Current Affairs NEET

  Flower   It can be defined as modified dwarf shoot which is meant for sexual reproduction. It is characteristics feature of angiosperm whease reproductive organs have been aggregated as flowers. (1) Parts of a typical flower : It comprises a stalk called pedicel which arises in the axis of leaves called bracts. Upon the pedicel there may be one to many small scaly structures called bracteoles. The terminal part of the pedicel is the thalamus or torus. It is a modified and condensed axis of the flower. Modified leaves called floral leaves or floral parts arises from the nodes of the thalamus as successive whorls. A typical flower of an angiosperm consists of four types of floral parts namely calyx, corolla, androecium and gynoecium. (i) Calyx : It is the outermost whorl composed of sepals. (ii) Corolla : It is composed of petals and is the second whorl. (iii) Androecium : It is the third whorl composed of stamens. (iv) Gyneocium : It is the innermost whorl and is also called pistil. It shows carpels. (2) General description of a flower : The flowers are termed pedicellate if they possess stalks and sessile if they lack them. The flower may be described as complete if it bears all the floral parts and incomplete, when one or more floral parts are absent. Flowers are called bisexual if they bear both androecium and gynoecium. The unisexual flowers have either androecium or gynoecium. The unisexual flowers may be male flowers or female flowers. The male flower are also called staminate flowers as they have stamens only. The female flowers have only the carpels and hence called pistillate flowers. Flowers with sterile sex organs are described as neutral flowers. According to the distribution of male, female and bisexual flowers, various pattern are recognized. (i) Monoecious : Presence of male and female flowers on the same plant, e.g., Acalypha, Cocos and Ricinus. (ii) Dioecious : Presence of male and female flowers on different plants, namely, male plants and female plants. e.g., Cycas, Carica papaya and Vallisneria. (iii) Polygamous : Presence of unisexual and bisexual flowers on the same plant, e.g., Mangifera and Polygonum. (3) Symmetry of flower : The number, shape, size and arrangement of floral organs in a flower determines its symmetry. On the basis of symmetry flowers can be of the following types : (i) Actinomorphic (Regular = Symmetrical) : Actinomorphic flowers can be divided (passing through center) by any vertical plane in to two equal and similar halves. e.g., Mustard, Brinjal, Catharanthus roseus. (ii) Zygomorphic (Monosymmetrical) : Zygomorphic flowers can be divide into two equal halves by only one verticle division e.g., Pea, Larkspur, Ocimum. (iii) Asymmetrical (Irregular) : Asymmetrical flowers can not be divided into two equal halves by any vertical division. e.g., Canna, Orchids.       (4) Arrangement of floral organs : On the basis of arrangement of floral organs, three types of flowers more...

  Secondary Growth           The increase in thickness or girth due to the activity of the cambium and the cork cambium is known as secondary growth. (1) Secondary growth in stem : On the basis of the activities of cambium and cork-cambium, secondary growth in stem can be discussed under the following heads : (i) Activity of cambium                (ii) Activity of cork-cambium (i) Activity of cambium : The vascular cambium in between xylem and phloem is called intrafascicular or fascicular cambium which is primary in origin. At the time of secondary growth the parenchymatous cells of medullary rays between the vascular bundles become meristematic and form strip of cambium called as interfascicular cambium which is secondary in origin. Both inter and intrafascicular cambium joins together and form cambium ring which is partly primary and partly secondary in origin. By anticlinal divisions the circumference of the cambium increase. By periclinal division cambium produced the secondary xylem and phloem tissues on innerside and outerside. The amount of sec. xylem produced is 8-10 times greater than sec. phloem. The cambium has two types of cells: (a) The fusiform initials which are elongated and form fibres, sieve cells, sieve tubes, tracheids. (b) Ray initials which produce parenchyma cells of the rays in wood and phloem. Ray initials are much shorter than fusiform initials. Certain cells of cambium form some narrow bands of living parenchyma cells passing through secondary xylem and secondary phloem and are called secondary medullary rays. These provide radial conduction of food from the phloem, and water and mineral salts from the xylem.

