Cell overview .
All the living things are composed of cells. A single cell is the smallest unit that has all the characteristics of life. Cell is defined as the structural and functional unit of the living body.
General Characteristics of Each cell in the body:-
1. Needs nutrition and oxygen
2. Produces its own energy necessary for its growth, repair and other activities
3. Eliminates carbon dioxide and other metabolic wastes
4. Maintains the medium, i.e. the environment for its survival .
5. Shows immediate response to the entry of invaders like bacteria or toxic substances into the body.
6. Reproduces by division. There are some exceptions like neuron, which do not reproduce.
Structure of Cell .
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| Structure of the cell |
Each cell is formed by a cell body and a membrane covering the cell body called the cell membrane. Cell body has two parts, namely nucleus and cytoplasm surrounding the nucleus . Thus, the structure of the cell is studied under three Components :
1. Cell membrane
2. Cytoplasm
3. Nucleus.
Cell Membrane .
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| cell membrane |
[1]. Cell membrane is a protective sheath, enveloping the cell body. It is also known as plasma membrane or plasmalemma. This membrane separates the fluid outside the cell called extracellular fluid (ECF) and the fluid inside the cell called intracellular fluid (ICF).
[2]. The cell membrane is a semipermeable membrane. So, there is free exchange of certain substances between ECF and ICF. Thickness of the cell membrane varies from 75 to 111Å .
[3]. Cell membrane is composed of three types of substances: 1. Proteins (55%) 2. Lipids (40%) 3. Carbohydrates (5%).
Structure of Cell membrane .
[1]. On the basis of structure, cell membrane is called a unit membrane or a three-layered membrane. The electron microscopic study reveals three layers of cell membrane, namely, one central electron-lucent layer and two electron-dense layers.
[2]. The two electron-dense layers are placed one on either side of the central layer. The central layer is a lipid layer formed by lipid substances. The other two layers are protein layers formed by proteins. Cell membrane contains some carbohydrate molecules also.
Structural Model of the Cell Membrane .
1. Danielli-Davson model .
Danielli-Davson model was the first proposed basic model of membrane structure. It was proposed by James F Danielli and Hugh Davson in 1935. And it was accepted by scientists for many years. This model was basically a ‘sandwich of lipids’ covered by proteins on both sides.
2. Unit membrane model .
In 1957, JD Robertson replaced ‘Danielli-Davson model’ by ‘Unit membrane model’ on the basis of electron microscopic studies.
3. Fluid mosaic model .
Later in 1972, SJ Singer and GL Nicholson proposed ‘The fluid mosaic model’. According to them, the membrane is a fluid with mosaic of proteins (mosaic means pattern formed by arrangement of different colored pieces of stone, tile, glass or other such materials). This model is accepted by the scientists till now. In this model, the proteins are found to float in the lipid layer instead of forming the layers of the sandwich-type model.
Lipid Layers of the Cell Membrane .
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| Lipids of the cell membrane |
The central lipid layer is a bilayered structure. This is formed by a thin film of lipids. The characteristic feature of lipid layer is that, it is fluid in nature and not a solid structure. So, the portions of the membrane move from one point to another point along the surface of the cell. The materials dissolved in lipid layer also move to all areas of the cell membrane. Major lipids are Phospholipids & Cholesterol.
1. Phospholipids .
[1]. Phospholipids are the lipid substances containing phosphorus and fatty acids. Aminophospholipids, sphingomyelins, phosphatidylcholine, phosphatidyletholamine, phosphatidylglycerol, phosphatidylserine and phosphatidylinositol are the phospholipids present in lipid layer of cell membrane.
[2]. Phospholipid molecules are arranged in two layers . Each phospholipid molecule resembles the headed pin in shape. The outer part of the phospholipid molecule is called the head portion and the inner portion is called the tail portion.
[3]. Head portion is the polar end and it is soluble in water and has strong affinity for water (hydrophilic). Tail portion is the non-polar end.
[4]. It is insoluble in water and repelled by water (hydrophobic). Two layers of phospholipids are arranged in such a way that the hydrophobic tail portions meet in the center of the membrane. Hydrophilic head portions of outer layer face the ECF and those of the inner layer face ICF (cytoplasm).
2. Cholesterol .
Cholesterol molecules are arranged in between the phospholipid molecules. Phospholipids are soft and oily structures and cholesterol helps to ‘pack’ the phospholipids in the membrane. So, cholesterol is responsible for the structural integrity of lipid layer of the cell membrane.
Functions of Lipid Layer in Cell Membrane .
Lipid layer of the cell membrane is a semipermeable membrane and allows only the fat-soluble substances to pass through it. Thus, the fat-soluble substances like oxygen, carbon dioxide and alcohol can pass through this lipid layer. The water-soluble substances such as glucose, urea and electrolytes cannot pass through this layer.
Protein Layers of the Cell Membrane .
[1]. Protein layers of the cell membrane are electron-dense layers. These layers cover the two surfaces of the central lipid layer. Protein layers give protection to the central lipid layer. The protein substances present in these layers are mostly glycoproteins.
