# Current Affairs 11th Class

#### Cytoplasm

The substance occur around the nucleus and inside the plasma membrane containing various organelles and inclusions is called cytoplasm. (1) The cytoplasm is a semisolid, jelly – like material. It consists of an aqueous, structureless ground substance called cytoplasmic matrix or hyaloplasm or cytosol. (2) It forms about half of the cell’s volume and about 90% of it is water. (3) It contains ions, biomolecules, such as sugar, amino acid, nucleotide, tRNA, enzyme, vitamins, etc. (4) The cytosol also contains storage products such as glycogen/starch, fats and proteins in colloidal state. (5) It also forms crystallo – colloidal system. (6) Cytomatrix is differentiated into ectoplasm or plasmagel (outer) and endoplasm or plasmasol (inner). (7) Cytomatrix is three dimensional structure appear like a network of fine threads and these threads are called microfilaments (now called actin filaments or microtrabecular lattice) and it is believed to be a part of cytoskeleton. It also contains microtubules and inter mediate cytoplasmic filaments. (8) Hyaloplasm contains metabolically inactive products or cell inclusions called deutoplast or metaplasts. (9) Cytoplasmic organelles are plastid, lysosome, sphaerosome, peroxisome, glyoxysomes, mitochondria, ribosome, centrosome, flagellum or cilia etc. (10) The movement of cytoplasm is termed as cyclosis (absent in plant cells).

#### Cilia and Flagella

Discovery : Flagellum presence was first reported by Englemann (1868). Jansen (1887) was first scientist to report the structure of sperm flagellum. Definition : Cilia and flagella are microscopic, hair or thread-like motile structures present extra-cellularly but originate intra-cellularly from the basal body. Occurrence : Cilia are found in all the ciliate protozoans e.g., Paramecium, Vorticella etc. Flagella are found in all the flagellate protozoans e.g., Euglena, Trichonympha etc. Structure : Both cilia flagella are structurally similar and possess similar parts-basal body, rootlets, basal plate and shaft.     (1) Basal body : These are also termed as blepharoplast (kinetosome) or basal granule. It is present below the plasma membrane in cytoplasm. The structure is similar to centriole made of 9 triplets of microtubules. (2) Rootlets : Made of microfilament and providing support to the basal body. (3) Basal plate : Central fibril develop in this area. It is highly dense and lie above plasma-membrane. The basal body and the shaft at the level of plasma membrane. (4) Shaft : It is the hair like projecting part of cilia and flagella which remains outside the cytoplasm. It has 9 doublet of microtubules in radial symmetry. These are called axonema. Each axonema has 11 fibrils, 9 in the periphery and 2 in the centre. The arrangement is called 9 + 2 pattern. Chemical composition : Chemically, the central tubules are formed of dynein protein while the peripheral microtubules are formed of tubulin protein. Type of flagella : There are two types of flagella. (1) Tinsel type : In this, flagellum has lateral hair-like processes, called flimmers or mastigonemes. (2) Whiplash type : In this, flagellum has no flimmers. Functions (1) They help in locomotion, respiration, cleaning, circulation, feeding, etc. (2) Being protoplasmic structure they can function as sensory organs. (3) They show sensitivity to changes in light, temperature and contact.   Difference between cilia and flagella
Cilia Flagella
More in number (may be upto 14,000 per cell). Less in number (1-8).
Small sized $(5-10\,\mu m).$ Large sized (upto $100-200\,\mu m).$
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#### Centrosome

