Thursday, January 17, 2013



The organisms whose cells contain a nucleus. A saclike structure that encloses the cell’s hereditary materials. The presence of a nucleus distinguishes eukaryotes from prokaryotes, those simple, one-celled organisms in which the hereditary material floats free within the cell. Only the eukaryotic cell is capable of a high degree of specialization, and specialization is what makes multi cellular organisms possible. Just as banks, post offices, and other specialized workplaces are intrinsic to a city, cells tailored for certain jobs are intrinsic to more-complex organisms. Working in concert, specialized cells can create a higher level of organization known as tissues, such as the growing shoot of a plant or the spiny skin of a sea star. Coordinated tissues form organs and, in animals, these organs combine to form complex organ systems, such as the circulatory, digestive, and respiratory systems. The orchestration of these organ systems makes up the organism. 

 Size and Structure of Eukaryotes

The complexity of eukaryotic cells is reflected in their size. In general, the diameter of eukaryotic cells, which range in size from 0.01 mm to 1 mm (0.000394 in to 0.0394 in), is 10 to 100 times that of typical prokaryotic cells. An average-sized animal cell measures about 0.020 mm (0.0008 in), about one-fifth the thickness of the page of a book, while a typical plant cell is slightly larger, about 0.035 mm (about 0.0014 in). The eukaryotic cell with the greatest diameter is the ostrich egg, which measures about 120 mm (4.72 in). The longest eukaryote cells on record are the nerve cells that extend 3 m (10 ft) down a giraffe’s neck.
Ostrich egg

The largest and most conspicuous organelle is the nucleus. The nucleus encloses and protects the cell’s genetic material, deoxyribonucleic acid (DNA), so that it is not damaged by biochemical reactions in the cell. Within the eukaryotic nucleus, DNA is wrapped around specialized proteins called histones, like a thread wound around a series of spools. Each DNA strand and its histones fold back and forth several times to form a compact, stick-shaped structure called a chromosome. Depending on the organism, the nucleus contains from one to over a thousand chromosomes. Surrounding the nucleus is the nuclear envelope, a membrane with numerous pores. The pores, ringed by special protein, regulate the flow of substances into and out of the nucleus.

Eukaryotic Nutrition

To function, eukaryotes need organic molecules: carbohydrates such as sugar and starch; proteins; lipids, which include fats and oils; and nucleic acids such as DNA. Sugars such as glucose are particularly important because eukaryotes use energy from sugar to build proteins, lipids, and other organic molecules. Photosynthetic eukaryotes are known as autotrophs, a group that includes plants, seaweeds, and microscopic algae, all of which can make their own sugar. Those that must take in sugar from outside sources are called heterotrophs. Among the heterotrophs are many single-celled eukaryotes, and all fungi and animals.

Heterotrophic eukaryotes typically absorb the nutrients in food through the plasma membrane. To accomplish this task, they must first break down, or digest, the food. Fungi secrete digestive enzymes onto the surface of their food—often decaying leaves or branches—and then absorb the enzyme-released nutrients across the cell wall and plasma membrane. In contrast, animals first ingest their food into some sort of digestive structure such as the stomach. There, digestive enzymes break down the food, and the nutrients are then absorbed into the cells.
Some single-celled eukaryotes, such as amoebas, use a process called endocytosis. In endocytosis, these organisms extrude part of the plasma membrane, scoop up a food particle, and drag it into the cell, where they digest it using enzymes within the cell. In these eukaryotes, large waste molecules typically are expelled from the cell by a reverse process called exocytosis. The waste is bundled into a sac called a vesicle and transported to the plasma membrane, where it fuses with the membrane. The waste is then expelled through a hole in the fused membrane. In complex animals, cells generate wastes such as urea when nutrients are broken down within cells. These wastes are transported by blood to the kidneys. The kidneys process the waste and produce urine, which is removed from the body through the bladder. Undigested food travels through the tube like intestines and is eliminated through the digestive system.