[2]. Protein molecules are classified into two categories:
1. Integral proteins or transmembrane proteins.
2. Peripheral proteins or peripheral membrane proteins.
1. Integral proteins .
[1]. Integral or transmembrane proteins are the proteins that pass through entire thickness of cell membrane from one side to the other side. These proteins are tightly bound with the cell membrane.
[2]. Examples of integral protein:
i. Cell adhesion proteins .
ii. Cell junction proteins
iii. Some carrier (transport) proteins
iv. Channel proteins
v. Some hormone receptors
vi. Antigens
vii. Some enzymes.
2. Peripheral proteins .
[1]. Peripheral proteins or peripheral membrane proteins are the proteins which are partially embedded in the outer and inner surfaces of the cell membrane and do not penetrate the cell membrane.
[2]. Peripheral proteins are loosely bound with integral proteins or lipid layer of cell membrane. So, these protein molecules dissociate readily from the cell membrane.
[3]. Examples of peripheral proteins:
i. Proteins of cytoskeleton .
ii. Some carrier (transport) proteins .
iii. Some enzymes.
Functions of Proteins in Cell Membrane .
1. Integral proteins provide the structural integrity of the cell membrane .
2. Channel proteins help in the diffusion of water soluble substances like glucose and electrolytes .
3. Carrier or transport proteins help in the transport of substances across the cell membrane by means of active or passive transport .
4. Pump: Some carrier proteins act as pumps, by which ions are transported actively across the cell membrane .
5. Receptor proteins serve as the receptor sites for hormones and neurotransmitters .
6. Enzymes: Some of the protein molecules form the enzymes and control chemical (metabolic) reactions within the cell membrane .
7. Antigens: Some proteins act as antigens and induce the process of antibody formation .
8. Cell adhesion molecules or the integral proteins are responsible for attachment of cells to their neighbors or to basal lamina.
Carbohydrates of the Cell Membrane .
[1]. Some of the carbohydrate molecules present in cell membrane are attached to proteins and form glycoproteins (proteoglycans). Some carbohydrate molecules are attached to lipids and form glycolipids.
[2]. Carbohydrate molecules form a thin and loose covering over the entire surface of the cell membrane called glycocalyx.
Functions of Carbohydrates in Cell Membrance .
1. Carbohydrate molecules are negatively charged and do not permit the negatively charged substances to move in and out of the cell .
2. Glycocalyx from the neighboring cells helps in the tight fixation of cells with one another .
3. Some carbohydrate molecules function as the receptors for some hormones.
Functions of Cell membrane .
1. Protective function: Cell membrane protects the cytoplasm and the organelles present in the cytoplasm
2. Selective permeability: Cell membrane acts as a semipermeable membrane, which allows only some substances to pass through it and acts as a barrier for other substances
3. Absorptive function: Nutrients are absorbed into the cell through the cell membrane
4. Excretory function: Metabolites and other waste products from the cell are excreted out through the cell membrane
5. Exchange of gases: Oxygen enters the cell from the blood and carbon dioxide leaves the cell and enters the blood through the cell membrane
6. Maintenance of shape and size of the cell: Cell membrane is responsible for the maintenance of shape and size of the cell.
Cytoplasm .
[1]. Cytoplasm of the cell is the jellylike material formed by 80% of water. It contains a clear liquid portion called cytosol and various particles of different shape and size. These particles are proteins, carbohydrates, lipids or electrolytes in nature.
[2]. Cytoplasm also contains many organelles with distinct structure and function. Cytoplasm is made up of two zones:
1. Ectoplasm: Peripheral part of cytoplasm, situated just beneath the cell membrane
2. Endoplasm: Inner part of cytoplasm, interposed bet ween the ectoplasm and the nucleus.
Organelles of Cytoplasm .
Cytoplasmic organelles are the cellular structures embedded in the cytoplasm. Organelles are considered as small organs of the cell. Some organelles are bound by limiting membrane and others do not have limiting membrane . Each organelle is having a definite structure and specific functions .
Organelles with limiting Membrane .
Endoplasmic reticulum .
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| Endoplasmic reticulum |
[1]. Endoplasmic reticulum is a network of tubular and microsomal vesicular structures which are interconnected with one another. It is covered by a limiting membrane which is formed by proteins and bilayered lipids.
[2]. The lumen of endoplasmic reticulum contains a fluid medium called endoplasmic matrix. The diameter of the lumen is about 400 to 700Å. The endoplasmic reticulum forms the link between nucleus and cell membrane by connecting the cell membrane with the nuclear membrane.
[3]. Endoplasmic reticulum is of two types, namely rough endoplasmic reticulum and smooth endoplasmic reticulum.
[4]. Both the types are interconnected and continuous with one another. Depending upon the activities of the cells, the rough endoplasmic reticulum changes to smooth endoplasmic reticulum and vice versa.
Rough Endoplasmic Reticulum .
It is the endoplasmic reticulum with rough, bumpy or bead-like appearance. Rough appearance is due to the attachment of granular ribosomes to its outer surface. Hence, it is also called the granular endoplasmic reticulum . Rough endoplasmic reticulum is vesicular or tubular in structure.
