Current Affairs 11th Class

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.

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...

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...

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.

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.

(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...

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.

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.

Microscopy (Gk. Micros = small ; skopein = to see) It is practice of using microscopes for the study of finer details of small objects including cells and tissues. Microscope are instruments consisting of lenses (made of glass / Lithium fluoride / electromagnetic lens) which magnify and resolve small objects not visible to unaided eye for the study of their details. The term microscope was coined by Faber in 1625. Magnification : Is the power of enlargement, which is the ratio of \[\text{Magnification}=\frac{\text{Size of the image with}\,\text{the instrument}}{\text{Size of the image with unaided eye}}\] Magnification of a microscope is roughly equal to the multiple of magnifying power of objective lens and ocular lens (eye piece) e.g., if the magnification power of an ocular lens is \[10\,X\] and of the objective is \[40\,X,\] then the total magnifying power of a microscope is \[10\times 40=400\,X\] (the magnification power of a microscope is represented by the symbol 'X'). Resolving power : It is the ability of a system to distinguish two close objects as two distinct objects. Its values is calculated by Abbe equation - \[{{L}_{m}}=\frac{0.61\lambda }{NA}\] Here, \[\lambda -\] is wavelength of used light, \[NA-\] Numerical Aperture, \[(NA=n\sin \theta )\] Numerical aperture is multiple of refractive index of medium (n) and \[\sin \theta \], which is sine of angle substended by optical axis and outer ray covered by objective. The value for best objective \[\sin e\,70{}^\circ =0.94.\] The resolving power of human eye is 100mm or microns (0.1 mm). This means that two points less than 100mm apart appear as one point to our eyes. Father of microscopy is Leeuwenhoek. He built first 270 X magnification microscope in 1672.

Mitochondria (Gk. Mito = thread ; chondrion = granule) are semi autonomous having hollow sac like structures present in all eukaryotes except mature RBCs of mammals and sieve tubes of phloem. Mesosomes of prokaryotes (bacteria) is analogous to mitochondrion in eukaryotes. Mitochondria are also called chondriosome, chondrioplast, plasmosomes, plastosomes and plastochondriane. Discoveries (1) These were first observed in striated muscles (Voluntary) of insects as granules by Kolliker (1880), he called them “sarcosomes”. (2) Flemming (1882) called them “fila” for thread like structure. (3) Altman (1890) called them “bioplast”. (4) C. Benda (1897) gave the term mitochondria. (5) F. Meves (1904) observed mitochondria in plant (Nymphaea). (6) Michaelis (1898) demonstrated that mitochondria play a significant role in respiration. (7) Bensley and Hoerr (1934) isolated mitochondria from liver cells. (8) Seekevitz called them “Power house of the cell”. (9) Nass and Afzelius (1965) observed first DNA in mitochondria. Number of mitochondria : Presence of mitochondria depends upon the metabolic activity of the cell. Higher is the metabolic activity, higher is the number e.g., in germinating seeds. (1) Minimum number of mitochondria is one in Microasterias, Trypanosoma, Chlorella, Chlamydomonas (green alga) and Micromonas. Maximum numbers are found (up to 500000) in flight muscle cell, (up to 50000) in giant Amoeba called Chaos – Chaos. These are 25 in human sperm, 300 – 400 in kidney cells and 1000 – 1600 in liver cells. (2) Mitochondria of a cell are collectively called chondriome. Size of mitochondria : Average size is \[0.51.00\,\,\mu \,m\] and length up to \[110\,\,\mu \,m.\] Smallest sized mitochondria in yeast cells \[(1\,\mu \,{{m}^{3}}).\] and largest sized are found in oocytes of Rana pipiens and are \[2040\,\,\mu \,m.\] Ultrastructure : Mitochondrion is bounded by two unit membranes separated by perimitochondrial space (6 – 10nm wide). The outer membrane is specially permeable because of presence of integral proteins called porins. The inner membrane is selective permeable. The inner membrane is folded or convoluted to form mitochondrial crests. In animals these are called cristae and in plants these folding are called tubuli or microvili. The matrix facing face is called ‘M’ face and face towards perimitochondrial space is called ‘C’ face. The ‘M’ face have some small stalked particles called oxysomes or \[{{F}_{1}}\] particle or elementory particle or Fernandez – Moran Particles (\[{{10}^{4}}{{10}^{5}}\] per mitochondria). Each particle is made up of base, stalk and head and is about 10nm in length. Oxysomes have ATPase enzyme molecule (Packer, 1967) and therefore, responsible for ATP synthesis. These elementary particles are also called \[{{F}_{0}}\text{ }{{F}_{1}}\] particles. The \[{{F}_{1}}\]  particle is made up of five types of subunits namely \[\alpha ,\,\beta ,\,\gamma ,\,\delta \] and \[\varepsilon .\] of these \[\alpha \] is heaviest and \[\varepsilon \] is lightest. \[{{F}_{0}}\] particles synthesize all the enzymes required to operate Kreb’s cycle.       Semi-autonomous nature of mitochondrion : Mitochondria contain all requirements of protein synthesis : (1) 70 S ribosomes. (2) DNA molecules more...


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