• Annual rings: Activity of cambium is not uniform in those plants which grow in the regions where favourable climatic conditions (spring or rainy season) alternate regularly with unfavourable climatic conditions (cold water or dry hot summer). In temperate climates, cambium becomes more active in spring and forms greater number of vessels with wider cavities; while in winter it becomes less active and forms narrower and smaller vessels. The wood formed in the spring is known as spring wood and that formed in the dry summer or cold winter autumn wood or late wood. Both autumn and spring wood constitute a growth or annual ring. In one year only one growth ring is formed. Thus by counting the number of annual rings in the main stem at the base we can determine the age of a tree. This branch of science is known as dendrochronology. Age is determined by an instrument increment borer. Growth rings are distinct or sharply demarcated in the plants of temperate regions where as in tropical climate (near equator) they are not distinct or sharply demarcated in the trees.

(ii) Activity of cork cambium : Cork cambium or phellogen develops from outer layer of cortex. It produces secondary cortex or phelloderm on innerside and cork or phellum on outerside. The cells of phellem are dead, suberized and impervious to water. Cells of more...

  Tissue (General)     Introduction   Plant tissue is a collection of similar cells performing an organized function for the plant. Each plant tissue is specialized for a unique purpose, and can be combined with other tissues to create organs such as leaves, flowers, stems and roots. The following is a brief outline of plant tissues, and their functions within the plant.   Meristematic tissues or Meristems   The word “Meristem” originated from “Meristos” (Greek = continuous division) and the term meristem was introduced by Nageli (1858). A group of cells which are much active and capable of showing continuous divisions and redivisions, is called as meristematic tissue. The various characteristic features of the meristems are discussed below :  

• They contain immature and young cells and are capable of repeated divisions.
• Intercellular spaces are not present in meristematic tissue.
• They contain a homogeneous thin wall.
• They contain large nuclei associated with abundant cytoplasm.
• They are metabolically very active but they do not store food material.
• Only proto-plastids are present instead of plastids, chloroplast absent.
• Dense cytoplasm is present which contains several premature mitochondria.
• Vacuoles are absent.
• Meristematic cells are isodiametric in shape.
• Undifferentiated tissue in which all divides continuously

  (1) Types of meristems : The meristems may be classified on the basis of their mode of origin, position or function : (i) According to origin and development : On the basis of origin, meristematic tissues are of three types : (a) Promeristem or Primordial meristem : The promeristem originates from embryo and, therefore, called primordial or embryonic meristem. It is present in the regions where an organ or a part of plant body is initiated. A group of initial cells that lay down the foundation of an organ or a plant part, is called promeristem. This group consists of a limited amount of cells, which divide repeatedly to give rise primary meristem. It occupies a small area at the tips of stem and root. The promeristem gives rise to all other meristems including the primary meristem. (b) Primary meristem : A primary meristem originates from promeristem and retains its meristematic activity. It is located in the apices of roots, stems and the leaf primordia. Primary meristem gives rise to the primary permanent tissue. (c) Secondary Meristem : They always arise in permanent tissues and have no typical promeristem. Some living permanent cells may regain the meristematic nature. This process in which permanent tissue regains meristematic nature is called dedifferentiation. The secondary meristems are so called because they originate from permanent cells. The phellogen or cork cambium arising from epidermis, cortex or other cells during secondary growth, is an important example of secondary meristem. The secondary meristems produce secondary tissues in the plant body and add new cells for effective protection and repair. (ii) According to position : more...

