Current Affairs UPSC

Cell     Components of Cell? In the living organism there are two types of cellular organizations. If we look at very simple of cellular like bacteria and blue-green algae, we will discover cells that have no defined nucleus, these are prokaryotes cells. The cells which have definite nucleus are known as eukaryote. But the things which both have in common is that there are compartments surrounded by some type of membranes. These are called cell membranes.   Cell membranes: It is like a plastic bag with some tiny holes that bag holds all of the cell pieces foreign particles outside the cell. The holes are there to let some things move in and out of the cell. Compounds called proteins and phospholipids make the basic bag. The proteins are found around the holes and help move molecules in and out of the cell. Substances like  and  can move across the cell membranes by a process called diffusion. Diffusion is a process of movements of substance from a region of high concentration to a region where its concentration is low. Water also obeys the law of diffusion. The movement of water molecules is called osmosis.   Cytoplasm: It is the fluid that fills a cell. Scientists used to call the fluid proto plasm, cytoplasm contain many specialized cell called organ cells. Each of these organ cells performs a specific function for the cell.   Cell organelles: Organelles are living part of the cell have definite shape, structure and functions. To keep their function different from each other these organelles use membranes bound little structure with in themselves. Some of the important organelles are:   (a) Endoplasmic reticulum: It is a network of tulsular membranes connected at one plasma membranes. Endoplasmic reticular (ER) are two types:-rough endoplasmic reticular (RER) and Smooth endoplasmic reticulum (SER).   Functions of ER:

• It forms the supporting skeleton frame work of the cell.
• It provides a pathway for distribution of nuclear material.
• It provides surface for various enzymatic reactions.

  (b) Ribosomes: It synthesis protein, and ER sent these protein in various part of the cell. Where as SER helps in the manufacture of fats.   Functions of these proteins and fats:

• Protein and fat (lipid) help in building the cell membranes. This process is known as membranes biogenesis.
• Some other protein and fat functions as enzymes and hormones.
• SER plays a crucial role in detoxifying many poisons and drugs.

  (c) Golgi apparatus: It is found in most cell. It is another packaging organelle like the molecules and combines them to make molecules and combines them to make molecules that are more complex. It then takes those big molecules, packages them in vesicles and either stores them for faster use or sends them out of the cell.   Other functions:

• Its functions include the storage modifications and packaging of products in vesicles.
• It is also the organ Elle that builds more...

Tissue     Epithetical Tissue It is a tissue that is made up of tightly packed cells. Without much materials with in these cells. The reasons for the tightly packed cells are to act a barrier against mechanical injury, invading micro-organisms and fluid loss. We can define epithetical tissue by considering two points in mind one is the number of cells layers and two the shape of the cells. (i) On the basis of cell layers (a) When an epithelium has a single layer of cells it is called a simple epithelium. (b) Where as a multiple tier of cells are known as stratified epithelium.  (ii) On the basis of simple shape of cells:  

• Cuboidal: its occurrence is in kidney tubules, salivary glands, inner lining of the cheek. Its main function is give mechanical strength.

 

• Columnar: its occurrence is in sweat gland, tear gland, salivary gland its main function is to gives mechanical strength concerned with secretions.

     

• Squamous: when it forms a living as that of blood vessels, it is called endothelium. Its main function is to protect the underlying parts from injury, entry of germs, etc.

 

• Connective tissue: its main function is to bind and support other tissues. They have sparse populations of cells scattered through an extra cellular matrix. This extra cellular matrix is a web of fibers that is woven in a homogeneous ground substance they can be liquid, solid, or jelly like. There are a few types of connective tissue.

