# Current Affairs 11th Class

#### Chromosome (Gr. chroma = colour soma = body)

Chromosome were discover by Hofmeister (1848) in filament of pollen mother cells of tradescantia (Rhoeodiscolour) studied by strasburger (1875) and given the persent name by Waldeyer (1888). During interphase, chromatin threads are present in the form of a network called chromatin reticulum. At the time of cell division, these thread like structures of chromatin become visible as independent structures, called chromosomes. The haploid set of chromosomes is define as genome. Structure : Each chromosome consists of two coiled filaments throughout its length called chromonemata by Vejdovsky. These have bead like structures called chromomeres which bear genes. Chromatid is a half chromosome or daughter chromosome. The two chromatids are connected at the centromere or primary constriction. Primary constriction (centromere) and secondary constriction gives rise to satellite. The secondary constriction consists of genes which code for ribosomal RNA and nucleolus hence it is called as “nucleolar organizer region”. Chromosomes having satellite are called SAT chromosomes. The ends of chromosomes are called “telomeres” (which do not unite with any other structure). In 1928 Emile Heitz developed a technique for stainning of chromosomes. Staining property of chromosomes is called as heteropycnosis. Chromosomes can be stained with basic dye like janus green there are two types of regions are seen :– (1) Heterochromatin : It is formed of thick regions which are more darkly stained than others areas. It is with condensed RNA which is transcriptionally inactive and late replicating. It generally lies near the nuclear lamina. It is of two type : (i) Constitutive hetrochromative : Occurs in all cells in all stages. e.g., Centromere. (ii) Facultative hetrochromative : Formed by inactivation of some gene in some cell in some stages. e.g., Barr body.  (2) Euchromatin : It is true chromatin and is formed of thin, less darkly stained  areas. It is with loose DNA which is transcriptionally active and early replicating. Chemical chomposition : DNA - 40%. Histone – 50%. Other (acid) Proteins – 8.5%. RNA – 1.5%. Traces of lipids, Ca, Mg and Fe. Histone are low molecular weight basic proteins which occur alongwith DNA in ratio. Nonhistone chromosomal or NHC proteins are of three types– structural, enzymatic and regulatory. Structural NHC proteins form the core or axis of the chromosome. They are also called scaffold proteins.

#### Types Of Cells

Chatton gave the term prokaryote and eukaryote. Depending upon the nature of nucleus cells are classified. Incipient nucleus is present in prokaryotes, where as in eukaryotes well organised nucleus is present.   Differences between prokaryotic and eukaryotic cell more...

