Current Affairs 11th Class

Plant Movements

Plants show movements in response to a variety of stimuli. Stimulus can be defined “as a change in external or internal environment of an organism that elicits response in the organism”. The reaction of plant to a stimulus is known as response. The power or ability of a plant to respond to a stimulus is called sensitivity or reactivity or irritability. The movements which occur without the effect of external stimulus are called autonomic or spontaneous movements. Thus spontaneous movements are brought by definite internal stimulus, and if the movements are produced in response to external stimulus, they are known as paratonic or induced movements. The area which perceives a stimulus is called perceptive region, while the plants part showing the response is known as responsive region. The minimum duration or time required for a stimulus to be applied continuously on the perceptive region to produce visible response is called presentation time. The duration between the application of stimulus and production of visible response is called latent time or reaction time. Classification of plant movements Plants movements are broadly classified into two types: (1) Movements of locomotion: In this case, plant moves physically from one place to another. The movements of locomotion are of two type-autonomic (occurs spontaneously) or paratonic (induced by external stimuli). (i) Autonomic movement of locomotion : These movement of locomotion are due to internal stimuli they are of following types : (a) Ciliary movements : Certain motile algae (e.g., Chlamydomonas, Volvox, etc). Zoospores and gametes of lower plants move from one place to another by means of cilia or flagella. (b) Amoeboid movements : It is the movement of naked mass of protoplasm by means of producing pseudopodia like process e.g., members of Myxomycetes (slime fungi).     (c) Cyclosis : These are movements of cytoplasm with in a cell (also called protoplasmic streaming).  These are of two types :
• Rotation : When the protoplasm moves around a single central vacuoles in either clockwise or anticlockwise direction e.g., leaf cells of Hydrilla, Vallisneria.
• Circulation : When the movement of protoplasm accurs around different vacuoles in different directions within the cell e.g., staminal hair of Tradescantia, shoot hairs of gourds.
(d) Excretory movements : Apical part of Oscillatoria is like a pendulum. It is considered that such movements are due to excretion of substances by the plants. (movements opposite to the side of excretion). (ii) Paratonic movement of locomotion (Tactic movement) : These movements take place in whole small plants.  e.g., chlamydomonas or small free ciliated organs e.g., gametes. These movements are due to external factors like light, temperature or chemicals and are of following types : (a) Phototactic movements or phototaxisms : It is the movement of free living organism towards or away from light. e.g., movement of Chlamydomonas, Ulothrix, Cladophora, Volvox etc. towards suitable light intensity. Three types of arrangement present in columular cells in more...