  The Tissue System   The various types of tissues present in the body of a plant perform different functions. Several tissues may collectively perform the same function. A collection of tissues performing the same general function is known as a “Tissue System''. According to Sachs (1975) there are three major tissue systems in plants as follows: (1) Epidermal tissue system    (2) Ground or fundamental tissue system   (3) Vascular tissue system   (1) Epidermal tissue system: The tissues of this system originate from the outermost layer of apical meristem. It forms the outermost covering of various plant organs which remains in direct contact with the environment.   (i) Epidermis: Epidermis is composed of single layer cells. These cells vary in their shape and size and form a continuous layer interrupted by stomata. In some cases epidermis may be multilayered e.g. Ficus, Nerium, Peperomia, Begonia etc.   The epidermal cells are living, parenchymatous, and compactly arranged without intercellular spaces.   Certain epidermal cells of some plants or plant parts are differentiated into variety of cell types :   (a) In aerial roots, the multiple epidermal cells are modified to velamen, which absorb water from the atmosphere (e.g., Orchids). (b) Some of the cells in the leaves of grasses are comparatively very large, called bulliform or motor cells. It is hygroscopic in nature. e.g., Ammophila. They are thin-walled and contain big central vacuoles filled with water. They play an important role in the folding and unfolding of leaves. (c) Some members of Gramineae and Cyperaceae possess two types of epidermal cells : the long cells and the short cells. The short cells may be cork cells or silica cells. (ii) Cuticle and Wax : In aerial parts, epidermis is covered by cuticle. The epidermal cells secrete a waxy substance called cutin, which forms a layer of variable thickness (the cuticle) within and on the outer surface of its all walls. it helps in reducing the loss of water by evaporation. Usually the cuticle is covered with wax which may be deposited in the form of granules, rods, crusts or viscous semiliquid masses. Other substances deposited on the cuticle surface may be oil, resin, silicon and salts (cystoliths are crystals of calcium carbonate, e.g., Ficus. Druse and Raphides, e.g., Pistia) are crystals of calcium oxalate. Thick cuticle are found in leaves of dry habitats plants. (iii) Stomata : Stomata are minute apertures in the epidermis. Each aperture is bounded by two kidney shaped cells, called guard cells. Stomata are absent in roots. In xerophytes the stomata are sunken in grooves due to which rate of transpiration is greatly reduced (e.g. Nerium). Usually there is a large air cavity below each aperture, it is called substomatal cavity. In some species the guard cells are surrounded by subsidiary cells or accessory cells which differ morphologically from the other epidermal cells. In monocots e.g., Doob, Maize guard cells are dumb bell shape. Stomata are scattered in dicots leaves but they more...

  Epithelial Tissue   Introduction   Body of an animal is formed of several kinds of cells. There are about 200 different types of specialized cells in the human body. The cell of one or more kinds are arranged together in a characteristic manner and cooperate to perform a specific role. Such a group of cells is called a tissue. The cell of a tissue may secrete between them a nonliving intercellular material. Thus, a tissue may be defined as a group of one or more types of cells having a similar origin and specialized for a specific function or functions along with the intercellular material.    Branch of biology dealing with the study of tissue is called histology. The term ‘tissue’ was introduced by Bichat and also known as ‘Father of histology’. Mayer coined the term ‘histology’ and the founder of histology is Marcello Malpighi. Histological study of an organ called Microscopic Anatomy. Marcello Malpighi is the father of microscopic anatomy.  Hertwig introduced the term ‘mesenchyme’ for mesodermal tissue. The formation of tissues from germinal layer is called as histogenesis. The tissue classified into four main groups on the basis their location and functions, are Epithelial tissue, Connective tissue, Muscular tissue and Nervous tissue.   Epithelial Tissue An epithelium is a tissue composed of one or more layers of cells that cover the body surface and lines its various cavities. It serves for protection, secretion and excretion. The word ‘epithelium’ was introduced by Ruysch. It was applied originally to thin skin covering the nipple (G. epi = upon, thele = nipple). They are located on the outer surfaces of organs, including the skin. They form the linings of tracts, cavities and vessels. Epithelial tissue evolved first in animal kingdom. It originate from all the three primary germ layers. e.g. Epidermis arises from ectoderm, Coelomic epithelium from the mesoderm and epithelial lining of alimentary canal from the endoderm.              (i) Structure: Cells are arranged in one or more layers, cells are compactly arranged and there is no inter cellular matrix between them. Neighbouring cells are held together by intercellular junctional complexes like desmosomes, tight junctions, interdigitations etc. the cells of lowermost - layers always rest on a non living basement membrane or basal lamina. Basement membrane is made up of no cell product of epithelial tissue. It is formed of mucopolysacharides, glycoprotein and collagen or reticular fibres. The epithelial cells are held together by small amounts of cementing substances is mainly composed of glycoprotein secreted by the cell themselves. Blood vessels are absent in the epithelial tissues. However, nerve endings may penetrate the epithelium. The free surface of cells may be smooth or may have fine hair like cilia, sterocilia and microvilli. Epithelium is subjected to continuos wear and tear and injury. Hence it posses very high capacity of renewal (mitotic cell division). The following types of modifications and junctions are found in the plasma membrane of adjacent epithelial cells to more...