Functions of Rough Endoplasmic Reticulum .
1. Synthesis of proteins .
[1]. Rough endoplasmic reticulum is concerned with the synthesis of proteins in the cell. It is involved with the synthesis of mainly those proteins which are secreted from the cells such as insulin from βcells of islets of Langerhans in pancreas and antibodies from B lymphocytes.
[2]. Ribosomes arrange the amino acids into small units of proteins and transport them into the rough endoplasmic reticulum. Here, the carbohydrates are added to the protein units forming the glycosylated proteins or glycoproteins, which are arranged in the form of reticular vesicles.
[3]. These vesicles are transported mainly to Golgi apparatus for further modification and processing. Few vesicles are transported to other cytoplasmic organelles.
2. Degradation of worn-out organelles .
Rough endoplasmic reticulum also plays an important role in the degradation of worn-out cytoplasmic organelles like mitochondria. It wraps itself around the worn-out organelles and forms a vacuole which is often called the autophagosome. Autophagosome is digested by lysosomal enzymes .
Smooth Endoplasmic Reticulum .
It is the endoplasmic reticulum with smooth appearance. It is also called agranular reticulum. It is formed by many interconnected tubules. So, it is also called tubular endoplasmic reticulum.
Functions of Smooth Endoplasmic Reticulum .
1. Synthesis of non-protein substance .
[1]. Smooth endoplasmic reticulum is responsible for synthesis of non-protein substances such as cholesterol and steroid. This type of endoplasmic reticulum is abundant in cells that are involved in the synthesis of lipids, phospholipids, lipoprotein substances, steroid hormones, sebum, etc.
[2]. In most of the other cells, smooth endoplasmic reticulum is less extensive than the rough endoplasmic reticulum.
2. Role in cellular metabolism .
Outer surface of smooth endoplasmic reticulum contains many enzymes which are involved in various metabolic processes of the cell.
3. Storage and metabolism of calcium .
Smooth endoplasmic reticulum is the major site of storage and metabolism of calcium. In skeletal muscle fibers, it releases calcium which is necessary to trigger the muscle contraction.
4. Catabolism and detoxification .
Smooth endoplasmic reticulum is also concerned with catabolism and detoxification of toxic substances like some drugs and carcinogens (cancer-producing substances) in the liver.
Golgi apparatus .
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| Golgi apparatus |
[1]. Golgi apparatus or Golgi body or Golgi complex is a membrane-bound organelle, involved in the processing of proteins. It is present in all the cells except red blood cells. It is named after the discoverer Camillo Golgi.
[2]. Usually, each cell has one Golgi apparatus. Some of the cells may have more than one Golgi apparatus. Each Golgi apparatus consists of 5 to 8 flattened membranous sacs called the cisternae. Golgi apparatus is situated near the nucleus.
[3]. It has two ends or faces, namely cis face and trans face. The cis face is positioned near the endoplasmic reticulum.
[4]. Reticular vesicles from endoplasmic reticulum enter the Golgi apparatus through cis face. The trans face is situated near the cell membrane. The processed substances make their exit from Golgi apparatus through trans face .
Functions of Golgi Apparatus .
Major functions of Golgi apparatus are processing, packing, labeling and delivery of proteins and other molecules like lipids to different parts of the cell.
1. Processing of materials .
Vesicles containing glycoproteins and lipids are transported into Golgi apparatus. Here, the glycoproteins and lipids are modified and processed.
2. Packaging of materials
All the processed materials are packed in the form of secretory granules, secretory vesicles and lysosomes, which are transported either out of the cell or to another part of the cell. Because of this, Golgi apparatus is called the ‘post office of the cell’.
3. Labeling and delivery of materials .
Finally, the Golgi apparatus sorts out the processed and packed materials and labels them (such as phosphate group), depending upon the chemical content for delivery (distribution) to their proper destinations. Hence, the Golgi apparatus is called ‘shipping department of the cell’.
Lysosomes .
[1]. Lysosomes are the membrane-bound vesicular organelles found throughout the cytoplasm. The lysosomes are formed by Golgi apparatus. The enzymes synthesized in rough endoplasmic reticulum are processed and packed in the form of small vesicles in the Golgi apparatus.
[2]. Then, these vesicles are pinched off from Golgi apparatus and become the lysosomes. Among the organelles of the cytoplasm, the lysosomes have the thickest covering membrane. The membrane is formed by a bilayered lipid material. It has many small granules which contain hydrolytic enzymes.
Types of Lysosomes .
1. Primary lysosome, which is pinched off from Golgi apparatus. It is inactive in spite of having hydrolytic enzymes
2. Secondary lysosome, which is the active lysosome. It is formed by the fusion of a primary lysosome with phagosome or endosome .
Functions of Lysosomes .
Lysosomes are often called ‘garbage system’ of the cell because of their degradation activity. About 50 different hydrolytic enzymes, known as acid hydroxylases are pre sent in the lysosomes, through which lysosomes execute their functions.
Important lysosomal enzymes .