    A. Areolar tissue: It fills spaces inside organs found around muscles, blood vessels and nerves. Its main function is to join skin to muscles, support internal organs, help in the repair of tissues. Whereas tendon’s main function is to connect muscles to bones and ligament is connects bones to each other.   B. Adipose tissue: its occurrence is below skin, between internal organs and in the yellow bone Marrow. Its main function is to storage of fat and to conserve heat.   C. Skeletal tissue: Bone & Curtilage cartilage occurrences is in nose pic, epiglottis and in intervertebral disc of mammals. Its main function is to provide support and flexibility to body part. Whereas bone protects internal delicate organs provides attachments for muscles, bone marrow makes blood cells.   D. Fluid tissue: Blood & Lymph blood transport nutrients, hormones to tissues and organs. Where as leucocytes fight diseases and platelets help in clothing blood.   Lymph transport nutrients into the heart and it also forms the defense system of the body.   Muscular Tissue It is specialized for an ability to contract muscle cells. These are elongated and referred to as muscles fibers. When a stimulates is received at one end of a muscle cell, a wave of excitation is conducted through more...

Muscular and Skeletal System     Skeletal Systems of Various Animals (i)   Movement is a major characteristic of animals. This movements is a result of contraction of muscles. The skeleton helps transmit that movement. Skeletons are either a fluid-filled body cavity, exoskeletons or internal skeletons.   (ii)   Hydrostatic skeletons consist of fluid-filled closed chambers. Internal pressures generated by muscles contraction cause movement as well as maintain the shape of the animals, such as the sea anemone and worms. The sea anemone has one set of longitudinal muscles in the outer layer of the body, and a layer of circular muscles in the inner layer of the body. The anemone can elongate or contract its body by contracting one or the other set of muscles.   (iii)  Exoskeletons are characteristic of the Phylum Arthropoda. Exoskeletons are hard segments that cover the muscles and visceral organs. Muscles for movement attach to the inner surface of the exoskeleton       Exoskeletons restrict the growth of the animal, thus it must shed its exoskeleton (or molt) to form a new one that has room for growth. The bulk and weight of the exoskeleton and associated mechanical attain.   Note: Spiders use a combination of an exoskeleton for protection and fluid pressure for movement.   Vertebrates have developed an internal mineralized (in most cases) endoskeleton composed of bone and/or cartilage. Muscles are on the outside of the endoskeleton.   Cartilage and bone are types of connective tissue.  

• Sharks, and rays have skeletons composed entirely of cartilage; other vertebrates have an embryonic cartilage skeleton progressively replaced by bone as they mature and develop.
• Some areas of the human body, however, retain cartilage in the adult: in joints and flexible structures such as the ribs, trachea, nose and ears.

  The Skeleton and Muscles

• The skeleton and muscles function together as the musculoskeletal system. This system (often treated as two separate systems, the muscular, and skeletal) plays an important homeostatic role: allowing the animal to move to move favorable external conditions.
• Certain cell in the bones produce immune cells as well as important cellular components of the blood.
• Bone also helps regulate blood calcium levels, serving as a calcium sink. Rapid muscular contraction is important in generating internal heat, another homeostatic function.

  Types of Skeletons

• The axial skeleton consists of the skull, vertebral column, and rib cage.
• The appendicular skeleton contains the bones of the appendages (limbs, wings, or flippers/fins), and the pectoral and pelvic girdles.
• The human skull, or cranium, has a number of individual bones tightly fitted together at immovable joints.
• At birth many of these joints are not completely structured together as bone, leading to a number of “soft spots” or fontanels, which do not completely join until the age of 14-18 months.
• The vertebral column has 33 individual vertebrae separated from each other by a cartilage disk. These disks allow a certain flexibility to the spinal column, more...

The Nervous System     Divisions of the Nervous System

• The nervous system monitors and controls almost every organ system through a series of positive and negative feedback loops.

 

• The Central Nervous System (CNS) includes the brain and spinal cord.

 

• The Peripheral Nervous System (PNS) connects the CNS to other parts of the body, and is composed of nerves (bundles of neurons).

 

• Not all animals have highly specialized nervous systems.

 

• Those with simple systems tend to be either small and very mobile or large and immobile.