Cell Division

Eukaryotes carry out cell division to make the new cells needed for growth, to repair damaged cells, and to replace worn out, dying cells. Most eukaryotic cells divide by mitosis, a process that produces two cells with the same genetic information as the original cell. Single-celled eukaryotes, such as amoebas and diatoms, commonly reproduce by mitosis.
Many eukaryotes also undergo a second type of cell division, called meiosis, which is designed for sexual reproduction, the union of male and female sex cells. In meiosis, two cell divisions occur in which the genetic material is rearranged, resulting in four genetically unique cells, each of which contains only half the number of chromosomes as the parent cell. When two cells with half the number of chromosomes unite, the new cell contains the full complement of chromosomes needed to produce the new organism.

Evolutionary Origin of Eukaryotic
Eukaryotes evolved much later than prokaryotes, whose origins date to about 3.5 billion to 3.8 billion years before present. Alga-like fossils from ancient rocks suggest that eukaryotes may have evolved about 2.1 billion years before present. Other fossil remains indicate that eukaryotes were well established 1.6 billion years before present. These fossils, called acritarchs, are hollow spheres that appear to be spores or cysts of eukaryotic algae.
Eukaryotic cells are thought to have evolved from primitive prokaryotes. Evidence for this view is found in the archaea, prokaryotes that resemble both bacteria and eukaryotes. Like bacteria, the archaea lack a nucleus and most other organelles. Like eukaryotes, they display flexible cell membranes and histone proteins, and have certain segments of DNA in common. This evidence, along with other molecular studies, leads many scientists to conclude that archaea, bacteria, and eukaryotes arose from a common ancestral prokaryote similar to the archaea. However, according to a theory developed by American microbiologist Carl Woese, the archaea, bacteria, and eukaryotes may have arisen, not from a single common ancestor, but from a group of genetically diverse, primitive prokaryotes.


Tuesday, January 15, 2013



Prokaryote, relatively simple unicellular organism, such as a bacterium, characterized by the absence of a nucleus and other specialized cell structures. Scientists distinguish prokaryotes from eukaryotes, which are more complex organisms with cells that contain a nucleus, such as plants and animals. 

Structure of prokaryotes

Prokaryotic cells are relatively small, ranging in size from 0.0001 to 0.003 mm (0.000004 to 0.0001 in) in diameter. With the exception of a few species, prokaryotic cells are surrounded by a protective cell wall. Just inside the cell wall of prokaryotes is the plasma membrane, a thin structure that is both flexible and strong. In both prokaryotes and eukaryotes, the plasma membrane is composed of two layers of phospholipid molecules interspersed with proteins, and regulates the traffic that flows in and out of the cell. The prokaryotic plasma membrane, however, carries out additional functions. It participates in replication of deoxyribonucleic acid (DNA) for cell division and synthesis of adenosine triphosphate (ATP), an energy molecule. In some prokaryotes, the plasma membrane is essential for photosynthesis, the process that uses light energy to convert carbon dioxide and water to glucose. 

Reproduction of Prokaryotes

Most prokaryotes multiply by the asexual process of binary fission, in which the DNA of the organism replicates in the cytoplasm, the cell divides in two, and one DNA molecule passes to each newly formed cell. In addition, some prokaryotes undergo various processes of genetic recombination. For example, in the process called transformation, a bacterium removes one or more genes from one organism and incorporates the genes into its own genetic makeup. In conjugation two organisms exchange genes. In transduction a virus transports bacterial genes from one organism to another. Gene transfers account for the appearance of new biochemical traits in prokaryotes.

Like most organisms, prokaryotes require carbon and energy to create nutrients such as carbohydrates, proteins, lipids, and nucleic acids. Prokaryotes obtain carbon and energy from a variety of sources. Certain prokaryotes use carbon dioxide as their carbon source. Called autotrophs, these prokaryotes derive energy from different sources, such as photosynthesis or inorganic molecules. Photoautotrophs, including the cyanobacteria and the green sulfur and purple sulfur archaebacteria, derive their energy from light. Chemoautotrophs, such as the soil bacteria Nitrobacter and Nitrosomonas, derive their energy from inorganic compounds such as hydrogen sulfide, ammonia, and iron. Heterotrophs are organisms that rely on ready-made organic compounds such as glucose or alcohol for their carbon source. Heterotrophs obtain energy by degrading organic molecules, such as plant or animal matter. A small group of bacteria, the photoheterotrophs, use light as their energy source, while chemo heterotrophs use organic compounds for both their carbon and energy sources. 

