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Biomolecules and the Cell, , , , , , he living matter Is composed of mainly six, , elements—carbon, hydrogen, oxygen,, rnltrogen, phosphorus and sulfur, These clements, together constitute about 90% of the dry weight, of the human body. Several other functionally, important elements ate also found in the cells,, These include Ca, K, Na, Cl, Mg, Fe, Cu, Co, I,, Zn, F, Mo and Se., , , , Carbon—a unique element of life, , Carbon is the most predominant and versatile, element of life. It possesses a unique property to, form infinite number of compounds. This is, attributed to the ability of carbon to form stable, covalent bonds and C=C chains of unlimited, length. It is estimated that about 90% of, compounds found in living system invariably, ‘contain carbon, , , , Chemical molecules of life, , Life is composed of lifeless chemical, molecules. A single cell of the bacterium,, Escherichia coli contains about 6,000 different, , ‘The cell speaks, , “Lam the unis of biological sctviey:, Organized into subcellular organelles:, Assigned to each are specific dies,, Tih, 1 truly represent life!”, , organic compounds. It is believed that man may, contain about 100,000 different types of, molecules although only 2 few of them have, been characterized., , Gomniex biomolecules, , ‘The organic compounds such as amino acids,, nucleotides and monosaccharides serve as the, ‘monomeric units or building blocks of complex, biomolecules—proteins, nucleic acids IDNA and, RNA) and polysaccharides, respectively, The, important biomolecules (macromolecules) with, their respective building blocks and major, functions are given in Table 1.1. As regards, lipids, it may be noted that they are not, biopolymers in a strict sense, but majority of, them contain fatty acids, , Structural, , , , rarchy of an arganisin, , ‘The macromolecules (proteins, lipids, nucleic, acids and polysaccharides) form supramolecular, assemblies (eg. membranes) which in tun, organize into organelles, cells, tissues, organs, and finally the whole organism,, , , , BIOCHEMISTRY, , , , , , Biomolecule Building block ‘Major functions, (repeating unit), 1. Proien ‘Amir acids Fundamental basis snare and, , functon ole (sabe end dynanic nein., , , , , , , , 3, ibonwclee cd (RNA), 4 Planer (09), , or, , , , , , , , , eands;sietual components of membranes, , , , Chemical composition of man, , The chemical composition of a normal man,, weighing 65 kg, is given in Table 1.2, Water is, the solvent of life and contributes to more than, 60% of the weight. This is followed by protein, (mostly in muscle) and lipid (mostly in adipose, tissue). The carbohydrate conte, , which is in the form of glycogen., , , , , , is rather low, , Prokaryotic and oukaryotic celts, , The cells of the living kingdom may be, divided into two categories, , 1. Prokaryotes (Greek : pro ~ before; karyon —, nucleus) lack a well defined nucleus and possess, relatively simple structure. These include the, various bacteria., , 2. Eukaryotes (Greck, , eu true; karyon ~, , nucleus) possess a well defined nucleus and are, , more complex in their structure and function,, ‘The higher organisms (animals and plants) are, , composed of eukaryotic cells., , The cell is the structural and functional unit, of life. It may be also regarded as the basic unit, of biological activity., , The concept of cell originated from the, contributions of Schleiden and Schwann (1838)., However, it only after 1940, the, , complexities of cell structure were exposed., , A comparison of the characteristics between, prokaryotes and eukaryotes is listed in Table 1.3,, , The human body is composed of about 104, cells. There are about 250 types of specialized, cells_in the human body”e.g. erythrocytes,, nerve" cells, muscle cells, cells of pancreas., , :, , , , Constituent Percent (%) Weight (lg) An cukaryotic cell is generally 10 to 190 jim, , in diameter. A diagrammatic representation, , see Se of a typical rat liver cell is depicted in, Protea m0 " Fig.A.1, , Leia 138 9 The plant cell differs from an animal cell by, , Cartchydrate 18 1 possessing a rigid cell wall (mostly composed of, , esas ti % Cellulose) and chloroplasts. The latter are the, , sites of photosynthesis.