Discovery : Centrosome was first discovered by Van Benden (1887) and structure was given by T. Boweri. Occurrence : It is found in all the animal cell except mature mammalian RBC’s. It is also found in most of protists and motile plant cells like antherozoids of ferns, zoospores of algae and motile algal forms e.g., Chlamydomonas but is absent in prokaryotes, fungi, gymnosperms and angiosperms. Structure : Centrosome is without unit membrane structure. It is formed of two darkly stained granules called centrioles, which are collectively called diplosome. These centrioles are surrounded by a transparent cytoplasmic area called centrosphere of Kinetoplasm. Centriole and centrosphere are collectively called centrosome. Each centriole is a microtubular structure and is formed of microtubules arranged in 9+0 manner (all the 9 microtubules are peripheral in position). Inside the microtubules, there is an intra-centriolar or cart-wheel structure which is formed of a central hub (about $25{AA}$ in diameter) and 9 radial spokes or radial fibres. Chemical composition : Centrosome is lipoproteinaceous structure. The microtubules of centriole are composed of protein tubulin and some lipids. They are rich in ATPase enzyme. Origin : The daughter centriole is formed from the pre-existing centriole in ${{G}_{2}}$ of interphase so called self-replicating organelle. Functions (1) The centrioles help organising the spindle fibres and astral rays during cell division. (2) They provide basal bodies which give rise to cilia and flagella.

#### Cell Wall

Discovery : It was first discovered by Robert Hooke in 1665 in Cork. Cell wall is the outer most, rigid, protective, non living and supportive layer found in all the plant cells, bacteria, cyanobacteria and some protists. It is not found in animal cells. Chemical composition : Mainly cell wall consists of two parts, matrix and cellulosic fibres (microfibrils). Matrix consists of hemicellulose, pectin, glycoproteins, lipids and water.In most of the plants cell wall is made up of cellulose                                                                            ${{({{C}_{6}}{{H}_{10}}{{O}_{5}})}_{n}},$ a polymer made-up of unbranched chain of glucose molecule linked by                                                                                                      $\beta ,\,1-4$ glycosidic bond. About 100 molecules of cellulose form a micelle, about 20 micelle form a microfibril and approx 200 microfibril form a fibril. The cell wall of bacteria and the inner layer of blue green algae is made-up mucopeptide. It is a polymer of two amino sugars namely N-acetyl glucosamine (NAG) and N-acetyl muramic acid (NAM) held alternately in                                                                                                     $\beta ,\,1-4-$  linkage. In higher fungi, the cell wall is made up of chitin, polymer of glucosamine. Structure : Cell wall consists of middle lamella, primary wall, secondary wall, tertiary wall. (1) Middle lamella : Middle lamella is the outermost region which functions as a common cementing layer between two cells. It is absent on the outer free surface. It ruptures to create intercellular spaces. Middle lamella is formed of calcium and magnecium pectate. Fruit softening is due to gelatinisation of pectic compounds of middle lamella. Pectin is used as commercial jellying agent. Which is present outside the primary wall.     (2) Primary wall : A young plant cell forms a single layer of wall material. This layer is known as the primary cell wall. The primary wall is thin, elastic and capable of expansion in a growing cell. It grows by intussusception. Meristematic and parenchymatous cells have primary cell wall only. The cells of leaves and fruits too have only primary wall. It has more hemicellulose and less cellulose. (3) Secondary wall : In mature cell, more layers of wall material are added internal to the primary wall. These are called the secondary cell wall. Growth by addition of new wall material on the primary wall is called accretion. It has more cellulose and less hemicellulose. The secondary wall is thick and rigid. It usually consists of three layers, which are often named                                                            ${{S}_{1}},{{S}_{2}}\,\text{and }{{\text{S}}_{\text{3}}}.$  It is found in collenchyma and sclerenchyma cells, xylem vesseles. (4) Tertiary wall : Sometimes tertiary wall is laid down on secondary wall, e.g., tracheids of gymnosperms. It is composed of cellulose and xylan. Origin :  A cell wall is originate at telophase stage of cell division. The plane and place of cell wall is determined by the microtubules. Fragments of ER and vesicles of golgi body alligned at the equator, called as phragmoplast, later which forms the cell plate. The synthesis of cellulose takes place by the help of enzyme cellulose synthase present in the plasma membrane. The more...