 

• Large, mobile animals have highly developed nervous systems: the evolution of nervous systems must have been an important adaptation in the evolution of body size and mobility.

  Nervous System in Various Organisms Coelenterates, cnidarians, and echinoderms have their neurons organized into a nerve net. These creatures have radial symmetry and lack a head. Although lacking a brain or either nervous system (CNS or PNS) nerve nets are capable of some complex behavior. Bilaterally symmetrical animals have a body plant that includes a defined head and a tail region. Development of bilateral symmetry is associated with cephalization of sensory organs at the front end of the organism. Flatworms have neurons associated into clusters known as ganglia, which in turn form a small brain. Vertebrates have a spinal cord in addition to a more developed brain. Chordates have a dorsal rather than ventral nervous system. Several evolutionary trends occur in chordates: spinal cord, continuation of cephalization in the form of larger and more complex brains, and development of a more elaborate nervous system.   The Neuron Nervous tissue is composed of two main cell types: neurons and glial cells. Neurons transmit nerve messages. Glial cells are in direct contact with neurons and often surround them.                             The neuron is the functional unit of the nervous system. Humans have about 100 billion neurons in their brain alone! While variable in size and shape,   a. Parts of Neuron:   All neurons have three parts. Dendrites receive information from another cell and transmit the message to the cell body. The cell body contains the nucleus, mitochondria and other organelles typical of eukaryotic cells. The axon conducts messages away from the cell body.   b. Types of Neuron:   Three types of neutrons occur, Sensory neurons typically have a long dendrite and short axon, and carry messages from sensory receptors to the central nervous system. Motor neurons have a long axon and short dendrites and transmit messages from the central nervous system to the muscles (or to glands). Interneurons are found only in the central nervous system where they connect neuron to neuron. Some axons are wrapped in a myelin sheath formed from the plasma membranes of specialized glial cells known as Schwann cells. Schwann more...

The Endocrine System     Hormones The endocrine system is a collector of glands that secrete chemical messages we call hormones. These signals are passed through the blood to arrive at target organ, which has cells processing the appropriate receptor. Exocrine glands (not part of the endocrine system) secrete products that the passed outside the body. Sweat glands, salivary glands, and digestive glands are examples of exocrine glands. Hormones are grouped into three classes based on their structure:

steroids

peptides

amines

 

Steroids

Steroids are lipids derived from cholesterol. Testosterone is the male sex hormone.   Estradiol, similar in structure to testosterone, is responsible for many female sex characteristics. Steroid hormones are secreted by the gonads, adrenal cortex, and placenta.  

Peptides and

Amines

Peptides are short chains of amino acids; most hormones are peptides. They are secreted by the pituitary, parathyroid, heart, stomach, liver, and kidneys. Amines are derived from the amino acid tyrosine and are secreted from the thyroid and the adrenal medulla. Solubility of the various hormone classes, varies.   Synthesis, Storage, and Secretion

• Steroid hormones are derived from cholesterol by a biochemical reaction series. Defects along this series often lead to hormonal imbalances with serious consequences. Once synthesized, steroid hormones pass into the bloodstream; they are not stored by cells, and the rate of synthesis controls them.

 

• Peptide hormones are synthesized as precursor molecules and processed by the endoplasmic reticulum and Golgi where they are stored in secretory granules. When needed, the granules are dumped into the bloodstream. Different hormones can often be made from the same precursor molecule by cleaving it with a different enzyme.

 

• Amine hormones (notably epinephrine) are stored as granules in the cytoplasm until needed.