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BIOMOLECULES ANO THE CELL, , Chapeer 4, , , , , , 110m), (Cals enveloped by atipd cel wat Cal's enveloped by a Pantie plasma mariana, , , , ‘Large (goneraty 10-100 er), , , , , , The cell consists of well defined subcellular, , organelles, enveloped by a plasma membrane, By differential centrifugation of tissue, homogenate, it is possible to isolate each, cellular organelle ina. relatively pure form, (Refer Chapter 41), The distribution of major, enzymes and metabolic pathways in different, cellular organelles is given in the chapter, ‘on enzymes (Refer Fig.6.6), The subcellular, organelles are briefly described in the following,, pares,, , , , Rough endoplasmic reticulum, , , , , , , , Mioois, ‘Conan organetis and yiesieleton, (patrons and amen), , Nucleus is the largest cellular organelle,, surrounded by a double membrane nuctear, envelope mbrane is continuous, with the membranes of endoplasmic reticulum., At cettain intervals, the two nuclear membranes, have nuclear pores with a diameter of about 90,, faim, These pores permit the free passage of the, products synthesized in the nucleus into the, surrounding cytoplasm., , The outer m, , , , Nuctous, , Nuceotus, }— smeors ondopiasmcretcutven, , , , , , , , , , |— Peroxisome, _oytoskototon, , —Coated pits, , , , Nucleus contains DNA, the repository of, genetic information, Eukaryotic ONA is, associated with basic protein (histones) in the, ratio of 1 1, to form nucleosomes, An assembly, of nucleosomes constitutes chromatin fibres of, ‘chromosomes (Greek: chroma ~ colour; soma =, body). Thus, a single human chromosome is, composed of about a million nucleosomes. The, number of chromosomes is a characteristic, feature of the species. Humans have 46, chromosomes, compactly packed in the nucleus., , , , The nucteus of the eukaryotic cell contains a, dense body known as mucleolus. It is rich in, RNA, particularly the ribosomal RNA which, enters the cytosol through nuclear pores., , The ground material of the nucleus is often, referred to as aucleoplasm. It is rich in enzymes, such as DNA polymerases and RNA, polymerases. To the surprise of biochemists, the, enzymes of glycolysis, citric acid cycle and, hexose monophosphate shunt have also been, detected in the nucleoplasm,, , Mitochondria, , The mitochondria (Greek thread;, chondros - granule) are the centres for the, cellular respiration and energy metabolism. They, fare regarded as the power houses of the cell, with variable size and shape. Mitochondria are, rod-like or filamentous bodies, usually sith, dimensions of 1,0 x 3. jum. About 2,000, mitochondria, occupying about |/sth of the total, cell volume, are present in a typical cell., , ‘The mitochondria are composed of a double, membrane system, The outer membrane ts, smoath and completely envelops the organelle, The inner membrane fs folded to form cristae, (Latin — crests) which occupy a larger surface, ‘area, The intemal chamber of mitochondria is, foforred to as matrix or mitosol., , ‘The componenis of electron transport chain, and. oxidative phosphorylation (lavoprote, cytochromes b, c), ¢ a and ay and coupling, factors) are buried in the inner mitochondrial, ‘membrane. The matrix contains several enzymes, concerned with the energy metabolism of, teabohydcates, lipids and amino acta (eg. citi, , , , , , BIOCHEMISTRY, , acid cycle, Broxidation). The matrix enzymes, also participate in the synthesis of heme and, trea. Mitochondria are the prineipal producers, of ATP in the aerobic cells. ATP, the energy, ‘currency, generated in mitochondria is exported, to all parts of the cell to provide energy for the, cellular work., , The mitochondrial matrix contains a circular, double stranded DNA (mDNA), RNA ond, tibosomes. Thus, the mitochondria are equipped, with an independent protein synthesizing, machinery, Its estimated that about 10% of the, ‘mitochondrial proteins are produced in the, ‘mitachondeia, , The structure and functions of mitochondria, closely resemble prokaryotic