Physiology Of Flowering Plant

Flowering in a plant occurs at a particular time of the year and controlled by many morphological and environmental conditions. Two important controlling factors are photoperiod or light period, i.e., photoperiodism, low temperature i.e., vernalization. (1) Photoperiodism (Light period) : The effects of photoperiods or daily duration of light periods (and dark periods) on the growth and development of plants, especially flowering is called photoperiodism. The role of photoperiodism in the control of flowering was demonstrated for the first time by W.W Garner and H.A. Allard (1920). They observed that Maryland Mammoth variety of tobacco could be made to flower in summer by reducing the light hours with artificial darkening. It could be made to remain vegetative in winter by providing extra light. On the basis of length of photoperiod requirements of plants, the plants have been classified into following categories. (i) Short day plants (SDP) : These plants initiate flowering when the day length (Photoperiod) become shorter than a certain critical period. Most of winter flowering plants belong to this category e.g., cocklebur (Xanthium), Chrysanthemum, sugarcane, tobacco (Mutant Maryland Mammoth), soyabean, strawberry etc., (ii) Long day plants (LDP) : These plants begin flowering when the day length exceeds a critical length. This length too differs from species to species. The long day plants fail to flower, if the day length is shorter than the critical period. e.g., spinach (Spinacea oleracea), henbane (Hyoscymus niger), radish, sugar-beet, wheat, lattuce, poppy, larkspur, maize etc. (iii) Day neutral plants : These plants can flower in all possible photoperiods. The day neutral plants can blossom throughout the year. e.g., cucumber, cotton, sunflower, tomato, some varieties of pea, etc.     (iv) Intermediate plants : These plants flower only under day lengths within a certain range usually between 12-16 hours of light but fail to flower under either longer or shorter photoperiods. e.g., Mikania scandens, Eupatorium hyssopifolium and Phaseolous  polystacous.  (a) Amphiphotoperiodic plants : Such plants remain vegetative on intermediate day length and flower only on shorter or longer day lengths. e.g., Media elegans. (b) Short long day plants : These plants require short photoperiods for initiation of flowering and long photoperiods for blossoming. e.g., Triticum vulgare, Secale cereale. (c) Long short day plants : These plants require long photoperiods for initiation of flowering and short photoperiods for blossoming. e.g., Bryophyllum, Cestrum. Critical period : Critical photoperiod is that continuous duration of light, which must not be exceeded in short day plants and should always be exceeded in long day plant in order to bring them to flower. There is no relation with the total day length. Thus, the real distinction between a SDP and LDP is whether flowering is induced by photoperiods shorter or longer than the critical period. The critical day length for Xanthium (a short day plant) is 15. 6 hours and that for Hyoscymus niger (a long day plant) is about 11 hours, yet the former is more...

Growth Hormones And Growth Regulators

The term hormone used by first Starling (1906). He called it stimulatory substance. The growth and development in plants is controlled by a special class of chemical substances called hormones. They are needed in small quantities at very low concentrations as compared to enzyme. They are rarely effective at the site of their synthesis. Thus, growth hormones also called phytohormones term given by Thimann (1948), it can be defined as ‘the organic substances which are synthesized in minute quantities in one part of the plant body and transported to another part where they influence specific physiological processes’. A group of plant hormones including auxins, gibberellins, cytokinins, ethylene and abscisic acid are presently known to regulate growth. Auxins : Auxins (Gk. auxein = to grow) are weakly acidic growth hormones having an unsaturated ring structure and capable of promoting cell elongation, especially of shoots (more pronounced in decapitated shoots and shoot segments) at a concentration of less than 100 ppm which is inhibitory to the roots. Among the growth regulators, auxins were the first to be discovered. Discovery : Julius Von Sachs was the first to indicate the presence of organ forming substances in plants. The existence of first plant growth hormone came from the work of Darwin and Darwin (1881). Darwin described  the effects of light and gravity in his book, “Power of movements in plants”. Darwin and his son found that bending movement of coleoptile of Canary grass (Phalasis canariensis) was due to exposure of tip to unilateral light. Boysen-Jensen (1910; 1913) found that the tip produces a chemical which was later named auxin. Paal (1914, 1919) removed coleoptile tip and replaced it asymmetrically to find a curvature. Auxin was first collected by Went (1928) from coleoptile tip of Avena. Went also developed Avena curvature test for bioassay of auxin. Types of auxins : There are two major categories of auxins natural auxins and synthetic auxins. (1) Natural auxins : These are naturally occurring auxins in plants and therefore, regarded as phytohormones. Indole 3-acetic acid (IAA) is the best known and universal auxin. It is found in all plants and fungi.     The first naturally occurring auxin was isolated by Kogl and Haagen-Smit (1931) from human urine. It was identified as auxin-a (auxenotriolic acid,${{C}_{18}}{{H}_{32}}{{O}_{5}}$). Later, in 1934 Kogl, Haagen-Smit and Erxleben obtained another, auxin, called auxin-b (auxenolonic acid,${{C}_{18}}{{H}_{30}}{{O}_{4}}$) from corn germ oil (extracted from germinating corn seeds), and heteroauxin from human urine. Heteroauxin (C10H9O2N) also known as indole-3-acetic acid (IAA), is the best known natural auxin, Besides IAA, indole-3-acetaldehyde, indole-3-pyruvic acid, indole ethanol, 4-chloro-idole actic acid  (4-chloro-IAA) etc., are some other natural auxins. Natural auxins are synthesized (Young) in physiologically active parts of plants such as shoot apices, leaf primordia and developing seeds, buds (apex), embryos, from amino acid tryptophan. In root apices, they are synthesized in relatively very small amount. Auxins show polar movement. It is basipetal (from apex to base) in stem more...