#### Ribosome

The ribosomes are smallest known electron microscopic without membrane, ribonucleo–protein particles attached either on RER or floating freely in the cytoplasm and are the sites of protein synthesis.  Discovery : In 1943 Claude observed some basophilic bodies and named them as microsome. Palade (1955) coined the term ribosome (form animal cell). Ribosomes in nucleoplasm were observed by Tsao and Sato (1959). First isolated by Tissieres and Watson (1958) from E. coli. Ribosomes found in groups are termed as polyribosomes or ergosomes (Rich and Warner 1963 observed first time polyribosomes). Occurrence : In prokaryotes ribosomes are found only in free form in the cytoplasm. While in the eukaryotes the ribosomes are found in two forms in the cytoplasm, free form and bind form (bound on RER and outer nuclear membrane). These are also reported inside some cell organelles like mitochondria and plastids respectively called mitoribosomes and plastidoribosomes. Types of ribosomes (1) 70S ribosomes : Found in prokaryotes, mitochondria and plastid of eukaryotes. (2) 80S ribosomes : Found in cytoplasm of eukaryotes. (3) 77S, 60S and 55S ribosomes : Levine and Goodenough (1874) observed 77S ribosomes in fungal mitochondria 60S ribosomes in animal mitochondria and 55S in mammalian  mitochondria. Structure : Each ribosome is formed of two unequal subunits, which join only at the time of protein synthesis. In 70S and 80S ribosomes, 50S and 30S, 60S and 40S are larger and smaller subunits respectively. Larger subunits is dome shaped and attached to ER by glycoproteins called “ribophorins”.     Smaller subunit is oval shaped and fits as a cap on flat side of larger subunit. Ribosomes are attached to ER through hydrophobic interactions. Chemical composition : Ribosomes are chemically composed of rRNA and proteins Ribonucleo-Protein (RNP). 70S ribosomes has 60-65% rRNA and $35-40%$ proteins (ratio is 1.5:1). rRNAs are of three types : 23S type and 5S type rRNAs in 50S and 16S type rRNA in 30S sub-units. 80S ribosome has 45% rRNA and 55% proteins (ratio is about 1 : 1). r-RNA are of four types : 28S, 5S and 5.8S types of rRNAs in 60S and 18S type rRNA in 40S sub-units. A $1\times {{10}^{-3}}\,(0.001M)$ molar concentration of $M{{g}^{++}}$ is needed for the structural cohesion of ribosomes i.e., for holding the two subunits together. If this concentration is increased by ten folds, two ribosomes unite to form a dimer. By decreasing the $M{{g}^{++}}$ conc. to normal, the dimer breaks into monomers (single ribosomes). Biogenesis of ribosome (1) In eukaryotes the ribosomal RNAs like 18S, 5.8S and 28S are synthesized by nucleolus and 5S RNA out of the nucleus.   (2) In prokaryotes both rRNA and its protein are synthesized as well as assembled by cytoplasm. Polyribosomes or Polysomes : When many ribosomes (generally $68$) are attached at some mRNA strand. It is called polysome. The distance between adjacent ribosomes is of 90 nucleotides. These are functional unit of protein synthesis. Functions (1) Ribosomes are also called protein factories of the cell or more...

#### Protoplasm (Proto = First, Plasm = Fluid)

Protoplasm is a complex, granular, elastic, viscous and colourless substance. It is selectively or differentially permeable. It is considered as “Polyphasic colloidal system”. Discoveries (1) J. Huxley defined it as “physical basis of life”. (2) Dujardin (1835) discovered it and called them “sarcode”. (3) Purkinje (1837) renamed it as “Protoplasm”. (4) Hugo Von Mohl (1844) gave the significance of it. (5) Max Schultz (1861) gave the protoplasmic theory for plants. (6) Fischer (1894) and Hardy (1899) showed its colloidal nature. (7) Altman (1893) suggested protoplasm as granular.   Chemically Composition
 Prokaryotic cell Eukaryotic cell It is a single membrane system. It is a double membrane system. Cell wall surrounds the plasma membrane. Cell wall surrounds the plasma membrane in some protists, most fungi and all plant cell. Animal cell lacks it. Cell wall is composed of peptidoglycans. Strengthening material is murein. It is composed of polysaccharide. Strengthening material is chitin in fungi and cellulose in others plants. Cell membrane bears respiratory enzymes. It lacks respiratory enzymes. Cytoplasm lacks cell organelles e.g., Mitochondria, ER, Golgi body etc. Cytoplasm contains various cell organelles. Ribosomes are only 70 S type. Ribosomes are both 80 S and 70 S type. There are no streaming movements of cytoplasm. Cytoplasm show streaming movements.
Water 75 - 85% Carbon 20%
Proteins 10 - 25% Oxygen 62%
Lipids 2 - 3% Hydrogen 10%
Inorganic Materials 1% Nitrogen 3%
Trace elements 5% (Ca, P, Cl, more...