  Evolution of Endocrine Systems Most animals with well-developed nervous and circulatory systems have an endocrine system. Most of the similarities among the endocrine systems of crustaceans, arthropods, and vertebrates are examples of convergent evolution. The vertebrate endocrine system consists of glands (pituitary, thyroid, adrenal), and diffuse cell groups secreted in epithelial tissues. More than fifty different hormones are secreted. Endocrine glands arise during development for all three embryologic tissue layers (endoderm, mesoderm, ectoderm). The type of endocrine product is determined by which tissue layer a gland originated in. Glands of ectodermal and endodermal origin produce peptide and amine hormones; mesodermal-origin glands secrete hormones based on lipids.   i. Endocrine Systems and Feedback Cycles The endocrine system uses cycles and negative feedback to regulate physiological functions. Negative feedback regulates the secretion of almost every hormone. Cycles of secretion maintain physiological and homeostatic control. These cycles can range from hours to months in duration.                                  ii. Mechanisms of Hormone Action The endocrine system acts by releasing hormones that in turn trigger actions in specific target cells. Receptors on target cell membranes bind only to one type more...

Lymphatic System and Immunity     The Lymphatic System

• The lymphatic system is composed of lymph vessels, lymph nodes, and organs. The functions of this system include the absorption of excess fluid and its return to the blood stream, absorption of fat (in the villi of the small intestine) and the immune system function.

 

• Lymph vessels are closely associated with the circulatory system vessels. Larger lymph vessels are similar to veins. Lymph capillaries are scatted throughout the body. Contraction of skeletal muscles causes movement of the lymph fluid through valves.

                   

• Lymph organs include the bone marrow, lymph nodes, spleen, and thymus.

 

• Bone marrow contains tissue that produces lymphocytes. B-lymphocytes (B-cells) mature in the bone marrow.

 

• T-lymphocytes (T-cells) mature in the thymus gland.

 

• Other blood cells such as monocytes and leukocytes are produced in the bone marrow.

 

• Lymph nodes are areas of concentrated lymphocytes and macrophages along the lymphatic veins.

 

• The spleen is similar to the lymph node except that it is larger and filled with blood.

 

• The spleen serves as a reservoir for blood, and filters of purifies the blood and lymph fluid that flows through it.

 

• If the spleen is damaged or removed, the individual is more susceptible to infections.

 

• The thymus secretes a hormone, thyroxin, that causes pre-T-cells to mature (in the thymus) into T-cells.

  Immunity

• Immunity is the body’s capability to repel foreign substances and cells.

 

• The nonspecific responses are the first line of defense.

 

• Highly specific responses are the second line of defense and are tailored to an individual threat.

 

• The immune response included both specific and nonspecific components. Nonspecific responses block the entry and spread of disease-causing agents.

 

• Antibody-mediated and cell-mediated responses are two types of specific response.

 

• The immune system is associated with defense against disease-causing agents, problems in transplants and blood transfusion, and diseases resulting from over-reaction (autoimmune, allergies) and under-reaction (AIDS).

  (A) General Defenses   Barriers to entry are the skin and mucous membranes.  

The skin is a passive barrier to infectious agents such as bacteria and viruses. The organisms living on the skin surface are unable to penetrate layers of dead skin at the surface. Tears and saliva secrete enzymes that breakdown bacterial cell walls. Skin glands secrete chemicals that retard the growth of bacteria.

 

Mucus membrane lining the respiratory, digestive, urinary, and reproductive tracts secrete mucus that forms another barrier. Physical barriers are the first line of defense.

  more...

The Respiratory System     Respiration in Single Cell Animals Single-celled organisms exchange gasses directly across their cell membrane. However, the slow diffusion rate of oxygen relative to carbon dioxide limits the size of single celled organisms. Simple animals that lack specialized exchange surfaces have flattened, tubular, or thin shaped body plans, which are the most efficient for gas exchange. However these simple animals are rather small in size.   Respiration in multicellular animals Large animals cannot maintain gas exchange by diffusion across their outer surface. They developed a variety of respiratory surfaces that all increase the surface area for exchange, thus allowing for larger bodies. A respiratory surface is covered with thin, moist epithelial cells that allow oxygen and carbon dioxide to exchange. Those gases can only cross cell membranes when they are dissolved in water or an aqueous solution, thus respiratory surfaces must be moist.   Respiratory System Principles 1. Movement of an oxygen containing medium so it contacts a moist membrane overlying blood vessels.                                2. Diffusion of oxygen from the medium into the blood.   3. Transport of oxygen to the tissues and cells of the body.   4. Diffusion of oxygen from the blood into cells.   5. Carbon dioxide follows a reverse path.

