasmic reticulum:

It plays an important role in the activity of cell. The functions are:
(1) Mechanical support: Due to flexible nature of plasma membrane and ability to extend into the cytoplasm, it has connections with nuclear envelope and Golgi apparatus which help to provide mechanical support to the cell.
(2) Transportation of material: As the endoplasmic reticulum has direct or indirect convection with the important organelles of the cell as well as with the cytoplasm and exterior, it acts as a transporter for the transportation of different material within the cell and from surrounding cells.
(3) Synthesis and transportation of proteins: The rough endoplasmic reticulum is involved in the synthesis and transportation of cellular proteins.
(4) Detoxificaiton of harmful substances: Smooth endoplasmic reticulum due to its own enzyme system metabolizes or destroys the toxic substances like steroids, carcinogens and toxins.
(5) Synthesis of lipids: Smooth endoplasmic reticulum synthesis different types of lipids that are used for the formation of plasma membrane and steroid hormones like testosterone and estrogens. Glycogen and glycolipids are also synthesized here.
(6) Site of new membranes: They are site for synthesis of proteins and lipids and are also considered to be primary site of new membranes.
(7) Storage of calcium ions: Smooth endoplasmic reticulum sores calcium ions in muscle cells which are required for muscle contraction.

Thursday, September 9, 2010

Plasma Membrane or Cell Membrane and its Function

Thursday, September 9, 2010 - 2 Comments

Cell surface is bounded by very thin osmotically active membrane called cell membrane or plasma lemma or plasma membrane. It is thin, elastic, porous and semi permeable in nature.

About the structure of plasma membrane different theories were proposed by some workers. To describe the structure of cell membrane, three structural models have been described which are as follows:

(1)        Bi-molecular lipid layer:
Gorter and Grendel in 1925 suggested that a membrane is made up of two layers of lipid molecules. Hydrophobic ends of each molecule point towards the interior while hydrophilic ends point towards the outer boundary of the membrane.






(2)        Trilaminar structure:
In 1935 Danieth and Davson proposed trilaminar nature of membrane which was further refined in 1938 by Harvey and Denieth indicating that bimolecular lipid layer is sandwiched by two protein layers. Roberson in 1959 confirmed the trilaminar structure of protein-lipid-protein. Middle layer which is about 35A thick is chemically made up of two layers of phospholipids. Hydrophobic ends of two lipid layers face each other while hydrophilic ends face outwards that is towards proteins. This lipid bi-layer is enclosed by 25A thick layer of protein or extended polypeptide chain. Under most circumstances the thickness of the membrane is 75A.

(3)        Fluid Mosaic Model:
In 1972 S. Jonathan Singer and Garth Nicloson developed fluid mosaic model of membrane structure. According to this model, a membrane is double layer of bi-layer of proteins and phospholipids and is fluid rather than solid. Phospholipids layer forms an ocean or sea in which specific proteins float like ice bergs. Being fluid the membrane is in constant state of flux shifting and changing, while retaining its uniform structure.
This word mosaic refers to many different kinds of proteins dispersed in phospholipids bi-layer. Important points are:
(1) Phospholipids have one polar end and one non-polar end. Polar ends are oriented on one side towards outside of cell and into the fluid cytoplasm on other side, and the non-polar ends face each other in middle of the layer. Taits of both layers of phospholipids molecules attrat each other and are repelled by water and as such are hydrophobic or water dreading. As a result the polar spherical leads the phosphate protein is located over the cell surfaces both the outer and inner and are hydrophobic or water attracting.
(2) Cholesterol is present in plasma membrane and organelle membranes of eukaryotic cells. Cholesterol molecules are embedded in interior of membrane and help to make the membrane less permeable to water soluble substances. Rigid structures of cholesterol molecules helps to stabilize the membrane.
(3) Membrane proteins are individual molecules attached to inner or outer membrane surfaces and termed as peripheral proteins or extrinsic proteins. Some are embedded within lipid bi-layer of membrane and is termed as intrinsic proteins. Some of them are linked to sugar protein makers on cell surface. Other intrinsic protein help to move ions or molecules across the membrane to cells inner part the cytoskeleton or to various molecule outside the cell.
(4) When carbohydrates unit with proteins they form glycoproteins and when they unite with lipids they form glycolipids on the surface of plasma membrane. Surface carbohydrates and portions of proteins and lipids make up glycocalyx or cell coat.
Function of plasma membrane:
(1) Regulation of material through cell membrane: Plasma membrane is differentially permeable membrane. It allows some selective molecules to pass through it. Different molecules may move from outside the cell to inside the cell or from inside to outside the cell.
(2) Transport material: Oil or fat soluble molecules enter through the lipid portion of membrane where as the water soluble molecules pass through the proteins.
(3) Cellular homeostasis: Plasma membrane help in homeostasis by regulating movement of water molecules and ions.
(4) Mechanical support: Plasma membrane provides mechanical support due to its flexible nature and helps in maintaining specific shape of the cell.
(5) Acts as barrier: It forms the boundary of the cell and separates the inside of the cell from outside. It also separates various organelles within the cell.
(6) Provide receptor site for specific chemicals: It provides large surface area on which specific chemical reactions can occur. Some proteins act as receptor sites, which bind specific chemicals. This binding helps the cell to begin specific function such as synthesis of a hormone.
(7) Cell identity makers: Cell identity makers are the sites proteins in nature that differentiate one type of cell form the other.
Movement across membrane: Molecules can cross membranes in number of ways both by using their energy and by relying on outside energy source. Various ways are diffusion & osmosis. Transport across membranes is necessary to maintain suitable pH, ionic concentration for enzymes activity and excrete toxic substances. For entry or exit, there are two main processes passive transport i.e. diffusion and osmosis and active transport. The passive processes do not require energy which active require energy with these. There are two other phenomenon i.e. endocytosis and exocytosis.
(i) Diffusion or Passive transport: It occurs spontaneously and no extra energy is required to bring it about. Few substances freely diffuse across plasma membrane e.g. the respiratory gases (O2 and CO2) diffuse in and out of cell.
(ii) Osmosis: It maintains a balance between osmotic pressure of the intracellular fluid and that of interstitial fluid known as osmoregulation.
(iii) Active transport: Movement of molecules from lower concentration to higher concentration by consuming energy called active transport.
(iv) Endocytosis: It is the process in which cell membrane helps to take in material by infolding in the form of vacuole endocytosis may be (a) phagocytosis in which solid particles are picked and ingested by the cell e.g. W. B. C. picked up foreign bodies from the blood stream. In this way they destroy harmful bacteria. It is also called cell eating process, (b) Pinocytosis when liquid material in bulk in the form of vesicles is taken in by endocytosis, the process is called pinocytosis which is also called cell drinking process.
(v) Exocytosis: The process of membrane fusion and the movement of material out of a cell is called exocytosis.

