# Current Affairs 11th Class

#### 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...

#### 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.

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