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Cell Biology and Cell Division, What is a Cell?, A cell is defined as the most basic, structural and functional unit of all living organisms. Essentially, a cell is a, structure that contains organelles which provide necessary functions to sustain itself. However, not all cells are, the same., •, •, , Antony Von Leeuwenhoek first saw and described a live cell., Robert Hooke saw a cell for the first time in a ‘Cork’. But it was a dead cell., , Cell Theory, The cell theory was formulated by two German Scientists, Matthias Schneider and Theodore Schwinn, independently., Schneider (1838) examined a large variety of plant tissues and observed that all plants are composed of, different kinds of cells., At about the same time, Schwinn (1839), closely studied different types of animal cells and found that the, animal cell had a very thin outer layer known as plasma membrane., •, , They postulated that- ‘All organisms are made up of cells and their products.’, , He also concluded, from his studies based on plant tissues that animal cells differ from plant cells in lacking cell, wall., Objections to Cell Theory, Cell theory failed to explain how and from where the new cells were formed. All these observations lead to a, major expansion of cell theory that was expressed by Rudolf Virchow in 1855 modified the hypothesis of, Schneider and Schwinn and explained in his statement that cells divide and new cells are formed from preexisting cells, i.e., Omnis cellula-e-cellula., Types of Cells, Based on the type of nucleus, cells are of two type:, 1., , 2., , Prokaryotic Cells, • Prokaryotic cells are unicellular microorganisms and one of the most ancient organisms on, earth., • Prokaryotic cells are placed in kingdom-Monera. These cells are represented by bacteria,, cyanobacteria (blue-green algae), mycoplasma or PPLO., • The bacteria are found in almost every place like deep in the soil, human intestine, deep in, seawater, etc., • Bacteria tends to vary greatly in size. It normally ranges from 0.3-1.5p.m with some, exceptions., • Components of a Prokaryotic (Bacterial) Cell- A bacterial cell is composed of various, components as genetic material, cell envelope, cytoplasm, nucleoid, inclusion bodies,, ribosomes, flagella, pili, fimbriae, etc., • Genetic Material- Nucleoid represents the genetic material incase of prokaryotes that is, naked, not enveloped by a nuclear membrane., Eukaryotic Cells, • Unlike prokaryotic cells, eukaryotic cells possess a nucleus enclosed within a membrane., • All multicellular organisms are eukaryotic and they fall under the domain Eukaryota., • Structurally, eukaryotic cells are much larger than prokaryotic cells, typically having a, volume 10,000 times greater than prokaryotic cells.

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•, , Eukaryotic cells also possess many membrane-bound organelles devoted to various life, functions., , Size of a Cell, The cells exhibit an endless variation in size, life span and cellular activities, e.g., Mycoplasma (smallest cell) or, PPLOs (Pleuro-Pneumonia Like Organisms) is only 0.3 Jim in length and bacteria are approx. 3-5 Jim in size., An ostrich egg, which is known to be the largest isolated single cell measures about 170 X 135 mm. Human Red, Blood Cells (RBCs) are about 7 Jim in diameter and the nerve cell of human being is the longest cell having, length of 90-100 cm., Shape of a Cell, The cells also vary in their shapes. They may be polygonal, disc-like amoeboid, thread-like, cuboid or irregular., The cell shape is always related and vary with the function they perform., , Generalised Structure, •, •, •, , An extensive compartmentalisation of cytoplasm is seen through the presence of membrane bound, organelles., Eukaryotic cells also possess a variety of locomotory and cytoskeletal structures., All eukaryotic cell are not-identical, instead they differ from each other on the basis of structure and, function., , Differences between Plant and Animal Cell