#### Cell As A Unit Of Life

Cytology : (Gk Kyios = cell ; logas = study) It is the branch of biology. Which comprises the study of cell structure and function. “Cell is the structural and functional unit of all living beings”. Study of metabolic aspects of cell components is called cell biology. Robert Hooke (1665) discovered hollow cavities (empty boxes) like compartments in a very thin slice of cork (cell wall) under his microscope. He wrote a book “Micrographia” and coined the term cellula, which was later changed into cell. Grew and Malpighi also observed small structures in slice of plants and animals. Leeuwenhoek was the first to see free cells and called them “wild animalcules” and published a book “The secret of nature”. He observed bacteria, protozoa, RBCs, sperms, etc. under his microscope. Cell theory : H.J. Dutrochet (1824) a French worker gave the idea of cell theory. The actual credit for cell theory goes to two German scientists, a Botanist M.J. Schleiden (1838) and a Zoologist T. Schwann (1839). They gave the concept “all living organisms are composed of cell”. Schleiden and Schwann both supported the theory of “spontaneous generation”. They also mentioned that “the new cell arises from nucleus by budding”. Exceptions to the cell theory : Viruses, viroids and prions are an exception to the cell theory as they are obligate parasites (sub–cellular in nature). Modification of cell theory : Modification of cell theory was done by Rudolf Virchow (1855). He proposed the “law of cell lineage” which states that cell originates from pre-existing cells. i.e., (omnis cellula-e-cellula). It is also called “cell principle” or “cell doctrine”. It states :   (1) Life exists only in cells. (2) Membrane bound cell organelles of the protoplasm do not survive alone or outside the protoplasm. (3) Cells never arise de novo. The new cells are like the parent cell in all respect. (4) All cells have similar fundamental structure and metabolic reactions. (5) Cells display homeostasis and remain alive. (6) Genetic information is stored in DNA and expressed within the cells. (7) DNA controls structure and working of a cell. The cell as a self contained unit : Autonomy of a cell is believed due to presence of DNA and its expressibility, otherwise, cell components have different shape and function. It has two positions. (1) Autonomy in unicellular organisms : Unicellular organisms leads to a totally independent life due to different shape, size and role of different organelles shows division of labour. All these display homeostasis. Unicellular organisms are more active due to large surface volume ratio. (2) Autonomy in multicellular organisms : In multicellular organisms life activities are displayed by each of the cells independently. Multicellular organisms have one thing advantage over unicellular organisms is division of labour. Cellular totipotency : Totipotency was suggested by Haberlandt (1902). When cells have tendency or ability to divide and redivide the condition of the cell is called totipotent and this phenomenon is called totipotency. Steward et.al. showed the phenomenon of cellular totipotency in carrot culture. Surface more...

#### X-Ray Crystallography

It was developed by the Bragg (1913). They can be used as a tool for determining the arrangement of atoms in various biological molecules. When the X-rays pass through a molecule, they are scattered by the atoms. The diffraction pattern of the X?rays is photographed. Wavelength of X-rays is ${{1}^{10}}{AA}.$ The nature of diffraction is related to the orientation of the atoms in the molecule. By using this technique Wilkins et al., 1953 found out details of the DNA molecule for which he was also awarded Nobel Prize along with Watson and Crick in 1962. Kendrew, 1957 by using the same technique studied the molecules of myoglobin.

#### Units of measurement used in microscopy

1 micron $(\mu )={{10}^{-6}}$ or one millionth 1 micrometer $(\mu m)={{10}^{-6}}m,\,\,{{10}^{-4}}cm,\,\,{{10}^{-3}}mm=1000nm$ 1 Nanometer$(nm)={{10}^{-9}}m,\,\,{{10}^{-7}}cm,\,\,{{10}^{-6}}mm,\,\,{{10}^{-3}}\mu m=10\overset{{}^\circ }{\mathop{A}}\,$  1 Angstrom $(\overset{{}^\circ }{\mathop{A}}\,)={{10}^{-10}}m,{{10}^{-8}}cm,\,{{10}^{-7}}mm,\,\,{{10}^{-4}}\mu m.$ 1 Picometer $(pm)={{10}^{-12}}m,\,{{10}^{-3}}nm$ 1 Femtometer $(fm)={{10}^{-15}}m,\,{{10}^{-6}}nm$ 1 Attometer $={{10}^{-18}}m,\,\,{{10}^{-9}}nm$ Common unit of measurement in Microscopy and cytology is nanometer while unit of measurement of cell is micron.