Abscission

The process of shedding of leaves, fruits or flowers by a plant is called abscission. The shedding of plant parts takes place by the formation of a special layer of cells called abscission layer, within the region of attachment. The middle lamella between certain cells in this layer in often digested by polysaccharide hydrolyzing enzymes such as cellulase and pectinases.

Thyroid Gland

The name "thyroid" was introduced by Thomas Wharton (1656). It is derived from Greek "Thyreos" a shield. Location : This is the largest endocrine gland of our body. It is located in our neck upon the ventral aspect of larynx (sound box or Adam's apple) and a few anteriomost tracheal rings. It is a dark brown and H-shaped/butterfly bilobed gland. Origin : It is endodermal in origin and arises in the embryo as a midventral process from the floor of the tongue in pharyngeal region between the first and second pharyngeal pouches. Later, the duct-like connection (thyroglossal duct) of the process degenerates, so that the process is separated from the tongue and becomes endocrine. Probably, the gland is homologous to the endostyle of lower chordates. Structure of thyroid gland : In adult human beings, thyroid gland measures about 5 cm in length and 3 cm in width. It's average weight is 30 grams. It is somewhat larger in women. In old age, it becomes somewhat smaller as age advances. Its two lobes are connected by a narrower isthmus formed of nonglandular connective tissue. A small, conical pyramidal lobe is often found extended forwards from the isthmus. The whole gland is enveloped by a fibrous capsule. Thin septa or trabeculae, extending inwards from the capsule, divide the gland into a number of lobules. Each lobule, in turn, consists of a large number of small and hollow, spherical follicles (acini) embedded in a small amount of a loose connective tissue that forms the stroma of the gland. The wall of each thyroid consists of a single-layered cuboidal epithelium suspended from a basal lamina, while its cavity is filled with a yellowish, jelly-like and iodinated colloid glycoprotein substance, called iodothyroglobulin. Besides containing a dense network of blood capillaries, the stroma contains small clusters of specialized parafollicular or 'C' cells. The latter are remnants of ultimobranchial bodies derived from the fifth pharyngeal (branchial) pouches in the embryo.     Synthesis and storage of iodothyroglobulin : Synthesis of a glycoprotein thyroglobulin (TGB) - occurs continuosly in the follicular cells under genic control. The cells keep extruding thyroglobulin in follicular cavity by exocytosis. Each molecule of thyroglobulin contains about 500 amino acid momoners of which 123 monomers are of tyrosine at fixed places. Soon as the molecules of iodine and thyroglobulin come out of follicular cells, these interact in such a way that 15 tyrosine monomers of each thyroglubulin molecule at fixed places become iodinated. Certain tyrosine monomers bind with single atoms of iodine, forming monoiodotyrosine (MIT or ${{T}_{1}}$). Other tyrosine monomers bind with two atoms of iodine, forming diiodotyrosine (DIT or ${{T}_{2}}$). This is called organification of thyroglobulin. Molecules of iodothyroglobulin keep accumulating in follicular cavity, forming the jelly-like colloid. Within the colloid, molecules of iodothyroglobulin undergo conformational changes and may even interact with each other. This results in a coupling of most of the iodinated tyrosine monomers in pairs. This more...