1. Proteases, which hydrolyze the proteins into amino acids
2. Lipases, which hydrolyze the lipids into fatty acids and glycerides
3. Amylases, which hydrolyze the polysaccharides into glucose
4. Nucleases, which hydrolyze the nucleic acids into mononucleotides.
Mechanism of lysosomal function .
Lysosomal functions involve two mechanisms:
1. Heterophagy: Digestion of extracellular materials engulfed by the cell via endocytosis
2. Autophagy: Digestion of intracellular materials such as worn-out cytoplasmic organelles.
Specific functions of lysosomes .
1. Degradation of macromolecules.
[1]. Macromolecules are engulfed by the cell by means of endocytosis (phagocytosis, pinocytosis or receptor mediated endocytosis: . The macromolecules such as bacteria, engulfed by the cell via phagocytosis are called phagosomes or vacuoles.
[2]. The other macromolecules taken inside via pinocytosis or receptor-mediated endocytosis are called endosomes. The primary lysosome fuses with the phagosome or endosome to form the secondary lysosome.
[3]. The pH in the secondary lysosome becomes acidic and the lysosomal enzymes are activated. The bacteria and the other macromolecules are digested and degraded by these enzymes.
[4]. The secondary lysosome containing these degraded waste products moves through cytoplasm and fuses with cell membrane. Now the waste products are eliminated by exocytosis.
2. Degradation of worn-out organelles.
The rough endoplasmic reticulum wraps itself around the worn-out organelles like mitochondria and form the vacuoles called autophagosomes. One primary lysosome fuses with one autophagosome to form the secondary lysosome. The enzymes in the secondary lysosome are activated. Now, these enzymes digest the contents of autophagosome.
3. Removal of excess secretory products in the cells.
Lysosomes in the cells of the secretory glands remove the excess secretory products by degrading the secretory granules.
4. Secretory function – secretory lysosomes .
[1]. Recently, lysosomes having secretory function called secretory lysosomes are found in some of the cells, particularly in the cells of immune system. The conventional lysosomes are modified into secretory lysosomes by combining with secretory granules (which contain the particular secretory product of the cell).
[2]. Examples of secretory lysosomes:
(i.) Lysosomes in the cytotoxic T lymphocytes and natural killer (NK) cells secrete perforin and granzymes, which destroy both viral-infected cells and tumor cells. Perforin is a pore-forming protein that initiates cell death. Granzymes belong to the family of serine proteases (enzymes that dislodge the peptide bonds of the proteins) and cause the cell death by apoptosis
(ii.) Secretory lysosomes of melanocytes secrete melanin .
(iii.) Secretory lysosomes of mast cells secrete serotonin, which is a vasoconstrictor substance and inflammatory mediator.
Peroxisomes .
Peroxisomes or microbodies are the membrane limited vesicles like the lysosomes. Unlike lysosomes, peroxisomes are pinched off from endoplasmic reticulum and not from the Golgi apparatus. Peroxisomes contain some oxidative enzymes such as catalase, urate oxidase and D-amino acid oxidase.
Functions of Peroxisomes .
[1]. Breakdown the fatty acids by means of a process called beta-oxidation : This is the major function of peroxisomes .
[2]. Degrade the toxic substances such as hydrogen peroxide and other metabolic products by means of detoxification. A large number of peroxisomes are present in the cells of liver, which is the major organ for detoxification.
[3]. Hydrogen peroxide is formed from poisons or alcohol, which enter the cell. Whenever hydrogen peroxide is produced in the cell, the peroxisomes are ruptured and the oxidative enzymes are released.
[4]. These oxidases destroy hydrogen peroxide and the enzymes which are necessary for the production of hydrogen peroxide
[5]. Form the major site of oxygen utilization in the cells .
[6]. Accelerate gluconeogenesis from fats .
[7]. Degrade purine to uric acid
[8]. Participate in the formation of myelin .
[9]. Play a role in the formation of bile acids.
Centrosome & Centrioles .
Centrosome is the membrane-bound cellular organelle situated almost in the center of cell, close to nucleus. It consists of two cylindrical structures called centrioles which are made up of proteins. Centrioles are responsible for the movement of chromosomes during cell division.
Secretory vesicles .
Secretory vesicles are the organelles with limiting membrane and contain the secretory substances. These vesicles are formed in the endoplasmic reticulum and are processed and packed in Golgi apparatus. Secretory vesicles are present throughout the cytoplasm. When necessary, these vesicles are ruptured and secretory substances are released into the cytoplasm.
Mitochondrion .
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| Structure of mitochondrion |
[1]. Mitochondrion (plural = mitochondria) is a membrane bound cytoplasmic organelle concerned with production of energy. It is a rod-shaped or oval-shaped structure with a diameter of 0.5 to 1 μ. It is covered by a bilayered membrane .
[2]. The outer membrane is smooth and encloses the contents of mitochondrion. This membrane contains various enzymes such as acetyl-CoA synthetase and glycerolphosphate acetyltransferase.
[3]. The inner membrane is folded in the form of shelf-like inward projections called cristae and it covers the inner matrix space.