Saturday, September 4, 2010

Various Organelles Present in the Cell

Saturday, September 4, 2010 - 0 Comments

Cell is the fundamental unit of life. It is tiny microscopic mass of protoplasm enclosing a denser spherical or oval body the nucleus. Cell is bounded by cell membrane, protoplasm, nucleus and cell membrane are all living parts of cell. Recently cell is defined, as structural and fundamental unit of living organism or the cell is the basic unit of life.
Animal cell consists of plasma membrane, nucleus, cytoplasm and its organelles.
Plasma membrane: All cells are covered by membrane that serves as their outer boundary separating the cytoplasm from external environment. This is called plasma membrane. It allows the cell to take up and retain certain substances while excluding others. It consists of double layer of phospholipids bearing proteins.
Nucleus: Nucleus lies in the cytoplasm of the cell. Nucleus is most important part of the cell and controls all the activities of the cell. Nucleus is spherical or oval in shape surrounded by nuclear membrane which bears nuclear pores. Certain substances pass freely through these pores between the nucleus and surrounding cellular substances.
Nucleus is filled with protein rich substance called nucleoplasm or karyolymph. In nucleoplasm are numerous fine strands in the form of network called chromatin network or nuclear reticulum composed of nucleic acid DNA and protein. During cell division chromatin changes its shape to form chromosomes which contain hereditary units called genes that carry the heredity information from generation to generation. Within nucleus is spherical body called nucleolus which may be more than one in one nucleus. It disappears during cell division and reappears afterwards.
Nucleolus is believed to play important role in the synthesis of ribonucleic acid and ribosome in eukaryotic cells.
Cytoplasm: Protoplasm outside nucleus is called cytoplasm. Cytoplasm is composed of several types of organelles and a blind matrix the cytosal in which organelles reside. Cytoplasm contains following organelles:

(1) Endoplasmic reticulum:
It extends from plasma membrane to the nuclear membrane. It is an elaborate tube like system of lipoprotein. There are two types of it (a) Agramulated or smooth endoplasmic reticulum and (b) Rough or granulated endoplasmic reticulum. Smooth endoplasmic reticulum is not associated with ribosomes. Endoplasmic reticulum serves as supporting platform for ribosome. It forms structural framework of the cell with increased surface for various metabolic reaction.



(2) Mitochondria or chondriosomes:
They are present in the cytoplasm and appear as minute granules, vesicles, rodlets, threads or strings. They are the centre of aerobic respiration. There are two this membranes of mitochondrion formed of lipids and proteins. Interior of mitochondrion contains blind like organic matrix with a number of chemical compounds. Mitochondria have semi autonomous existence in the cell. They have their own DNA. Mitochondria are called ‘power house of the cell’.





(3) Goolgi apparatus (Dictyosome):
It is canalicular system with sacs and has parallel arranged flattered membrane bound vesicles which lack ribosomes. Golgi was scientist who first found out these bodies in the cytoplasm. Each golgi body is dise shaped and has central flattered plate like compartments called histernae peripheral network of inter connecting tubules and peripherally occurring vesicles and Golgian vauoles. Golgi bodies are especially prominent in glandular cells. Golgi bodies manufacture certain macromolecules by itself.





(4) Lysosomes:
These are spherical bodies, few micrometer in diameter, surrounded by single membrane originated by Golgi apparatus and containing digestive enzymes. They occur only in the cytoplasm of animal cells and function the digestion of material taken into the cell by phagocytosis. Normally they function as destroyers of foreign particles and worn out cellular components. Then the membrane of lysosomes in ruptured the cell undergoes chemical break down or lysis.







(5) Ribosomes:
They are so named because they contain high concentration of Ribonucleic acid (RNA). These small structures are the sites of protein synthesis in all cells. Ribosomes are found freely dispersed in cytoplasm in prokaryotic cells. Ribosomes are composed of 50 or more different kinds of proteins. There are millions of these per cell and are all similar. Ribosomes are regarded as protein factories. Under direction of nucleus they produce protein needed by the cell. Ribosomes are among the most vital cellular components. Recent investigations reveal that ribosomes are manufactured in the nucleolus from where they are transferred to the cytoplasm through nucleopores.

(6) Vacuoles:
They are non protoplasmic liquid filled cavities in the cytoplasm and are surrounded by membrane called tonoplast which permeable; it allows certain substances to enter in the vacuole. In animal cell they are temporarily formed at the time of need.

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