Thymus Gland

Origin and position : The thymus is bilobed gland, is located in the upper part of the thorax near the heart in the mediastinum. It is endodermal in origin, arising in the embryo from the epithelium of outer part of third branchial pouches. Structure : Structurally, it is like lymph gland enveloped by a thin, loose and fibrous connective tissue capsule. Septa, or trabeculae extending inwards from the capsule, divide the two lobes of the gland into a number of small lobules. Each lobule is distinguished into a cortical parenchyma containing numerous lymphocytes, and a medullary mass of large, irregularly branched and interconnected epithelial cells (reticular cells), a few lymphocytes and some phagocytic cells called macrophages or Hassal's corpuscles.     Function of thymus glands (1) Thymus is haemopoietic, as well as, an endocrine gland. Thymus is the "seedbed" of "thymic lymphocytes (T-lymphocytes). Certain "stem cells", originating in yolk sac and liver in early embryo, but only in bone marrow in late embryo, migrate into the thymus and proliferate to form a large number of lymphocytes. (2) The major function of thymus is to secrete thymosin hormone, thymic humoral factor (THF), thymic factor (TF), thymopoietin. These compounds induce, not only the proliferation of lymphocytes, but also their differentiation into a variety of clones differently specialized to destroy different specific categories of antigens and pathogens likely to get into the body. This is called maturation of lymphocytes. (3) As is clear from above account, thymus is essential in neonatal (newly born) infant and postnatal child for normal development of lymphoid organs and cellular immunity. That is why, the thymus, small at birth, progressively grows in size about three or four-folds upto about the age of puberty. By this time lymphoid organs and tissues are well-developed. The thymus, therefore, starts gradually diminishing in size and its tissue is progressively infiltrated by yellowish adipose tissue. This is known as the "immunity theory of ageing". By the old age, the thymus is reduced to quite a thin, yet functional chord of tissue.

Pituitary Gland (Hypophysis)

Pituitary is known as hypophysis cerebri, its name pituitary was given by Vesalius. Muller’s gland of amphioxus and subneural gland of hardmania is homologous to pituitary of vertebrates. Weight of pituitary is 0.5 gm. Removal of pituitary is knows as hypophysectomy. Position and origin : Pituitary gland is the smallest (about 1 to 1½ cm in diameter) endocrine gland of the body. It is pea-shaped, ovoid, radish brown gland situated at the base of the brain in a cavity, hypophyseal fossa of the sella turcica of sphenoid bone. It is connected by a short stalk called Infundibulum, to the ventral wall (Hypothalamus) of diencephalon. That is why it is also called hypophysis cerebri. It weight about 0.5 to 1 gm. It control most of the endocrine glands. Hence, it is also called leader of endocrine orchestra or master gland. Pituitary gland is closely related with hypothalamus, hence, it is also called hypothalamo-hypophyseal gland, pituitary is ectodermal in origin. Parts and component Adenohypophysis       Structure of pituitary gland : Pituitary gland is comprised of two main lobes – Adenohypophysis and Neurohypophysis. Adenohypophysis is arises as hypophysial or Rathke's pouch from dorsal wall of embyronic stomodeum. It is the anterior lobe of pituitary. The neurohypophysis (Pars nervosa or Posterior lobe) form as an outgrowth from the infundibulum of the floor of hypothalamus.     In pituitary following types of cells are found : (1) Chromophobes cells : Found in adenohypophysis of pituitary. These are not stained by acid and base dye. Pigment granules are absent. These are colourless may change into chromophils. (2) Chromophil cells : Found in adenohypophysis of pituitary. These are stained by acid and base dye. Pigment granules are filled in these cells. These may be two types : (i) Acidophils : It is also known as a-cells synthesize and secretes growth hormone and prolactin. (ii) Basophils : It is also known as cyanophils or b-cells synthesize and secretes TSH, ACTH, FSH, LH and MSH hormones. (3) Pituicyte cells : These cells found in neurohypophysis of pituitary. These are supporting neuroglia cells and gives support to herring bodies. (4) Herring bodies : Herring bodies Are dilated terminal portion of Neurosecretory axon constituting hypothalamohypophyseal tract. They are hormone precursors for oxytocin and vasopressin.     Blood supply to pituitary or Hypophyseal portal system : A pair of posterior hypophysial arteries and a pair of anterior hypophysial arteries provide blood to the pituitary gland. Posterior arteries supply blood to the pars nervosa, and anterior arteries supply blood to the hypothalamus and pars distalis. Adenohypophysis has dual blood supply by means of a "circle of willis". The anterior hypophysial artery which bring blood into this circle big ureates in to two branches outside the lobe. One more...