[4]. Cristae contain many enzymes and other protein molecules which are involved in respiration and synthesis of adenosine triphosphate (ATP). Because of these functions, the enzymes and other protein molecules in cristae are collectively known as respiratory chain or electron transport system.
[5]. Enzymes and other proteins of respiratory chain are Succinic dehydrogenase , Dihydronicotinamide adenine dinucleotide (NADH) dehydrogenase , Cytochrome oxidase , Cytochrome C & ATP synthase.
[6]. Inner cavity of mitochondrion is filled with matrix which contains many enzymes. Mitochondrion moves freely in the cytoplasm of the cell. It is capable of reproducing itself.
[7]. Mitochondrion contains its own deoxyribonucleic acid (DNA), which is responsible for many enzymatic actions. In fact, mitochondrion is the only organelle other than nucleus, which has its own DNA.
Functions of Mitochondrion .
1. Production of energy .
[1]. Mitochondrion is called the ‘power house’ or ‘power plant’ of the cell because it produces the energy required for cellular functions. The energy is produced during the oxidation of digested food particles like proteins, carbohydrates and lipids by the oxidative enzymes in cristae.
[2]. During the oxidative process, water and carbon dioxide are produced with release of energy. The released energy is stored in mitochondria and used later for synthesis of ATP.
2. Synthesis of ATP .
The components of respiratory chain in mitochondrion are responsible for the synthesis of ATP by utilizing the energy by oxidative phosphorylation. ATP molecules diffuse throughout the cell from mitochondrion. Whenever energy is needed for cellular activity, the ATP molecules are broken down.
3. Apoptosis .
Cytochrome C and second mitochondria-derived activator of caspases (SMAC)/diablo secreted in mitochondria are involved in apoptosis .
Organelles without limiting membrane .
Ribosomes .
Ribosomes are the organelles without limiting membrane. These organelles are granular and small dot-like structures with a diameter of 15 nm. Ribosomes are made up of 35% of proteins and 65% of ribonucleic acid (RNA). RNA present in ribosomes is called ribosomal RNA (rRNA). Ribosomes are concerned with protein synthesis in the cell.
Types of Ribosomes .
Ribosomes are of two types:
1. Ribosomes that are attached to rough endoplasmic reticulum
2. Free ribosomes that are distributed in the cytoplasm.
Functions of Ribosomes.
[1]. Ribosomes are called ‘protein factories’ because of their role in the synthesis of proteins. Messenger RNA (mRNA) carries the genetic code for protein synthesis from nucleus to the ribosomes. The ribosomes, in turn arrange the amino acids into small units of proteins.
[2]. Ribosomes attached to rough endoplasmic reticulum are involved in the synthesis of proteins such as the enzymatic proteins, hormonal proteins, lysosomal proteins and the proteins of the cell membrane. Free ribosomes are responsible for the synthesis of proteins in hemoglobin, peroxisome and mitochondria.
Cytoskeleton .
[1]. Cytoskeleton is the cellular organelle present throughout the cytoplasm. It determines the shape of the cell and gives support to the cell. It is a complex network of structures with varying sizes.
[2]. In addition to determining the shape of the cell, it is also essential for the cellular movements and the response of the cell to external stimuli.
[3]. Cytoskeleton consists of three major protein components are microtubule, Intermediate filaments and Microfilaments.
1. Microtubules .
[1]. Microtubules are the straight, hollow and tubular structures of the cytoskeleton. These organelles without the limiting membrane are arranged in different bundles. Each tubule has a diameter of 20 to 30 nm.
[2]. Length of microtubule varies and it may be 1000 times more than the thickness. Structurally, the microtubules are formed by bundles of globular protein called tubulin .
[3]. Tubulin has two subunits, namely αsubunit and βsubunit.
Functions of microtubules .
[1]. Microtubules may function alone or join with other proteins to form more complex structures like cilia, flagella or centrioles and perform various functions.
[2]. Microtubules determine the shape of the cell .
[3]. Microtubules give structural strength to the cell.
[4]. Microtubules act like conveyer belts which allow the movement of granules, vesicles, protein molecules and some organelles like mitochondria to different parts of the cell.
[5]. Microtubules form the spindle fibers which separate the chromosomes during mitosis
[6]. Microtubules are responsible for the movement of centrioles and the complex cellular structures like cilia.
2. Intermediate Filaments.
[1]. Intermediate filaments are the structures that form a network around the nucleus and extend to the periphery of the cell. Diameter of each filament is about 10 nm.
[2]. The intermediate filaments are formed by ropelike polymers, which are made up of fibrous proteins .
[3]. Intermediate filaments are divided into five subclasses:
i. Keratins (in epithelial cells)
ii. Glial filaments (in astrocytes)
iii. Neurofilaments (in nerve cells)
iv. Vimentin (in many types of cells)
v. Desmin (in muscle fibers).
Functions of intermediate filaments.
[1]. Intermediate filaments help to maintain the shape of the cell.
[2]. These filaments also connect the adjacent cells through desmosomes.
3. Microfilaments .
[1]. Microfilaments are long and fine threadlike structures with a diameter of about 3 to 6 nm. These filaments are made up of non-tubular contractile proteins called actin and myosin. Actin is more abundant than myosin.