#### Types of microscopes

(1) Simple microscope : It is also known as magnifying glass and consists of a convergent lens. Leeuwenhoek (1683) designed a primitive microscope and discovered cells with it. It was the first tool ever used to observe biological objects. Its magnification power was 14 – 42 times only, so it is considered as simple microscope. (2) Compound microscope or Light microscope : The first compound microscope was assembled by Zacharias Janssen and J. Janssen, the Dutch spectacles makes in 1590. The compound microscope was prepared by Kepler and Galileo in 1611. However, it was not used for laboratory study. It is simplest, widely used microscope having three lens i.e., condensor, which collects the light rays and precisely focuses them on the objects; objective lens, which magnifies the image by three objective lenses, i.e., low power (10x), high power (45x) and oil immersion lenses. In a compound microscope an object can be magnified upto 1000 times and the magnification is independent of intensity of light, size of microscope and numerical aperture. The light microscope is also called bright field microscope because it forms the image when light is transmitted through the object. (3) Fluorescent microscope : It was developed by Coons (1945). It is observed that when ultraviolet light is irradiated on certain chemical substances, they absorb it and emit visible light. These chemical substances are called fluoro-chromes. The fluorescent substances e.g., quinine sulphate, rhodamine and auramine are used to stain the cellular objects and these objects are easily visible as fluorescent areas when illuminated with ultraviolet light. (4) Polarizing microscope : It was invented by Tolbart. In this microscope the plane polarised light is used as a source of illumination. Unlike the ordinary light, plane polarised light vibrates only in one direction and the cellular objects are easily visible as they appear bright against the dark ground. Polarizing microscope is helpful in studying the spindle fibres in the cells. (5) Ultraviolet microscope : It was invented by Caspersson. In this microscope the source of illumination is ultraviolet radiations having shorter wavelengths $(1500\text{ }{AA}\text{ }\text{ }3500\text{ }{AA})$ as compared to ordinary visible light. In this microscope, the lenses are made of fluoride, lithium fluoride or quartz instead of glass. Ultraviolet microscope is helpful in quantitative determination of all those cell components which absorb ultraviolet rays. (6) Phase contrast microscope (i) Discovered by Dutch man Fredericke Zernicke (1935). (ii) Source of illumination is visible light. (iii) It is used to study living cells and tissues without staining and effect of chemical and physical agents on the living cells. (iv) It is also used to study spindle formation, pinocytosis, karyokinesis, cytokinesis etc. (v) It is draw back is low magnification power so subcellular organelles smaller than $0.2\,\mu ,$ like ribosomes, lysosomes, ER, cannot be visualised. (7) Interference microscope (Morten et.al.) (i) It’s principle is similar to that of the phase contrast microscope and gives / studies quantitative data. (ii) Nomarski interference contrast microscope is useful to study mitosis /cell components in living state. (iii) more...

#### Radioisotope Or Tracer Technique

They are unstable isotopes which function like normal elements but emit positive or negative particles, e.g., $^{3}H$(Tritium), $^{14}C$(Carbon), $^{32}P$(Phosphorus), $^{35}S$(Sulphur), $^{42}K$(Potassium), $^{131}I$(Iodine). Radioactivity is recorded in different parts by Geiger counter or scintillation counter or autoradiography to know regions of use and transport. The tracers have been used for knowing pathway of mineral transport (Stout and Hoagland, 1939), organic solute transport (Vernon and Aronoff, 1952), carbon assimilation (Calvin, 1955). Where radioactive elements are not available, heavy isotopes are used, e.g., $^{15}N{{,}^{18}}O.$ Their fate is recorded by mass spectroscopy and density gradient centrifugation. Meselson and Stahl (1958) studied DNA replication and Ruben et al (1941) evolution of oxygen (photolysis of water) in photosynthesis by using heavy isotopes.

#### Pulse-Labelling Technique

Some of the biological molecules undergo changes after their synthesis. We can cite here the case of RNA. The transcription of hnRNA from DNA ultimately leads to the formation of m-RNA. These changes can be studied through pulse-labelling technique.

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