  Connective Tissue   It connects and supports all the other tissues, the intercellular element predominating. The cellular element is usually scanty. In function this tissue may be mechanical, nutritive and defensive. It is a tissue made up of matrix (abundant intercellular substance or ground substance) and living cells that connects and support different tissues. Connective tissue was called mesenchyme by Hertwig (1893). Connective tissue is one of the most widespread tissue in the body, found in or around every organ of the body constituting about 30% of body mass and present between ectoderm and endoderm. All connective tissues in the body are formed by mesoderm.   (i) Structure: There are large intercellular spaces between the cells. Intercellular spaces are filled with large amount of extracellular materials formed of insoluble protein fibres lying in an amorphous, transparent ground substance called matrix. Ground substances is formed of mucopolysaccharides chondritin-6-sulphate of hyaluronic acid. Ageing of an animal body is associated with deterioration in its connective tissues. With advancing age, the amount of the jelly like amorphous ground substance in connective tissues of the body decreases, while the fibres become thicker and more numerous. Moreover, calcium salts get deposited in the elastin fibres of all connective tissues and particularly those of the wall of blood vessels. Consequently, the connective tissues gradually lose elasticity, resiliency and normal tone. Wrinkles in skin, poor blood supply to various tissues due to hardening of the wall of blood vessels etc., are the ultimate results of this ageing process.             (ii) Functions (a) Attachment: Their chief function is to bind other tissues together in the organs. (b) Storage: Certain connective tissues such as adipose tissues store fat. (c) Support: Skeletal connective tissues like bones and cartilages provide the body with a supporting skeletal frame work. (d) Transport: Fluid connective tissues such as blood and lymph transport various materials in the body. (e) Defence and Scavenging: Plasma cells synthesize antibodies, viz., macrophages. Lymphocytes ingest cell debris, harmful bacteria and foreign matter. Thus these cells of connective tissues are protective in function. (f) Shock-proof cushions: The jelly-like ground substance of connective tissues acts as shock absorber around some organs such as eye balls and kidneys.  (g) Formation of blood corpuscles: The bone marrow produces blood cells. (h) Packing material: Areolar tissue acts as packing material in various organs. (i) Repair: Collagen fibres of connective tissue help in repair of injured tissues. (iii) Classification of connective tissues:  It is classified into three types -         Connective tissue proper             Connective tissue proper possess soft viscous semisolid or semi-fluid matrix.           (i) Areolar Tissue: Areolar tissue is loose connective tissue, possess transparent gelatinous, highly vascular and sticky matrix which have variety of cells and fibres. It allows movement of part connected by it (Muscle and their compound). Areolar tissue mainly consist more...

  Skeletal Tissue   It provide support and surface for attachment of muscle. Skeletal connective tissue form the frame work of body. It provide rigidity to body. These protect the various organ and help in locomotion. It is of three types. (i) Cartilage: Cartilage is a solid but semi-rigid and flexible connective tissue. Cartilage is a nonvascular connective tissue, consisting of cells embeded in a resilent matrix of chondrin. Chondrin is a protein of cartilage. Cartilage differs from other connective tissue in that only one cell type the chondrocytes is present; Chondrocytes are found in small opening called lacunae. Chondroblast, a cartilage forming cells are embeded in firm, translucent matrix younger cartilage are possesing phagocytic cells called chondroclasts which eats up extra matrix of cartilage to provide new shape to the cartilage. It is a vascular so, nutrient must reach by process of diffusion movement is through the matrix from blood vessels located in a specialized connective tissue membrane  called perichondrium, a outer covering of cartilage. Regeneration of cartilage can occur from its peri-chondrium. Cartilage is said to be metabolically nearly inactive. In kids the cartilage cells show 2 types of growth.

• Appositional or Perichondral or Secondary or Exogenous growth: It is due to deposition of matrix and division of chondrogenic cells of periphery. It leads to growth in thickness.
• Endogenous or Interstitial growth: It is due to deposition of matrix and division in inner cells of cartilage. It leads to growth in size.