#### Plastids

Plastids are semiautonomous organelles having DNA, RNA, Ribosomes and double membrane envelope. These are largest cell organelles in plant cell. History (1) Haeckel (1865) discovered plastid, but the term was first time used by Schimper (1883). (2) A well organised system of grana and stroma in plastid of normal barley plant was reported by de Von Wettstein. (3) Park and Biggins (1964) gave the concept of quantasomes. (4) The term chlorophyll was given by Pelletier and Caventou, and structural details were given by Willstatter and Stall. (5) The term thylakoid was given by Menke (1962). (6) Fine structure was given by Mayer. (7) Ris and Plaut (1962) reported DNA in chloroplast and was called plastidome. Types of plastids : According to Schimper, Plastids are of 3 types: Leucoplasts, Chromoplasts and Chloroplasts. Leucoplasts : They are colourless plastids which generally occur near the nucleus in nongreen cells and possess internal lamellae. Grana and photosynthetic pigments are absent. They mainly store food materials and occur in the cells not exposed to sunlight e.g., seeds, underground stems, roots, tubers, rhizomes etc. These are of three types. (1) Amyloplast : Synthesize and store starch grains. e.g., potato tubers, wheat and rice grains. (2) Elaioplast (Lipidoplast, Oleoplast) : They store lipids and oils e.g., castor endosperm, tube rose, etc. (3) Aleuroplast (Proteinoplast) : Store proteins e.g., aleurone cells of maize grains. Chromoplasts : Coloured plastids other than green are kown as chromoplasts. These are present in petals and fruits. These also carry on photosynthesis. These may arise from the chloroplasts due to replacement of chlorophyll by other pigments. Green tomatoes and chillies turn red on ripening because of replacement of chlorophyll molecule in chloroplasts by the red pigment lycopene in tomato and capsanthin in chillies. Thus, chloroplasts are changed into chromatoplast. All colours (except green) are produced by flavins, flavenoids and cyanin. Cyanin pigment is of two types one is anthocyanin (blue) and another is erythrocyanin (red). Anthocyanin are water soluble pigments and found in cell sap of vacoule. Chloroplast : Discovered by Sachs and named by Schimper. They are greenish plastids which possess photosynthetic pigments. Number : It is variable. Number of chloroplast is 1 in Spirogyra indica, 2 in Zygnema, 16 in S.rectospora, up to 100 in mesophyll cells. The minimum number of one chloroplast per cell is found in Ulothrix and species of Chlamydomonas. Shape : They have various shapes
Shape Example
Cup shaped more...

#### Plasma Membrane

Every living cell is externally covered by a thin transparent electron microscopic, elastic regenerative and selective permeable membrane called plasma membrane. It is quasifluid in nature. Membranes also occur inside the cells. They are collectively called biomembranes. The term cell membrane was given by C. Nageli and C. Cramer (1855) for outer membrane covering of the portoplast. It was replaced by the term plasmalemma by Plower (1931). Chemical composition : Proteins lipoprotein (Lipid +Protein) are the major component forming 60% of the plasma membrane. Proteins provide mechanical strength and responsible for transportation of different substances. Proteins also act as enzyme. Lipids account may 28%-79% depending upon the type of cell and organism involved (in humans, myelin 79%). The lipids of plasma membrane are of three types namely phospholipids, glycolipids and sterols. The sterol found in the membrane may be cholesterol (Animals), phytosterol (Plants) or ergosterol (Microorganisms). Carbohydrates form 2%–10%. Oligosaccharides are the main carbohydrates present in plasma membrane. The carbohydrates of plasma membrane are covalently linked to both lipid and protein components. Ultrastructure : Under electron microscope the plasma membrane appears three layered, i.e., trilaminar or tripartite. One optically light layer is of lipid and on both sides two optically dense protein layers are present. Molecular structure and different models : Several models have been proposed to explain the structure and function of the plasma membrane. (1) Overton’s model : It suggests that the plasma membrane is composed of a thin lipid single layer. (2) Sandwitch model : It was proposed by Davson and Danielli (1935). According to this model the light biomolecular lipid layer is sandwitched between two dense protein layers (globular a type protein). This model was also said to be unit membrane hypothesis. (3) Robertson’s unit membrane model : It states that all cytoplasmic membranes have a similar structure of three layers with and electron transparent phospholipid bilayer being sandwitched between two electron dense layer of proteins (extended or $\beta$  type protein). Its thickness is about 75 Å with a central lipid layer of $35\text{ }{AA}$ thick and two peripheral protein layers of $\text{20 }{AA}$ thick. (4) Fluid mosaic model : The most important and widely accepted latest model for plasma membrane was given by Singer and Nicolson in 1972. According to them it is “protein iceberg in a sea of lipids.” According to this model, the cell membrane consists of a highly viscous fluid matrix of two layers of phospholipid molecules. Protein molecules occur as separate particles asymmetrical arranged in a mosaic pattern. Some of these are loosely bound at the polar surfaces of lipid layers, called peripheral or extrinsic proteins. Others penetrate deeply into the lipid layer called integral or intrinsic proteins. Some of the integral proteins penetrate through the phospholipid layers and project on both the surface. These are called trans membrane or tunnel proteins (glycophorins). Singly or in groups, they function as channels for passage of water ions and other solutes.     The more...