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Components of a Eukaryotic Cell, An eukaryotic cell is composed of various cell components as cell membrane, cell wall (only in plants),, mitochondria, chloroplast, Golgi bodies, ribosomes, centrioles (only in animals), etc. All these are described, here under in detail., Cell Organelles, Eukaryotes possess many cellular organelles that perform various functions. Some of these organelles, such as, the cell wall, are exclusive to plant cells., •, •, •, •, •, •, •, •, •, •, •, •, , Cell Membrane, Cell wall, Mitochondria, Plastids, Ribosomes, Cytoskeletons, Centrosome, Centrioles, Nucleus, Microbodies, Cilia and Flagella, Endomembrane systems which consist of the endoplasmic reticulum and Golgi apparatus. Plant cells, have centrally placed vacuoles., , Cell Envelope and Its Modifications, Cell envelope is the outermost covering of protoplasm of the cell. It is known to protect the cell from, mechanical shocks and injuries., It may be of three types:

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1., •, •, •, 2., •, , •, , •, •, , •, , a., b., , •, , •, , •, , Glycocalyx (Mucilage Sheath), It is the outermost layer, made up of macromolecules that gives sticky character to the bacterial cell., Glycocalyx differs in composition and thickness among different bacteria., It could be in the form of loose mucilaginous sheath called slime layer or thick and tough covering, called capsule., Function- help in resisting phagocytosis., Cell Membrane, Every living cell is covered by a thin, elastic, transparent, semi-permeable and regenerative, membrane called cell membrane also called plasma membrane or plasmalemma. The plasma, membrane separates the internal environment of the cell from external environment. As this, membrane helps in regulating the entrance and exit of molecules into and out of the cell., In 1950s with the advancement of electron microscope the detailed structure of the membrane was, studied. Most of the initial studies on cell membrane structure, i.e., especially on the human Red, Blood cells (RBCs), which enabled the scientists to deduce the possible structure of plasma, membrane., Human RBCs are considered to be the best material for the : study of biochemical composition of the, cell membrane because they lack nucleus as well as cytoplasmic organelles., Structure, Studies on human RBCs concluded that the cell membrane is composed of lipid which forms a bilayer, with protein molecules embedded in it at places. Later it was revealed that cell membranes also, possess protein and carbohydrates., Lipid, The lipid molecules are amphipathic in nature and are arranged within the membrane by the help of, two types of ends. These are as follows, Polar Hydrophilic End This region is in the form of (water loving) head, which faces towards the outer, sides of the cell membrane to interact with the aqueous environments on both sides., Non-polar Hydrophobic End This region is in the form of (water repelling) tail, both ends of which, faces each other that occur towards the centre of the cell membrane., The proportion of lipid molecules varies in plasma membrane of different cell types. These are formed, of cholesterol (25-32%) and mainly of phospho- glycerides or phospholipids (55-75%)., Proteins, Depending upon the ease of extraction, the ratio of protein and lipid varies considerably in different, cell types. In human beings, the membrane of the erythrocytes (RBCs) has approximately 52% protein, and 40% lipid., The membrane proteins can be classified as, (i) Integral Proteins (intrinsic protein) They have stronger association and bound firmly to the, membrane. These proteins are buried partially or totally in the phospholipid bilayer., (ii) Peripheral Proteins (extrinsic protein) They have weaker association and are bound to lipids of, membrane by electrostatic interactions., Carbohydrates, These constitute about 1-5% of chemical composition of plasma membrane. These are associated, with the phospholipids or with the peripheral proteins to form glycolipids and glycoproteins, respectively., To understand the structure of plasma membrane various models are given out of which the most, accepted model is Fluid Mosaic Model., , Fluid Mosaic Model, This model was given by Singer and Nicholson (1972)., According to this model, the lipid bilayer and integral proteins appear like a mosaic arrangement and the, quasi-fluid nature of lipid enables the lateral movement of the proteins within the overall bilayer., This ability of proteins to move within the membrane indicate the fluidity of the lipid part.