[2]. Microfilaments are present throughout the cytoplasm. The microfilaments present in ectoplasm contain only actin molecules and those present in endoplasm contain both actin and myosin molecules.
Functions of microfilaments .
1. Give structural strength to the cell
2. Provide resistance to the cell against the pulling forces
3. Are responsible for cellular movements like contraction, gliding and cytokinesis (partition of cytoplasm during cell division).
Nucleus .
[1]. Nucleus is the most prominent and the largest cellular organelle. It has a diameter of 10 µ to 22 µ and occupies about 10% of total volume of the cell. Nucleus is present in all the cells in the body except the red blood cells.
[2]. The cells with nucleus are called eukaryotes and those without nucleus are known as prokaryotes. Presence of nucleus is necessary for cell division. Most of the cells have only one nucleus (uninucleated cells).
[3]. Few types of cells like skeletal muscle cells have many nuclei (multinucleated cells). Generally, the nucleus is located in the center of the cell. It is mostly spherical in shape. However, the shape and situation of nucleus vary in some cells.
Structure of Nucleus .
Nucleus is covered by a membrane called nuclear mem – brane and contains many components. Major components of nucleus are nucleoplasm, chromatin and nucleolus.
Nuclear Membrane .
[1]. Nuclear membrane is double layered and porous in nature. This allows the nucleoplasm to communicate with the cytoplasm. The outer layer of nuclear membrane is continuous with the membrane of endoplasmic reticulum.
[2]. The space between the two layers of nuclear membrane is continuous with the lumen of endoplasmic reticulum. Pores of the nuclear membrane are guarded (lined) by protein molecules.
[3]. Diameter of the pores is about 80 to 100 nm. However, it is decreased to about 7 to 9 nm because of the attachment of protein molecules with the periphery of the pores. Exchange of materials between nucleoplasm and cytoplasm occurs through these pores.
Nucleoplasm .
[1]. Nucleoplasm is a highly viscous fluid that forms the ground substance of the nucleus. It is similar to cytoplasm present outside the nucleus. Nucleoplasm surrounds chromatin and nucleolus.
[2]. It contains dense fibrillar network of proteins called the nuclear matrix and many substances such as nucleotides and enzymes.
[3]. The nuclear matrix forms the structural framework for organizing chromatin. The soluble liquid part of nucleoplasm is known as nuclear hyaloplasm.
Chromatin .
[1]. Chromatin is a thread-like material made up of large molecules of DNA. The DNA molecules are compactly packed with the help of a specialized basic protein called histone. So, chromatin is referred as DNA-histone complex.
[2]. It forms the major bulk of nuclear material. DNA is a double helix which wraps around central core of eight histone molecules to form the fundamental packing unit of chromatin called nucleosome.
[3]. Nucleosomes are packed together tightly with the help of a histone molecule to form a chromatin fiber. Just before cell division, the chromatin condenses to form chromosome.
Chromosomes .
[1]. Chromosome is the rod-shaped nuclear structure that carries a complete blueprint of all the hereditary characteristics of that species. A chromosome is formed from a single DNA molecule coiled around histone molecules.
[2]. Each DNA contains many genes. Normally, the chromosomes are not visible in the nucleus under microscope. Only during cell division, the chromosomes are visible under microscope. This is because DNA becomes more tightly packed just before cell division, which makes the chromosome visible during cell division.
[3]. All the dividing cells of the body except reproductive cells contain 23 pairs of chromosomes. Each pair consists of one chromosome inherited from mother and one from father.
[4]. The cells with 23 pairs of chromosomes are called diploid cells. The reproductive cells called gametes or sex cells contain only 23 single chromosomes. These cells are called haploid cells.
Nucleolus .
[1]. Nucleolus is a small, round granular structure of the nucleus. Each nucleus contains one or more nucleoli. The nucleolus contains RNA and some proteins, which are similar to those found in ribosomes.
[2]. The RNA is synthesized by five different pairs of chromosomes and stored in the nucleolus. Later, it is condensed to form the subunits of ribosomes.
[3]. All the subunits formed in the nucleolus are transported to cytoplasm through the pores of nuclear membrane. In the cytoplasm, these subunits fuse to form ribosomes, which play an essential role in the formation of proteins.
Function of Nucleus .
[1]. Major functions of nucleus are the control of cellular activities and storage of hereditary material .
[2]. Control of all the cell activities that include metabolism, protein synthesis, growth and reproduction (cell division) .
[3]. Synthesis of RNA .
[4]. Formation of subunits of ribosomes.
[5]. Sending genetic instruction to the cytoplasm for protein synthesis through messenger RNA (mRNA)
[6]. Control of the cell division through genes
[7]. Storage of hereditary information (in genes) and transformation of this information from one generation of the species to the next.
Deoxyribonucleic acid (DNA) .
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| Structure of DNA |
[1]. Deoxyribonucleic acid (DNA) is a nucleic acid that carries the genetic information to the offspring of an organism. DNA forms the chemical basis of hereditary characters.