#### Microbodies

(1) Sphaerosomes Discovery : These were first observed by Hanstein (1880) but discovered by Perner (1953). Term sphaerosomes was given by Dangeard. Occurrence : These are found in all the plant cells which involves in the synthesis and storage of lipids i.e., endosperm and cotyledon of oil seeds. Shape, size and structure : These are spherical or oval in shape about $0.5-2.5\,\mu \,m$in diameter. They contain hydrolytic enzymes like protease, ribonuclease, phosphatase, esterase etc. They are bounded by a single unit membrane. Function : The main function of sphaerosomes is to help in lipid metabolism. These are also known as plant lysosomes. (2) Peroxisomes (Uricosomes) Discovery : These were first discovered by J. Rhodin (1954) in the cells of mouse kidney and were called microbodies. De Duve (1965) isolated certain sac like organelles from various types of animals and plants. These were called peroxisomes because these contain peroxide producing enzymes (oxidases) and peroxide destroying enzymes (catalases). Occurrence : These are found in photosynthetic cells of plants. In animals peroxisomes are found in vertebrates (cells of liver, kidney), brain, small intestine, testis and adrenal cortex), invertebrates and protozoans e.g., Paramecium. Shape, size and structure : These are spherical in shape, about $1.5\,\mu \,m$ in size. They are bounded by a single unit membrane. Their membrane is permeable to amino acids, uric acids, etc. They contain four enzymes of ${{H}_{2}}{{O}_{2}}$ metabolism. The enzymes urate oxidase, d-amino oxidase, a-hydroxy acid oxidase produce ${{H}_{2}}{{O}_{2}}$ whereas the catalases plays a significant protective role by degrading H2O2 because ${{H}_{2}}{{O}_{2}}$ is toxic for cells. Function : These are involved in the formation  and degrading of ${{H}_{2}}{{O}_{2}}$. Plant peroxisomes are also involved in photorespiration. (3) Glyoxysomes Discovery : These were discovered by Beevers in 1961 and Briedenbach in 1967. Occurrence : These are found in fungi, some protists and germinating fatty seeds where insoluble lipid food reserves must be turned into soluble sugars. Absent in animal cell. Shape, size and structure : These are spherical in shape, about $0.5-1\,\mu \,m$in size, they contain enzymes of metabolism of glycolic acid via glyoxylate cycle and bounded by a unit membrane. These are also contain enzymes for $\beta -$oxidation of fatty acids. Produced acetyl CoA. The better is metabolised in glyoxlate cycle to produced carbohydrates. Functions : The main function of glyoxysomes is conversion of fats into carbohydrates. (4) Lomasomes : These are sac like structures found between cell wall and plasmalemma in the haustoria of fungal hyphae. These were first discovered by Bowen and Berlin. Webster called them border bodies.