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Fluidity of Membrane, The fluid nature of the membrane is important from the point of view of interactions of molecules within the, membrane as well as other functions like formation of inter cellular junctions, cell growth, secretion,, endocytosis, cell division, etc., Passage of substances across the membrane occurs mainly by two methods;, I., , II., , Active Transport, Active transport is the movement of the molecules across the membrane against their concentration, gradient, i.e., from lower to the Tighter concentration. It is an energy dependent process, in which, ATP is utilised. It occurs in few ions and molecules,, e.g., Na+ / K+ pump., Polar molecules requires a carrier protein of the membrane to facilitate their transport across the, membrane because they cannot pass through the non-polar lipid bi-layer., Passive Transport, Passive transport is the mode of movement of molecules or substances across the membrane without, any requirement of energy., , It can be further of following three types, (a) Osmosis -It is the process by which water molecules pass through a membrane from a region of higher, concentration to a lower concentration., (b) Simple Diffusion- In this process, neutral molecules move across the membrane along the concentration, gradient (from higher to lower concentration), e.g., Gases and small molecules., (c) Facilitated Diffusion- In this process, the molecules are transported along concentration gradient by the, help of ion channels and permeases. Energy is not required in this process., Differences between Active and Passive Transport, , Functions, Cell membrane possess the following functions, (i) It is a selectively permeable or semi-permeable membrane, allows only selected substances to pass, inwardly., (ii) It protects the cell from injury., (iii) Membranes have carrier proteins for active transport., (iv) Cell membrane contain enzymes which perform certain reaction on their surface, e.g., ATPase,, phosphatase, etc., 3., , •, •, , Cell Wall, It was first discovered by Robert Hooke (1665). It is a rigid and a non-living structure which forms an, outer covering of the plasma membrane in plants and fungi. It is absent in animal cells., Cell wall is metabolically active in nature and is capable of growth. Its thickness varies from 0.1-10 pm., Cell wall not only gives shape to cell and protects the cell from mechanical damage and infection, it, also helps in cell to cell infraction and provides barrier to undesirable macromolecules.

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Chemical Composition, The cell wall of algae is made up of cellulose, galactans, mannans and minerals like calcium carbonate, etc.,, while cell wall of plant is composed of cellulose, hemicellulose, pectins and proteins., Structure of Cell Wall, On the basis of the structure, cell wall is differentiated into the following three parts;, I., , Middle Lamella, It is the layer mainly made up of calcium and magnesium pectates. It cements the cell walls of two, adjoining cells together. It is absent on the outer side of surface cells middle lamella along with a cell, wall transversed by plasmodesmata which connects cytoplasm of ^ neighbouring cells., Primary Cell Wall, It is produced inner to the middle lamella in a young and growing cell. It is capable of growth and, extension. It tends to diminish gradually as the cell attain maturity., Secondary Cell Wall, The thick secondary wall is formed inner towards membrane to the primary wall. As the cell gets fully, matured. Its composition is similar to the primary wall., , II., , III., , Functions, Cell wall possess the following Junctions, (i) It helps in providing a definite shape to the cell and also protects protoplasm against any mechanical injury,, i.e., damage and infection., (ii) It also helps in cell-to-cell interaction., (iii) It provides barrier to undesirable macro-molecules and attack of pathogens., Mitochondria, •, , •, , •, , •, •, , •, •, , Mitochondria are membrane bound cell organelles, essential for aerobic respiration of eukaryotic, cells. These are also known as power house of the cell. Thus, they produces cellular energy in the, form of ATP., Mitochondria are present in all living cell except, prokaryotic cell and certain specialised eukaryotic, cell such as anaerobic cells and mature RBCs., It is revealed from the studies that mitochondria is not easily visible, unless it is specifically stained., Mitochondria vary considerably according to the shape and size. They have varying shape such as, granular fibrillar, spherical, oval, discodial, etc. Average size of mitochondria is 2-6 |im in length and, 0.5 pm in diameter (typical cylindrical or sausage-shaped mitochondria has diameter of 0.2-1.0 Jim)., A mitochondrion contains two membranes, i.e., outer and inner. Out of which outer membrane is, smooth and forms the continuous boundary of the organelle. The inner membrane is semipermeable, to some metabolities. It is infolded into the matrix as incomplete partitions called cristae., The cristae are responsible for increasing the physiological active area or surface area. The density of, cristae determines the intensity of respiration., The outer and the inner membranes divides its lumen into two aqueous compartments separately,, i.e., the outer and the inner compartment., Inner compartment is also called matrix, which forms the inner core of the mitochondrion. The matrix, also possesses single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the, components required for the synthesis of proteins. The mitochondria divide by fission., The two membranes of mitochondria have their own specific enzymes associated with mitochondrial, function., Functions, Mitochondria possess the following functions, (i) Mitochondria provide important intermediates for the synthesis of several biochemicals like, pyrimidines, alkaloids, etc., (ii) The inner chamber matrix of the mitochondria has enzyme for the synthesises of fatty acids., (iii) Helps in regulation of cellular metabolism., (iv) Helps in apoptosis (programmed cell death).