cells. is, hypothesized that mitochondria have evolved, from aerobic bacteria, Further, itis believed that, during evolution, the aerobic bacteria developed, ‘2 symbiotic relationship with primordial, anaerobic eukaryotic cells that ultimately led to, the arrival of aerobic eukaryotes, , , , Endoplasmic reticulum, , The network of membrane enclosed spaces, that extends throughout the _ cytoplasm, constitutes endoplasmic reticulum (ER). Some of, these thread-ike structures 4 from the, nuclear pores to the plasma membrane., , , , A large portion of the ER is studded with, ribosomes to give a granular appearance which, is referred to as rough endoplasmic, Ribosomes are the factories of protein, biosynthesis. Outing the process of cell, fractionation, rough ER is disrupted to form small, vesicles known as microsomes. it may be noted, that microsomes as such do not occur in the, cell, , The smooth endoplasmic reticulum does not, contain ribosomes. It fs involved in the synthesis, Of lipids (triacylglycerols, phospholipids, sterols), ‘and. metabolism of drugs, besides supplying Ca, for the cellular functions., , , , , , Golgi apparatus, Eukaryotic cells contain a unique cluster of, membrane vesicles known as dictyosomes

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Ghopeer 1: BIOMOLECULES AND THE CELL, , , , which, in tum, constitute Golgi apparatus (or, Golgi complex), The newly synthesized proteins, are handed over to the Golgi apparatus which, catalyse the addition of carbohydrates, lipids or, sulfate moieties to the proteins. These chemical, modifications are necessary for the transport of, proteins across the plasma membrane,, , Certain proteins and enzymes are enclosed in, membrane vesicles of Golgi apparatus and, secreted from the cell after the appropriate, signals, The digestive enzymes of pancreas are, produced in this fashion,, , Golgi apparatus are also involved in the, membrane synthesis, particularly forthe, formation of intracellular organelles (e.g, peroxisomes, lysosomes),, , , , Lysosomes, , Lysosomes are spherical vesicles enveloped, , , , , , hy a single membrane, Lysosomes are regarded, as the digestive tract of the cell, since they are, actively involved in digestion of cellular, substances—namely proteins, lipids, carbo:, , hydrates and nuctelc acids. Lysosomal enzymes:, ine categorizes as hydrotases. These include the, following enzymes (with substrate in brackets), , , , a-Glucosidase (giveogen), Cathepsins (proteins), Lipases (lipids), , Ribonucleases (RNA), , The pH of the lysosomal matrix is more acidic, (PH<5) than the cytosol (pH-7) and. this, facilitates the degradation of diferent compounds., The lysosomal enzymes are responsible for, ‘maintaining the cellular jn a dynamic, state, by their degradation and recycling, The, degraded products leave the lysosomes, usually, by diffusion, for reutilization by the cell, Sometimes, however, certain residual products,, tich in lipids and proteins, collectively known as, Fpotuscin accumulate in the cell. Lipofuscin is, the age pigment or wear and tear pigment which, has been implicated in ageing process., , The digestive enzymes of cellular compounds, are confined ta the lysosomes in the best interest, of the cell, Escape of these enzymes into cytosol, will destroy the functional macromolecules of the, cell and result in many complications. The, occurrence of several diseases (e.g. arthritis,, muscle diseases, allergic disorders) has been pat, attributed to the release of lysosomal enzymes., , , , Peroxisome, , Peraxisomes, also known as microbodies, are, single membrane cellular organelles. They are, spherical or oval in shape and contain the, ‘enzyme catalase. Catalase protects the cell from, the toxic effects of H.O, by converting it to H,O., and ©,. Peroxisomes are also involved in the, oxidation of long chain fatty acids (>C,.), and, synthesis of plasmalogens and glycotipids. Plants, contain glyoxysomes, a specialized type of, , peers, , , , © A ling cell Is a true representative of life with Its own organization