#### Membrane Transport

It is passage of metabolites, by-products and biochemicals across biomembrane. Membrane transport occurs through four methods–passive, facilitated, active and bulk. Size of the particles passing through plasmalemma is generally $115\text{ }{AA}.$ Passive transport : No energy spent. Passive transport occurs through diffusion and osmosis. (1) Diffusion : It is movement of particles from the region of their higher concentration or electrochemical potential to the region of their lower concentration or electrochemical potential. Electrochemical potential operates in case of charged particles like ions. Simple diffusion does not require carrier molecules. (2) Osmosis : It is diffusion of water across a semipermeable membrane that occurs under the influence of an osmotically active solution. Mechanism of passive transport : Passive transport can continue to occur if the absorbed solute is immobilised. Cations have a tendency to passively pass from electropositive to electronegative side. While anions can pass from electronegative to electropositive side. There are two modes of passive transport. (1) Lipid matrix permeability : Lipid soluble substances pass through the cell membrane according to their solubility and concentration gradient, e.g., triethyl citrate, ethyl alcohol, methane. (2) Hydrophillic membrane channels : They are narrow channels formed in the membrane by tunnel proteins. The channels make the membrane semipermeable. Water passes inwardly or outwardly from a cell through these channels according to osmotic gradients. $C{{O}_{2}}$ and ${{O}_{2}}$ also diffuse through these channels as per their concentration gradients. Facilitated transport or Facilitated diffusion : It is passage of substances along the concentration gradient without expenditure of energy that occurs with the help of special permeating substances called permeases. Permeases form pathways for movement of certain substances without involving any expenditure of energy. Facilitated transport occurs in case of some sugars, amino acids and nucleotides. Active transport : It occurs with the help of energy, usually against concentration gradient. For this, cell membranes possess carriers and gated channels. At times certain substances are transported alongwith the ones requiring active transport. The latter phenomenon called cotransport. (1) Carrier particles or Proteins : They are integral protein particles which have affinity for specific solutes. A solute particles combines with a carrier to form carrier solute complex. The latter undergoes conformational change in such a way as to transport the solute to the inner side where it is released into cytoplasm. (2) Gated channels : The channels are opened by either change in electrical potential or specific substances, e.g., Calcium channels. Active transport systems are also called pumps. The pumps operate with the help of ATP.${{K}^{+}}-\,{{H}^{+}}$exchange pump occurs in guard cells. $N{{a}^{+}}-\,{{K}^{+}}$exchange pump operates across many animal membranes. Active transport of one substance is often accompanied by permeation of other substances. The phenomenon is called secondary active transport. It is of two main types, cotransport (e.g., glucose and some amino acids alongwith inward pushing of excess $N{{a}^{+}})$ and counter-transport $(C{{a}^{2+}}$and ${{H}^{+}}$movement outwardly as excess $N{{a}^{+}}$passes inwardly). Bulk transport : It is transport of large quantities of micromolecules, macromolecules and food particles through the membrane. It is accompanied by formation of more...

#### Lysosomes

Lysosomes are electron microscopic, vesicular structures of the cytoplasm, bounded by a single membrane (lipoproteinous) which are involved in intracellular digestive activities, contains hydrolytic enzymes, so called lysosomes. Discovery (i) These were first discovered by a Belgian biochemist, Christian de Duve (1955) in the liver cells and were earlier named pericanalicular dense bodies. (ii) Terms Lysosome was given by Novikoff under the study of electron microscope. (iii) Matile (1964) was first to demonstrate their presence in plants, particularly in the fungus Neurospora. Polymorphism in lysosomes were described by De Robertis et. al (1971). Occurrence : These are absent from the prokaryotes but are present in all eukaryotic animal cells except mammalian RBCs. They have been recorded in fungi, Euglena, cotton and pea seeds. Shape : These are generally spherical in shape but are irregular in plant root tip cells. Size : Size range is $0.2-0.8\,\,\mu m$ while size is $0.5\,\,\mu \,m\,\,(500nm).$ Types of lysosomes : On the basis of their contents, four types of lysosomes are recognised. (1) Primary Lysosomes : A newly formed lysosome contains enzymes only. It is called the primary lysosomes. Its enzymes are probably in an inactive state. (2) Secondary Lysosomes : When some material to be digested enters a primary lysosome, the latter is named the secondary lysosome, or phagolysosome or digestive vacuole, or heterophagosome. (3) Tertiary lysosomes/Residual bodies : A secondary lysosome containing indigestible matter is known as the residual bodies or tertiary lysosome. The latter meets the cell by exocytosis (ephagy). (4) Autophagosomes/Autolysosomes : A cell may digest its own organelles, such as mitochondria, ER. This process is called autophagy. These are formed of primary lysosomes. The acid hydrolases of lysosomes digest the organelles thus, it is called autophagosome. The lysosome are sometimes called disposal units/suicidal bags. Sometime they get burst and causes the distruction of cell or tissue. Chemical composition : Matrix of primary lysosome is formed of hydrolases, which is involved in hydrolysis or polymeric compounds, that operate in acidic medium at pH 5, so called acid hydrolases. Upto now 50 types of enzyme have been reported. These are as : Proteases (cathepsin and collagenase), Nucleases (DNAse and RNAse), Glycosidases ($\beta -$galactosidase, $\beta -$glucoronidase), Phosphatases (ATPase, acid phosphatase /marker enzyme). Functions (1) Lysosomes of sperms provide enzyme for breaking limiting membrane of egg e.g., hyaluronidase enzyme. (2) Lysosomes functions as trigger of cell division or initiate cell division by digesting repressor molecules. (3) Nucleases (DNAse) of lysosomes may cause gene mutations which may cause disease like leukemia or blood cancer (partial deletion of 21st chromosome). (4) Sometimes residual bodies accumulate inside the cells leading to storage diseases e.g., a glycogen storage disease called Pompe’s disease, polynephritis Hurler’s disease (deformed bones due to accumulation of mucopolysaccharides). (5) Lysosomes also engulf the carcinogens.