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(v) Each of membrane potential., Mitochondrion is the second largest cell organelle and are more in animal cells than in plant cells., Plastids, These are semi-autonomous organelles that have double membrane envelope. Plastids have their own genetic, material (i.e., DNA). Due to their large size, they are easily seen under the microscope., Plastids are found in all plant cells and euglenoides except in some protistans (e.g., Euglena, Dinophyceae, etc)., Plastids are differentiated into three different types on the basis of the colour, i.e., type of pigments found in, them., 1., , Leucoplasts, These are the colourless plastids of varied shapes and sizes with stored nutrients in the form of, carbohydrates lipids and proteins., , These are of following three types;, (а) Amyloplasts are the carbohydrates (starch) containing leucoplast, e.g., Rice, wheat, potato, etc., Amyloplasts are larger than the normal/original size of leucoplast., (b) Elaioplasts are the leucoplast which stores oils and fats, e.g, Tuberose endosperm of castor seeds, etc., (c) Aleuroplasts are the protein storing leucoplast., e.g., Maize (aleurone cells)., 2., , 3., , •, •, •, •, , •, , •, , •, , •, , Chromoplasts, These are the leucoplast, whioh are yellow or reddish in appearance because of the presence of fat, soluble carotenoid pigment carotene., Xanthophyll and some other pigments are also present as the fat soluble carotenoid pigment other, than carotene, e.g., Orange colour of carrot, etc., Chloroplasts, These are the plastids which are greenish in colour containing photosynthetic pigments chlorophyll, and carotenoids. These pigments are responsible for trapping the light energy, essential for the, photosynthesis, i.e., the synthesis of organic food from an inorganic raw materials in the presence of, sunlight., Chloroplasts occur in major number in the photosynthetic mesophyll cells of leaves and green stem., They may be lens-shaped, oval, spherical, discoid or even ribbon-like organelles. They also have, variable length (5-10 mm) and width (2-4 mm)., Their number also varies from one per cell of the Chlamydomonas (a green alga) to 2-40 per cell in, mesophyll., Chloroplasts are also bounded by double membrane envelope like mitochondria, the two membranes, are smooth and are thick of about 90-100 A. The inner membrane of chloroplast is less permeable, than the other one., The inner membrane is grounded by a space known as stroma or matrix, a dense, colourless and a, granular substance mainly formed of soluble proteins. It also contains enzymes which are essential for, the synthesis of carbohydrates, lipids and proteins., Thylakoids are number of membranous like flattened structures that run throughout the matrix or, stroma. When several thylakoids are arranged or organised in the stack (like the piles of coins), called, grana or the intergranal thylakoids. Many flat membranous tubules interconnect the thylakoids of, different grana known as stroma lamellae., The space between the two membrane is called intermembrane space, which separates the two, membrane. This space contains a narrow fluid. Stroma also contains small, double-stranded circular, DNA, molecules and ribosomes., Ribosomes of chloroplasts are smaller (70S) than the ribosomes of cytoplasm (80S).Ribosomes, These are the small sub-spherical granular organelles, not bounded by any membrane. Ribosomes, were first observed by George Palade (1953), as the dense particles under the electron microscope., Hence, are also called Palade particles.