and specialted, , functions., , © Accumulation of lipofuscin, a pigment rich in lipids and proteins, in the cell has been, , implicated in ageing process., © Leakage of, , ‘enzymes into the cell degrades several functional macromolecules, , lysosomal, ‘and this may lead to certain disorders (e.g. arthritis), © Zellweger syndrome is a rare diseose characterized by the absence of functional, peroxisomes., , , , BIOCHEMISTRY, , , , peroxisomes, which are, slyoxylate pathway,, , involved in, , Peroxisome biogenesis disorders (PBDs), are, a group of rare diseases involving the enzyme, The biochemical, PADS include, ong. chain fatty acids, (Cag and Cop) and decreased concentrations of, , activities of peroxisomes., abnormalities associated with, increased levels of very, , plasmalogens. The most severe form of PBDs is, Zellweger syndrome, a condition characterized, by the absence of functional peroxisomes, The, victims of this disease may die within one year, after bith,, , Gytosol and cytoskeleton, , The cellular matrix is collectively reterted 10, as cytosol, Cytosol is basically a compartment, containing several enzymes, metabolites and, salts in an aqueous gel like medium, More recent, studies however, indicate that the cytoplasm, actually contains a complex network of protein, filaments, spread throughout, that constitutes, cytoskeleton, The cytoplasmic filaments are of, , the, , , , three types microtubules, actin filaments an, intermediate filaments. The filaments which are, polymers of proteins are responsible for the, structure, shape and organization of the cell, , , , INTEGRATION OF, CELLULAR FUNCTIONS, , , , The eukaryotic cells perform a wide range of, complex reactions/lunctions to maintain tissues,, and for the ultimate well-being of the whole, organism. For this purpose, various, , , , intracellular processes and biochemical reactions, are tightly controlled and integrated. Division of, 2 cell into two daughter cells is good example of, the orderly occurrence of an integrated series of, cellular reactions., , , , Apoptosis is the programmed cell death or, cell suicide. This occurs when the cell has, fulfilled its biological functions. Apoptosis may, bbe regarded as a natural cell death and it differs, from the cell death caused by injury due to, radiation, anoxia ete. Programmed cell death is, a highly regulated process, , | itniamer x cal, , 1. Life is composed of lifeless chemical molecules. The complex biomolecules, proteins,, nucleic acids (DNA and RNA), polysaccharides and lipids are formed by the monomeric, units amino acids, nucleotides, monosaccharides and fatty acids, respectively., , 2. The cell ts the structural ond functional unit of life. The eukaryotic cell consists of well, defined subcellular organelles, enveloped in a plasma membrane., , 3. The nucleus contains DNA, the repository of genetic information, DNA, In association, «sith proteins (histones), forms nucleosomes which, in turn, make up the chromosomes,, , The mitochondria are the centres for energy metabollam. They are the prineipal producers, , of ATP which is exported to oil parts of the cell to provide energy for cellular work., , Endoplasmic reticulum (ER) is the network of membrane enclosed spaces that extends, , throughout the cytoplasm, ER studded with ribosomes, the foctories of protein, , ‘losynthesis, is referred to as rough ER. Golgi apparatus are a cluster of membrane, , vesicles to which the newly synthesised proteins are handed over for further processing, , ‘and export, , Lysosomes are the digestive bodies of the cell, actively involved in the degradation of, , cellular compounds. Peroxisomes contain the enayme catalase that protects the cell from, , the toxle effects of H,O, The cellular ground matrix is referred to as cytosol which, in, fact, 1s composed of a network of protein filaments, the, , The eukaryote calls perform a ude range of complex functions in a well coordinated and, integrated fashion. Apoptosis is the process of programmed cell death or cell suicide., , , , oe, , , , 7