[2]. It contains the instruction for the synthesis of proteins in the ribosomes. Gene is a part of a DNA molecule.
[3]. DNA is present in the nucleus (chromosome) and mitochondria of the cell. The DNA present in the nucleus is responsible for the formation of RNA.
[4]. RNA regulates the synthesis of proteins by ribosomes. DNA in mitochondria is called non-chromosomal DNA.
Structure of DNA .
[1]. DNA is a double stranded complex nucleic acid. It is formed by deoxyribose, phosphoric acid and four types of bases. Each DNA molecule consists of two polynucleotide chains, which are twisted around one another in the form of a double helix.
[2]. The two chains are formed by the sugar deoxyribose and phosphate. These two substances form the backbone of DNA molecule. Both chains of DNA are connected with each other by some organic bases .
[3]. Each chain of DNA molecule consists of many nucleotides. Each nucleotide is formed by:
1. Deoxyribose – sugar
2. Phosphate
3. One of the following organic (nitrogenous) bases:
Purines – Adenine (A) – Guanine (G) .
Pyrimidines – Thymine (T) – Cytosine (C) .
[4]. The strands of DNA are arranged in such a way that both are bound by specific pairs of bases. The adenine of one strand binds specifically with thymine of opposite strand.
[5]. Similarly, the cytosine of one strand binds with guanine of the other strand. DNA forms the component of chromosomes, which carries the hereditary information.
[6]. The hereditary information that is encoded in DNA is called genome. Each DNA molecule is divided into discrete units called genes.
Gene .
[1]. Gene is a portion of DNA molecule that contains the message or code for the synthesis of a specific protein from amino acids. It is like a book that contains the information necessary for protein synthesis. Gene is considered as the basic hereditary unit of the cell.
[2]. In the nucleotide of DNA, three of the successive base pairs are together called a triplet or a codon. Each codon codes or forms code word (information) for one amino acid.
[3]. There are 20 amino acids and there is separate code for each amino acid. For example, the triplet CCA is the code for glycine and GGC is the code for proline.
[4]. Thus, each gene forms the code word for a particular protein to be synthesized in ribosome (outside the nucleus) from amino acids.
Ribonucleic acid (RNA) .
Ribonucleic acid (RNA) is a nucleic acid that contains a long chain of nucleotide units. It is similar to DNA but contains ribose instead of deoxyribose. Various functions coded in the genes are carried out in the cytoplasm of the cell by RNA. RNA is formed from DNA.
Structure of RNA .
Each RNA molecule consists of a single strand of polynucleotide unlike the double stranded DNA. Each nucleotide in RNA is formed by:
1. Ribose – sugar.
2. Phosphate.
3. One of the following organic bases:
Purines – Adenine (A) – Guanine (G)
Pyrimidines – Uracil (U) – Cytosine (C). Uracil replaces the thymine of DNA and it has similar structure of thymine.
Types of RNA .
RNA is of three types. Each type of RNA plays a specific role in protein synthesis.
The three types of RNA are:
1. Messenger RNA (mRNA) .
Messenger RNA carries the genetic code of the amino acid sequence for synthesis of protein from the DNA to the cytoplasm.
2. Transfer RNA (tRNA) .
Transfer RNA is responsible for decoding the genetic message present in mRNA.
3. Ribosomal RNA (rRNA) .
Ribosomal RNA is present within the ribosome and forms a part of the structure of ribosome. It is responsible for the assembly of protein from amino acids in the ribosome.
Gene expression .
Gene expression is the process by which the information (code word) encoded in the gene is converted into functional gene product or document of instruction (RNA) that is used for protein synthesis. Gene expression involves two steps:
1. Transcription.
2. Translation.
Transcription of Genetic code .
[1]. The word transcription means copying. It indicates the copying of genetic code from DNA to RNA. The proteins are synthesized in the ribosomes which are present in the cytoplasm. However, the synthesis of different proteins depends upon the information (sequence of codon) encoded in the genes of the DNA which is present in the nucleus.
[2]. Since DNA is a macromolecule, it cannot pass through the pores of the nuclear membrane and enter the cytoplasm. But, the information from DNA must be sent to ribosome. So, the gene has to be transcribed (copied) into mRNA which is developed from DNA.
[3]. Thus, the first stage in the protein synthesis is transcription of genetic code, which occurs within the nucleus. It involves the formation of mRNA and simultaneous copying or transfer of information from DNA to mRNA.
[4]. The mRNA enters the cytoplasm from the nucleus and activates the ribosome resulting in protein synthesis. The formation of mRNA from DNA is facilitated by the enzyme RNA polymerase.
Translation of Genetic Code .
[1]. Translation is the process by which protein synthesis occurs in the ribosome of the cell under the direction of genetic instruction carried by mRNA from DNA. Or, it is the process by which the mRNA is read by ribosome to produce a protein.
[2]. This involves the role of other two types of RNA, namely tRNA and rRNA. The mRNA moves out of nucleus into the cytoplasm. Now, a group of ribosomes called polysome gets attached to mRNA.