The Circulatory System     Circulatory Systems in Single-celled Organisms Single-celled organisms use their cell surface as a point of exchange with the outside environment.   Sponges are the simplest animals, yet even they have a transport system. Seawater is the medium of transport and is propelled in and out of the sponge by biliary action.   Simple animals, such as the hydra and planarian lack specialized organs such as hearts and blood vessels, instead using their skin as an exchange point for materials. This, however, limits the size an animals can attain. To become larger, they need specialized organs and organ systems.   Circulatory Systems in Multicellular Organisms Multicellular animals do not have most of their cells in contact with the external environment and so have developed circulatory systems to transport nutrients, oxygen, carbon dioxide and metabolic wastes. Components of the circulatory system include            i. Blood: a connective tissue of liquid plasma and cells ii. Heart: a muscular pump to move the blood iii. Blood vessels: arteries, capillaries and veins that deliver blood to all tissues   Vertebrate Cardiovascular System The vertebrate cardiovascular system includes a heart, which is a muscular pump that contracts to propel blood out to the body through arteries, and a series of blood vessels.   The upper chamber of the heart, the atrium (pl. atria), is where the blood enters the heart. Passing through a valve, blood enters the lower chamber, the ventricle.   Contraction of the ventricle forces blood from the heart through an artery.   The heart muscles is composed of cardiac muscle cells.   Arteries are blood vessels that carry blood away from heart. Arterial walls are able to expand and contract. Arteries have three layers of thick walls. Smooth muscles fibers contract, another layer of connective tissue is quite elastic, allowing the arteries to carry blood under high pressure.   The aorta is the main artery leaving the heart.   The pulmonary artery is the only artery that carries oxygen-poor blood. The pulmonary artery carries deoxygenated blood to the lungs. In the lungs, gas exchange occurs, carbon dioxide diffuses out, oxygen diffuses in   Arterioles are small arteries that connect larger arteries with capillaries. Small arterioles branch into collections of capillaries known as capillary beds. Capillaries, are thin-walled blood vessels in which gas exchange occurs. In the capillary, the wall is only one cell layer thick. Capillaries are concentrated into capillary beds. Some capillaries have small pores between the cells of the capillary wall, allowing materials to flow in and out of capillaries as well as the passage of white blood cells.   Changes in blood pressure also occur in the various vessels of the circulatory system. Nutrients, wastes, and hormones are exchanged across the thin walls of capillaries. Capillaries are microscopic in size, although blushing is one manifestation of blood flow into capillaries. Control of blood flow into capillary beds is done by nerve-controlled sphincters.   The circulatory system functions in the delivery of more...

The Reproductive System     Asexual reproduction                         

• Asexual reproduction allows an organism to rapidly produce many offspring without the time and resources committed to courtship, finding a mate, and mating.

 

• Fission, budding, fragmentation, and the formation of rhizomes and stolon’s are some of the mechanisms that allow organisms to reproduce asexually.

 

• The hydra produces buds;

 

• Starfish can regenerate an entire body from a fragment of the original body.

 

• The lack of genetic variability in asexuality reproducing populations can be detrimental when environmental conditions change quickly.

  Sexual Reproduction

• In sexual reproduction new individuals are produced by the fusion of haploid gametes to form a diploid zygote.

 

• Sperm are male gametes, ova (ovum singular) are female gametes.

 

• Meiosis produces cells that are genetically distinct from each other.

 

• Fertilization is the fusion of two such distinctive cells.