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•, •, , •, a., b., c., d., e., , Ribosomes are mainly composed of ribonucleoproteins (i. e., RNA-t- proteins) and are also known as, protein factories, as they are primarily involved in the synthesis of proteins or polypeptides., As studied earlier, the prokaryotic ribosomes are 70S type, while the eukaryotic ribosomes are 80S, type. Here, ‘S’ (Svedberg’s unit) stands for sedimentation coefficient (measure of density and size)., Both 70S and 80S ribosomes contains two sub-units, i.e., the smaller and the larger sub-unit ., Differences between 70S and 80S Ribosomes, Functions, Chloroplasts possess the following functions, Helps in photosynthesis, i.e., formation of organic compounds., In consumption of C02 and release of 02 in photosynthesis., May also change into chromoplast in order to provide colour to many flowers and fruits., Helps in storing fat and lipids., Functions in transduction of energy., , Cytoskeleton, •, •, , The network of interconnected proteinaceous filaments and tubules, which extends from the nucleus, to the plasma membrane in eukaryotic cells., Functions, Cytoskeleton possess the following functions, (i) The cytoskeletal structures maintain the shape of the cell and its extensions., (ii) It is also involved in many functions in a cell as mechanical support, motility, etc., , Cilia and Flagella, •, , •, , •, , •, •, , •, , These are hair-like projections of cell membrane. Both cilia and flagella are almost identical in, structure but differ somewhat in length. As cilia are small structures, working as oars (causing the, movement of either the cell or the surrounding fluid), while flagella are comparatively longer in size, than cilia and are responsible for the movement of cell., According to the electron microscopic studies it is predicted that the cilium or the flagellum are, covered with plasma membrane. Their core called the axoneme, contains a number of microtubules,, running parallel to the long axis., Usually, the axoneme has nine pairs of doublets of peripheral microtubules that are radially arranged, and a pair of centrally located microtubules. This arrangement of axonemal microtubules is referred, to as the (9 + 2) array., The central tubules are connected by bridges and is also enclosed by a central sheath, which is, connected to one of the tubules of each peripheral doublets by a radial spoke., Thus, it has been estimated that there are nine radial spokes. The peripheral doublets are also, interconnected by linkers. Both the cilium and flagellum emerge from centriole like structure called, the basal bodies., Flagella are also present in prokaryotic bacteria but these are ’ structurally different from that of, eukaryotic flagella.

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Centrosomes and Centrioles, •, , Centrosome is an organelle that generally have two cylindrical structures known as centrioles. They, are basically surrounded by an amorphous pericentriolar materials., , •, , Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation, like the cartwheel., , •, , They are usually made up of nine evenly spaced peripheral fibrils (triplet in nature) of tubulin protein., With which adjacent triplets are also being linked., , •, , The centre part of the proximal region of the centriole possess rod-shaped proteinaceous mass, known as hub, which is connected with tubules of the peripheral triplets fibrils known as radial spokes, (made up of protein)., , •, , From the basal body of cilia or flagella the centrioles and spindle fibres give rise to spindle apparatus, during cell division in animal cells., Functions, Centrosomes and centrioles possess the following functions, (i) These forms spindle fibres and move to the poles, at the time of cell divisidn, which thus, help in, the movement of chromatids in daughter cells., (ii) Help in the formation of cilia and flagella of the cells., , •, , Nucleus, •, •, •, •, •, •, •, , •, , •, , •, •, •, a., , b., , It is a specialised and principle cell organelle of the cell, which contains all the genetic information for, controlling all essential processes related to metabolism and transmission., Nucleus was first described by Robert Brown as early as 1831., Later the name chromatin was given by Flemming when the material of the nucleus was stained by, the basic dyes., Nucleus is known to be the largest cell organelle also known as brain of the cell., A nucleus is known to be present in all eukaryotic cells except a few cell types such as RBCs of, humans, sieve cells of vascular plants, etc., Prokaryotic cell lack a well organised nucleus, instead they have a nucleoid., The interphase nucleus (nucleus of a cell when it is not dividing) has highly extended and elaborate, nucleoprotein fibres called chromatin, nuclear matrix and one or more spherical bodies called, nucleoli., It has been revealed from the studies of electron microscopy that the nuclear envelope, consists of, two parallel membranes with a space between 10-50 nm called the perinuclear space, which forms a, barrier between the materials present inside the nucleus and that of the cytoplasm., The outer membrane usually bears ribosomes on it andremains continuous with the endoplasmic, reticulum. The nuclear envelope is interrupted by minute nuclear pores, at a number nuclear of, places, which are produced by the fusion of its two membranes., These nuclear pores are the passages through which movement of RNA and protein molecules takes, place in I both directions between the nucleus and the cytoplasm., Normally, there is only one nucleus per cell, but variations in the number of nuclei can also be seen in, various organisms., Nucleus is differentiated into following four parts, Nuclear Envelope, It is a double membrane bound envelope that surround the nucleus and separates the latter from the, cytoplasm., Nucleoplasm, It is a clear, non-staining, fluid material present in the nucleus, which contains raw materials, (nucleotides), enzymes (DNA/RNA polymerases) and metal ions for the synthesis of RNAs and DNA., The nuclear matrix or the nucleoplasm is composed of nucleolus and chromatin (spherical structures, present in the nucleoplasm).