Pineal Gland (Epiphysis)

Origin, position and structure : This is a small, (0.1 to 0.29) whitish and somewhat flattened ectodermal gland situated at the tip of a small, fibrous stalk that arises from dorsal wall of diencephalon, i.e. the roof (epithalamus) of third ventricle of the brain. Due to its location, it is also called epiphysis cerebri. It is covered over by a thin capsule formed of the piamater of the brain. Septa from this membrane extend into the gland, dividing in into lobules having two types of branched cells, viz the large and modified nerve cells, called pinealocytes, and interstitial or neuroglial cells forming the supporting tissue. In the pineal gland starts degenerating after the age of about 7 years because of deposition of granules of calcium salts (brain sand) in it. Function of pineal body : Hormone, though the function of the gland is still the subject of current research, it is known to secrete one hormone, melatonin. Melatonin concentration in the blood appears to flow a diurnal (day-night) cycle as it arises in the evening and through the night and drops to a low around noon. Melatonin lightens skin colour in certain animals and regulates working of gonads (testes and ovaries). Light falling on the retina of the eye decreases melatonin production, darkness stimulates melatonin synthesis. Girls blind from birth attain puberty earlier than normal, apparently because there is no inhibitory effect of melatonin on ovarian function. Serotonin, a neurotransmitter found in other locations in the brain, is also found in the pineal gland. Research evidence is accumulating to support the idea that the pineal gland may be involved in regulating cyclic phenomena in the body. Melatonin also is a potent antioxidant. Melatonin causes atrophy of gonads in several animals.

Parathyroid Gland

(1) Position and structure : These are four in number which are wholly or partially embedded in the dorsal surface of the thyroid gland two glands in each lobe of thyroid gland. Each is oval shaped, small sized $(5\times 5\text{ }mm)$ and yellow coloured. Histologically, a parathyroid gland is formed of masses of polygonal cell arranged in cords. Endocrine cell are two types principal or chief and oxyphil cells. Parathyroid is endodermal in origin.     (2) Hormones of parathyroid : Active hormone secreted by parathyroids is parathormone (PTH), also called Collip's Hormone (Phillips collip, 1925). It was discovered and purified by Collip in 1925. Its crystals were first prepared by Craig and Ras mussen in 1960. Its molecular structure was worked out by potts and his associates in 1971. The latter is a protein of 84 amino acid monomer. It is a polypeptide hormone. Parathyroids are present in all vertebrates except fishes. Its secretion is stimulated by low level of calcium in blood than normal level through feedback control. Functions of parathormone : Parathormone is essential for survival, because it significantly contributes to "homeostatis" by regulating the amount of calcium and phosphate ions in ECF. Our body requires an optimum calcium level (10.0 to 11.5 mg per 100mL.) in ECF (total 1000 to 1120 grams in a 70 kg man), because calcium is a key element in many physiological functions like proper permeability of cell membranes, muscular activities, nerve impulse conduction, heartbeat, blood coagulation, bone formation, fertilization of ova, etc. Calcium is most abundant of all minerals found in the body and about 99% of calcium and phosphorous are contained in the bones. Maintenance of proper calcium level under 'homeostasis' is, in fact, a combined function of parathormone, thyrocalcitonin and vitamin D3 (cholecalciferol). Parathormone promotes absorption of calcium from food in the intestine and its reabsoption from nephrons in the kidneys. Simultaneously, it accelerates elimination of phosphates in urine (phosphaturic action). Thus, calcium level tends to rise in the ECF due to the effect of parathormone. This calcium is, then, utilized by bone-forming cells – osteoblast – in bone formation under the influence of vitamin D3. Bones are asymmetrical when first formed. Their unnecessary parts are, therefore, dissolved by bone-eating cells called osteoclasts. This process also proceeds under the influence of parathormone. It results in release of calcium and phosphate in blood. Vitamin D3, is a steroid hormone which is first synthesized in an inactive form in skin cells from 7-dehydrocholesterol under the influence of ultraviolet (UV) rays of sunlight. Skin cells release it in blood. Liver cells take it from blood, change in into 25-hydroxycholecalciferol and release back into blood. Finally, the cells of proximal convoluted tubules of nephrons in the kidneys change 25-hydroxycholecalciferol into 1-25-dihydroxycholecalciferol under the influence of parathormone. This last compound is released in blood as active vitamin D3 named as cholecalciferol (calcitriol). In addition to its role in bone-remodelling, D3 more...