#### Golgi Complex

Golgi complex is made up of various membranous system e.g., cisternae, vesicles and vacuoles. These are also called golgi bodies, golgisomes, lipochondrion, dictyosomes, Dalton complex, idiosomes or Baker’s body and “traffic police” of the cell. Discovery : First observed by George (1867) but it’s morphological details were given by Camillo Golgi (1898), in nerve cells of barn owl and cat. Occurence : It is present in all eukaryotic cells. In plants, these are scattered irregularly in the cytoplasm and called as “dictyosomes”. These are absent in bacteria and blue green algae, RBCs, spermatozoa of bryophytes and pteridophytes, and sieve tube cells of phloem of angiosperm. The number of golgi body increased during cell division. Average number 10 – 20 per cell. Golgi body surrounded by a zone of protoplasm which is devoid of cell organelles called zone of exclusion (Morre, 1977). Structure : Under transmission electron microscope the st. of golgibodies was study by Dalton and Felix (1954), golgi body is made of 4 parts. (1) Cisternae : Golgi apparatus is made up of stack of flat. Sac like structure called cisternae. The margins of each cisterna are gently curved so that the entire golgi body takes on a cup like appearance. The golgi body has a definite polarity. The cisternae at the convex end of the dictyosome comprises forming face (F. face) or cis face. While the cisternae at the concave end comprises the maturing face (M. face) or trans face. The forming face is located next to either the nucleus or endoplasmic reticulum. The maturing face is usually directed towards the plasma membranes. It is the functional unit of golgi body. (2) Tubules : These arise due to fenestration of cisternae and it forms a complex of network. (3) Secretory vesicles : These are small sized components each about 40 Å in diameter presents along convex surface of edges of cisternae. These are smooth and coated type of vesicles. (4) Golgian vacuoles : They are expanded part of the cisternae which have become modified to form vacuoles. The vacuoles develop from the concave or maturing face. Golgian vacuoles contain amorphous or granular substance. Some of the golgian vacuoles function as lysosomes.     Origin : Most accepted view is that golgi body originates from RER-that has lost its ribosomes from this RER arise transport vesicles that contain Golgi membrane and fuse with the saccule on the forming face of Golgi apparatus. This is why this face is called the forming face. Functions  (1) The main function of golgi body is secretion, so it is large sized among the secretory cells. (2) Glycosidation of lipids i.e., addition of oligosaccharides to produce glycolipids. (3) Glycosylation of proteins i.e., addition of carbohydrate to produce glycoproteins. (4) Formation of primary lysosomes. (5) Golgi body forms the cell plate. During cell division by secreting hemicellulose formation of enzyme and hormones (Thyroxine) etc. (6) In oocytes of animal, golgi apparatus functions as the centre around which more...

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