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c., , d., , •, , Nucleolus, It is a naked, round and slightly irregular structure, which is attached to the chromatin at a specific, region. The content of nucleolus is continuous with the rest of the nucleoplasm as it is not a, membrane bound structure., It is a site for active ribosomal RNA synthesis. Larger and more numerous nucleoli are present in cells, actively carrying out protein synthesis., Chromatin, It is named so, because it has the ability to get stained with certain basic dyes. It is known to be the, hereditary DNA protein fibrillar complex. The chromatin fibres are distributed throughout the, nucleoplasm., It has two distinct regions, (a) Euchromatin (lightly stained), (b) Heterochromatin (darkly stained), Functions, Nucleus possess the following functions, (i) It stores information that control cellular functions., (ii) It controls the synthesis of structural proteins., (iii) It also stores the genetic information for development reproduction and behaviour., (iv) It also induces genetic variations., , Chromosomes, •, , •, •, •, •, •, a., b., , c., , d., , •, •, , It has been already studied in the chapter that the nucleus in the interphase has a loose and indistinct, network of nucleoprotein fibres called chromatin. But during different stages of*cell division cells, show structured chromosomes in place of the nucleus. The chromosomes are meant for the equal, distribution of genetic material., Their number is fixed and is same in all individuals of a species., Chromatin is composed DNA and some basic proteins called histones. Some non-histone proteins and, RNA are also present in the chromatin., A single human cell has approximately two metre long thread of DNA distributed among its 46 (23, pairs) chromosomes., Each and every chromosome is composed of a primary constriction or the centromere. On the sides of, which the disc-shaped structures are present known as kinetochores., On the basis of the position of the centromere, the chromosomes can be classified into four following, types, Metacentric Chromosome, It has chromosome with equal arms and centromere lies in the centre., Sub-metacentric Chromosome, It has one shorter arm and one longer arm with centromere slighdy away from the middle of the, chromosome., Acrocentric, It forms one extremely short and one very long arm and centromere is located near the end of the, chromosome., Telocentric, It has the terminal centromere, i.e., centromere is placed at an extreme end., Telocentric chromosomes are not present in humans., Few chromosomes have a non-staining secondary constrictions being present at a constant location, at some or the other time which gives the appearance of a small fragment known a satellite., Functions, Chromosomes possess the following functions, (i) Control cellular differentiation., (ii) Contains all hereditary information located in the genes., (iii) Forms a link between the offspring and the parents.

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(iv) Introduce variations, through the process of crossing over., (v) Control cell metabolism., MIcrobodies, •, •, •, , These are the membrane bound cytoplasmic elements that are composed of enzymes and other, substances., These are minute vesicles found in both plant cells and animal cells, e.g., In the liver, kidney, Protozoa,, yeast and many other types of cells. Their shape can be ovoid, spherical, granular, etc., Peroxisomes and glyoxysomes are the types of microbodies being found in plant cell and animal cell, respectively.