Pancreas

Location and origin : Pancreas (Gr. pankreas = sweet bread; Fr., pan = all + kreas = flesh) is a flattened and pinkish mixed gland (both exocrine and endocrine) situated in the concavity formed by duodenum just behind the stomach. It measures about 15 cm in length and 4 to 5 cm in breadth. It forms by fusion of two bilateral endodermal processes of embyronic intestine (duodenum of future adult). Structure : About 99% part of the gland is exocrine and formed of hollow pancreatic acini or lobules embedded in a connective tissue stroma. In the stroma, there are numerous (approximately 1 to 2 million in human pancreas) small (0.1 to 0.2 mm in diameter) clusters of endocrine cells, called islets of Langerhans after the name of their discoverer, Paul Langerhans (1869).     Cells Types in the Pancreatic Islets Each pancreatic islet includes four types of hormone-secreting cells : (1) Alpha or A cells constitute about 15% of pancreatic islet cells and secrete glucagon. (2) Beta or B cells constitute about 80% of pancreatic islet cells and secrete insulin. (3) Delta or D cells : It’s constitute about 5% of pancreatic islet cells and secrete somatostatin (identical to growth hormone-inhibiting hormone secreted by the  hypothalamus). (4) F cells : It’s constitute the remainder of pancreatic islet cells and secrete pancreatic polypeptide. Hormones of pancreas and their role The $\beta$ and $\alpha$ cells of islets of Langerhans respectively secrete insulin and glucagon hormones which are important regulation of carbohydrate, protein and fat metabolism in the body. (1) Insulin : In 1889, Minkowski and Mehring discovered that pancreas is related with the disease of diabetes mellitus in humans. Normal concentration of glucose in blood is about 100 mg (0.1 gm) per 100 ml. It increases somewhat after a carbohydrate rich food. Then, the secretion of insulin increases. It increases the permeability of all cells for glucose several times, except that of brain cells and red blood corpuscles (RBCs). The brain cells and RBCs are already highly permeable to glucose. After taking more glucose from blood, the cells utilize it for energy-production. Consequently, the basal metabolic rate (BMR) and RNA and protein synthesis increases in cells. In 1923, two Canadian scientists, Banting and Best succeeded in preparing a pure extract of insulin from the pancreatic islets of a new born calf with the help of Macleod, Banting and Macleod won the 1923 Nobel prize for this work. Later, Abel (1926) succeeded in preparing pure crystals of insulin. F. Sanger (1955) worked out the molecular structure of bovine insulin and won the 1958 Nobel Prize. He discovered that insulin is a small protein whose molecule consists of two polypeptide chains, $\alpha$and $\beta ,$ joined by disulphide linkages and respectively formed of 21 and 30 amino acid residues. Insulin is the first protein to be crystallized in pure form, first protein whose molecular structure was worked out, more...

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