            (a) Types of cartilage: It is of following types - (1) Hyaline cartilage: It is most primitive and glass like cartilage. Its matrix is transparent homogenous and pearly white or bluish green in colour, contain chondrin. When the chondrocytes or cartilage cells are arranged in groups of two, four etc. in a single lacuna it is called a cell nest. It is slightly elastic and also known as articular cartilage because it forms the articular surface of joints. Hyaline cartilage is found in trachea, larynx and bronchi, limb bones (called hyaline cap), sternum, in the hyoid apparatus nasal septum, ribs (sternal parts) larynx (cricoid, thyroid), nasal cartilage (nasal septum).           (2) Fibro cartilage (White fibrous cartilage): In this cartilage, the small amount of matrix of cartilage is packed with large number of bundles of thick white (collagen) fibres. So it is toughest and less flexible. Between the bundles of white fibres, there are scattered lacunae, each containing a chondrocyte. It is found in intervertebral discs and acts as shock absorber. It is also found in pubic symphysis and helps in parturition (child birth). The intervertebral discs remain contracted when the body is active, but relaxed when the body is at rest. That is why, our body becomes a bit taller during sleep and after death.             (3) Elastic more...

  Vascular Tissues   As the size and organizational complexity of body increased in evolution, most cells of the body got separated away from organs that receive external supplies and organs of elimination. Hence, internal transport of materials between various parts of the body became a highly specialized and important function. A vascular system, therefore, evolved in higher invertebrates and vertebrates. Blood and lymph evolved as the fluid transportation media which circulate throughout the body, carrying materials from one part to the others. These have a common fluid intercellular substance or matrix, called plasma. Several types of numerous small cells, termed corpuscles, move above or float in the plasma. There are no fibres in the plasma. Unlike other connective tissues, the plasma is not formed by the corpuscles themselves.           (i) Blood: In chordates, and in annelids amongst the non chordates, the blood is a red and opaque fluid of salty taste and peculiar smell. It is a little heavier than water. Its specific gravity and viscosity is 1.04 - 1.07 and 4.7 respectively. Its  is 7.4 so it is slightly alkaline. In human beings, the quantity of blood is about 7% to 8% of total body weight. Thus a person, weighing about 70 kg has about 5 to 6 litres of blood, occupying about 1/13th part of the body by volume. Percentage of blood in women is slightly lower. The study of blood is called haematology. It is red coloured liquid connective tissue which originates from the mesoderm. It reaches into the various organs through the blood vessels and transports various chemical substances between different tissues. During embryonic state, the blood is mainly formed in the liver but little blood is also formed in the spleen and ribs. In adults, the blood is formed in the red bone marrow. The blood formation is called as haemopoiesis. (ii) Plasma: It constitutes about 5% of body weight. It represents matrix of blood. Plasma is slightly alkaline and transparent. It forms 55-60% by volume of blood. Plasma contains: Water (91-92%), Solid (8-9%). Plasma solid part consists of organic (7%) and inorganic (1%) substances which are as follows: (a) Organic constituents of plasma: Some are its own constituents, while others are those which are transported by it. All these are divisible into following categories: (1) Plasma proteins: Protein constitute about 7% part of plasma and remain in it as colloid particles. These mainly include albumins, globulins, prothrombin and fibrinogen.           Globulins are mainly formed by plasma cells in lymphoid organs. Other plasma proteins are mainly formed in liver. These render the plasma viscous, and maintain its osmotic pressure (7.5 atmospheric) and pH. Prothrombin and Fibrinogen are essential for blood clotting. Albumins are mainly more...