 

• Rotifers will reproduce asexually when conditions are favorable by having females produce eggs by mitosis. When conditions deteriorate, rotifers will reproduce sexually and encase their zygotes inside a resistant shell. Once conditions improve, these eggs hatch into diploid individuals. Rotifers thus use sexual reproduction as way to survive a deteriorating environment.

 

• Sexual reproduction offers the benefit of generating genetic variation among offspring, which enhances the chances of the population’s survival.

 

• Costs of this process include the need for two individuals to mate, courtship rituals, as well as a number of basic mechanisms described later.

  Human Reproduction and Development

• Human reproduction employs internal fertilization, and depends on the integrated action of hormones, the nervous system, and the reproductive system

 

• Gonads are sex organs that produce gametes. Male gonads are the testes, which produce sperm and male sex hormones. Female gonads are the ovaries, which produce eggs (ova) and female sex hormones.

  The Male Reproductive System

• Testes are suspended outside the abdominal cavity by the scrotum, a pouch of skin that keeps the testes close or far from the body at an optimal temperature for sperm development.

 

• Seminiferous tubules are inside each testis, and are where sperm are produced by meiosis. About 250 meters (850 fe.et) of tubules are packed into each testis.

 

• Spermatocytes inside the tubules divide by meiosis to produce spermatids that in turn develop into mature sperm.

 

• Sperm production begins at puberty and continues throughout life, with several hundred million sperm being produced each day. Once sperm form they move into the epididymis, where they mature more...

Plant Reproduction     Flowering plants Flowering plants, the angiosperms, were the last of the seed plant groups to evolve, appearing over 100 million years ago during the middle of the Age of Dinosaurs (late Jurassic). All flowering plants produce flowers and if they are sexually reproductive, they produce a diploid zygote and triploid endosperm.                 Flowers Flowers are collections of reproductive and sterile tissue arranged in a tight whorled array having very short internodes. Sterile parts of flowers are the sepals and petals. When these are similar in size and shape, they are termed tepals. Reproductive parts of the flower are the stamen (male, collectively termed the androecium) and carpel (often the carpel is referred to as the pistil, the female parts collectively termed the gynoecium).   Pollen Pollen grains (from the greek palynos for dust or pollen) contain the male gametophyte (micro gametophyte) phase of the plant. Pollen grains are produced by meiosis of microspore mother cells that are located along the inner edge of the anther sacs (microsporangia). The outer part of the pollen is the exime, which is composed of a complex polysaccharide, sporopollenin. Inside the pollen are two (or, at most, three) cells that comprise the male gametophyte. The tube cell (also referred to as the tube nucleus) develops into the pollen tube. The germ cell divides by mitosis to produce two sperm cells. Division of the germ cell can occur before or after pollination.   Pollination The transfer of pollen from the anther to the female stigma is termed pollination. This is accomplished by a variety of methods. Entomophily is the transfer of pollen by an insect. Anemophily is the transfer of pollen by wind. Other pollinators include birds, bats, water, and humans. Some flowers (for example garden peas) develop in such a way as to pollinate themselves. Others have mechanisms to ensure pollination with another flower. Flower color is thought to indicate the nature of pollinator: red petals are thought to attract birds, yellow for bees, and white for moths. Wind pollinated flowers have reduced petals, such as oaks and grasses.   Gynoecium The gynoecium consists of the stigma, style, and ovary containing one or more ovules. These three structures are often termed a pistil or carpel. In many plants, the pistils will fuse for all or part of their length.   The Stigma and Style The stigma functions as a receptive surface on which pollen lands and germinates its pollen tube. Corn silk is part stigma, part style. The style serves to move the stigma some distance from the ovary. This distance is species specific.   The Ovary The ovary contains one or more ovules, which in turn contain one female gametophyte, also referred to in angiosperm as the embryo sac. Some plants, such as cherry, have only a single ovary which produces two ovules. Only one ovule will develop into a seed.   Double Fertilization The more...