[3]. The sequence of codons in mRNA are exposed and recognized by the complementary sequence of base in tRNA. The complementary sequence of base is called anticodon.
[4]. According to the sequence of bases in anticodon, different amino acids are transported from the cytoplasm into the ribosome by tRNA that acts as a carrier.
[5]. With the help of rRNA, the protein molecules are assembled from amino acids. The protein synthesis occurs in the ribosomes which are attached to rough endoplasmic reticulum.
Growth factors .
[1]. Growth factors are proteins which act as cell signaling molecules like cytokines and hormones . These factors bind with specific surface receptors of the target cell and activate proliferation, differentiation and/or maturation of these cells.
[2]. The term growth factor is interchangeably used with the term cytokine. But growth factors are distinct from cytokines. Growth factors act on the cells of the growing tissues. But cytokines are concerned with the cells of immune system and hemopoietic cells.
[3]. Many growth factors are identified. The known growth factors are:
1. Platelet derived growth factor – PDGF
2. Colony stimulating factors – CSF
3. Nerve growth factors – NGF .
4. Neurotropins
5. Erythropoietin .
6. Thrombopoietin .
7. Insulin like growth factors – IGF .
8. Epidermal growth factor – present in keratinocytes and fibroblasts. It inhibits growth of hair follicles and cancer cells
9. Basic fibroblast growth factor – present in blood vessels. It is concerned with the formation of new blood vessels .
10. Myostatin – present in skeletal muscle fibers. It controls skeletal muscle growth .
11. Transforming growth factors (TGF) – present in transforming cells (cells undergoing differentiation) and in large quantities in tumors and cancerous tissue.
[4]. TGF is of two types:
1. TGFα secreted in brain, keratinocytes and macrophages. It is concerned with growth of epithelial cells and wound healing
2. TGFβ secreted by hepatic cells, T lymphocytes, B lymphocytes, macrophages and mast cells. When the liver attains the maximum size in adults, it controls liver growth by inhibiting proliferation of hepatic cells. TGFβ also causes immunosuppression.
Stem cells .
Stem cells are the primary cells capable of reforming themselves through mitotic division and differentiating into specialized cells. These cells serve as repair system of the body and are present in all multicellular organisms.
Types of Stem Cells .
Stem cells are of two types:
1. Embryonic stem cells derived from embryo
2. Adult stem cells derived from adults.
1. Embryonic Stem Cells .
[1]. Embryonic stem cells are derived from the inner cell mass of a blastocyst which is an early stage of embryo. It takes about 4 to 5 days after fertilization to reach the blastocyst stage and it has about 30 to 50 cells.
[2]. Embryonic stem cells have two important qualities:
1.Self-renewal capacity .
2.Pluripotent nature, i.e. these cells are capable of differentiating into all types of cells in ectodermal, endodermal and mesodermal layers.
[3]. In these two qualities, the embryonic stem cells can be used therapeutically for regeneration or replacement of diseased or destroyed tissues. In fact, embryonic pluripotent stem cells are now cultured and lot of research is going on to explore the possibility of using these cells in curing the disorders like diabetes mellitus by cell replacement technique.
[4]. But, ethical issues arise because the embryo has to be destroyed to collect the stem cells. Stem cells from umbilical cord blood Stem cells in umbilical cord blood are collected from the placenta or umbilical cord.
[5]. Use of these stem cells for research and therapeutic purposes does not create any ethical issue because it does not endanger the life of the fetus or newborn.
[6]. Because of vitality and easy availability, the umbilical cord blood stem cells are becoming a potent resource for transplant therapies.
[7]. Nowadays, these stem cells are used to treat about 70 diseases and are used in many transplants worldwide.
2. Adult Stem Cells .
[1]. Embryonic stem cells do not disappear after birth. But remain in the body as adult stem cells and play a role in repair of damaged tissues. However, their number becomes less.
[2]. Adult stem cells are the undifferentiated multipotent progenitor cells found in growing children and adults. These are also known as somatic stem cells and are found everywhere in the body.
[3]. These cells are capable of dividing and reforming the dying cells and regenerating the damaged tissues. So, these stem cells can also be used for research and therapeutic purposes.
[4]. Adult stem cells are collected from bone marrow.
[5]. Two types of stem cells are present in bone marrow:
1. Hemopoietic stem cells, which give rise to blood cells .
2. Bone marrow stromal cells, which can differentiate into cardiac and skeletal muscle cells.
Advantages of Stem Cells .
[1]. Adult stem cells from bone marrow are used in bone marrow transplant to treat leukemia and other blood disorders since 30 years.
[2]. Recently, it is known that these stem cells can develop into nerve cells, liver cells, skeletal muscle cells and cardiac muscle cells.
[3]. Recent discoveries also reveal that the stem cells are present in several tissues which include blood, blood vessels, skeletal muscle, liver, skin and brain. It is also found that these cells are capable of differentiating into multiple cell types.
[4]. So, the cell-based therapy using stem cells may be possible to treat many diseases such as heart diseases, diabetes, Parkinson’s disease, Alzheimer’s disease, spinal cord injury, stroke and rheumatoid arthritis.
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