  Muscular Tissues   Contractility and motility (movement) are fundamental properties of protoplasm. That is why, all cells possess potential motility. Contraction for motility in the cells results essentially from the interaction of two contractile proteins, actin and myosin. These proteins enter into the composition of microfilaments of cellular cytoskeleton. During evolution, organism achieved enhancement in motility by various means. For example, unicellular organisms and cells of lower metazoans (mesozoa and parazoa) acquired the ability to form pseudopodia, flagella, or cilia as  locomotory organelles. Then, in eumetazoans began the evolution of specialized contractile cells having much more of actin and myosin proteins. In the lowest eumetazoans (cnidarians), basal ends of epithelial cells are drawn out into elongated contractile processes. That is why, these cells are called epitheliomuscular or myoepithelial  cells. In the eumetazoans higher than cnidarians, independent contractile cells, called muscle cells (myocytes), occur and form Muscular Tissues (muscles). These tissues are obviously responsible for movements of organs and locomotion of the body in response to stimuli. These develop from embryonic mesoderm except for those of the iris and ciliary body of eyes, which are ectodermal in origin. About 40% to 50% of our body mass is of muscles. The muscle cells are always elongated, slender and spindle-shaped, fibre-like cells, These are, therefore called muscle fibres. These possess large numbers of myofibrils formed of actin and myosin. The myofibrils are obviously stuffed in a small amount of cytoplasm due to thin, fibre-like shape of muscle cells. Muscle cells are highly contractile (contracting to 1/3 or 1/2 the resting length). Muscle cells lose capacity to divide, multiply and regenerate to a great extent. Study of muscle is called myology. Types of muscle are following - (i) Striated or striped muscles : Most muscles of body are striated. These generally bring about voluntary movements under conscious control of brain and, hence, called voluntary muscles. Most of these are inserted at both ends upon bones in different parts of the body depend upon these muscles. Hence, these are also called skeletal muscles. Movements of limbs and the body solely depend upon these muscles. Hence these are also called somatic muscles. These are also called phasic type of muscles, because contraction in these is rapid, but brief and fatigue occurs quickly.             (a) Structure of striated muscles: Each striated muscle consists of numerous muscle fibres segregated into several small and parallel bundles, called Fasciculi. Fibres of each fascicule are bound together by a connective tissue sheath, called endomycium. All fasciculi of a muscle are bound together by a connective tissue termed perimycium which also forms a sheath around each fascicule. Similarly, the whole muscle itself is covered by a connective tissue sheath, called epimycium. The latter extends as a tendons at each end of the muscle to insert it on to bones. Endomycium, more...

  Nervous Tissue   A most complex tissue in the body, composed of densely packed interconnected nerve cells called neurons (as many as 1010 in the human brain). It specialized in communication between the various parts of the body and in integration of their activities. Nervous tissue is ectodermal (from neural plate) in origin. It forms the nervous system of the body which controls and coordinates the body functions. Nerve cells (neurons) are specialized to receive the external and internal stimuli. A stimulus of adequate strength (threshold stimulus) causes the depolarization or reversal of polarity of the neuron locally and initiates a nerve impulse. The neurons are capable of conducting this depolarization as a wave along their length in a particular direction either to other nerve cells or to effectors like muscles and glands which give the response. There response may be in the form of muscle contraction or glandular secretion. Therefore, excitability and conductivity two fundamental properties of nervous tissues. There is no intercellular matrix between neurons. These have permanently lost the power of division as have no centriole and have minimum power of regeneration. So these cannot be cultured in vitro. Irritability is the main function of nervous tissue. (i) Composition of Nervous Tissue: Nervous tissue is formed of four types of cells: (a) Neurons (nerve cells)                                  (b) Neuroglia (c) Ependymal cells                              (d) Neuro-secretory cells                                                                 (a) Structure of neurons: A neuron is a nerve cell with all its branches. Neuron is formed from neuroblast. It is the structural and functional unit of nervous system. It is the longest cell of the body. (1) Cyton: It is also called perikaryon or soma or cell body. Its granular cytoplasm is called neuroplasm which has following structures : (i) A large, spherical, centrally placed nucleus with a single nucleolus. (ii) Numerous fine threads called neurofibrils for the conduction of nerve impulses. (iii) A number of small, basophilic granules called Nissl’s granules formed of rough endoplasmic reticulum with ribosomes and are sites of protein synthesis. (iv) Neuroplasm has large number of mitochondria to provide high energy for impulse conduction. (v) Neuroplasm may have melanophores with melanin pigment and lipochromes with orange or yellow pigment. (vi) A mature neuron has no centriole, so it cannot divide. (vii) A “Barr body” is often seen abutting against the inner surface of nuclear membrane of cytons in females. This has been proved to be a transformed ‘X’ chromosome. (viii) Certain neurons having flask-shaped cytons and called purkinje cells, occur in the cerebellum of the brain.   (2) Neuron processes: The processes of neurons, called neurites, extend varying distances from the cyton and are of two types - dendrites or dendrons and an axon or axis cylinder (neuraxon).            (i) more...