Importance of Prokaryotes

Prokaryotes play significant roles in our daily lives. In a process called nitrogen fixation, many species of cyanobacteria convert atmospheric nitrogen to nitrogenous compounds that other organisms use as food sources. Moreover, the photosynthesis occurring in cyanobacteria still contributes substantial amounts of oxygen to the atmosphere and stores the Sun’s energy in carbohydrate molecules. Cyanobacteria are the foundation for aquatic ecosystems, providing food for protozoa and other aquatic organisms. Cyanobacteria are threatened, however, by ultraviolet radiation, which penetrates the atmosphere as a result of the thinning ozone layer.

Monday, January 14, 2013



Kingdom Monera OR Prokaryote

Prokaryote, relatively simple unicellular organism, such as a bacterium, characterized by the absence of a nucleus and other specialized cell structures. Scientists distinguish prokaryotes from eukaryotes, which are more complex organisms with cells that contain a nucleus, such as plants and animals.
 Virus, Bacteria, Cynobacteria (Blue Green Algae).

Kingdom Protista
All unicellular eukaryotic organisms included in this kingdom. They have distinct nucleus and perform all living activities with in singular cell. These have little complex cell structure then prokaryotes.
Amobea, Euglena, Paramacium etc

Kingdom Fungi

Fungi, also eukaryotic and long considered members of the plant kingdom, have now been placed in a separate kingdom because they lack chlorophyll and plastids and because their rigid cell walls contain chitin rather than cellulose. Unlike the majority of plants, fungi do not manufacture their own food; instead they are saprophytic, absorbing their food from either dead or living organic matter.
Mashroom, Pancillium etc

Kingdom Plantae

Plant cells have all the components of animal cells and boast several added features, including chloroplasts, a central vacuole, and a cell wall. Chloroplasts convert light energy—typically from the Sun—into the sugar glucose, a form of chemical energy, in a process known as photosynthesis. Chloroplasts, like mitochondria, possess a circular chromosome and prokaryote-like ribosome, which manufacture the proteins that the chloroplasts typically need.
Examples are All plants

Kingdom animalae
Eukaryotic cells are typically about ten times larger than prokaryotic cells. In animal cells, the plasma membrane, rather than a cell wall, forms the cell’s outer boundary. With a design similar to the plasma membrane of prokaryotic cells, it separates the cell from its surroundings and regulates the traffic across the membrane.

Sunday, December 2, 2012



Nose, organ of smell, and also part of the apparatus of respiration and voice. Considered anatomically, it may be divided into an external portion—the visible projection portion, to which the term nose is popularly restricted—and an internal portion, consisting of two principal cavities, or nasal Fosse, separated from each other by a vertical septum, and subdivided by spongy or turbinated bones that project from the outer wall into three passages, or meat uses, with which various sinuses in the ethmoid, spheroid, frontal, and superior maxillary bones communicate by narrow apertures.
The margins of the nostrils are usually lined with a number of stiff hairs that project across the openings and serve to arrest the passage of foreign substances, such as dust and small insects, which might otherwise be drawn up with the current of air intended for respiration. The skeleton, or framework, of the nose is partly composed of the bones forming the top and sides of the bridge, and partly of cartilages. On either side are an upper lateral and a lower lateral cartilage, to the latter of which are attached three or four small cartilaginous plates, termed sesamoid cartilages. The cartilage of the septum separates the nostrils and, in association posterior with the perpendicular plate of the ethmoid and with the vomer, forms a complete partition between the right and left nasal Fosse.

Smell, one of the five special senses by which odors are perceived. The nose, equipped with olfactory nerves, is the special organ of smell. The olfactory nerves also account for differing tastes of substances taken into the mouth, that is, most sensations that appear introspectively as tastes are really smells.
Sensations of smell are difficult to describe and classify, but useful categorizations have been made by noting the chemical elements of odorous substances. Research has pointed to the existence of seven primary odors—camphor like, musky, floral, pepper mint like, ethereal (dry-cleaning fluid, for example), pungent (vinegar like), and putrid—corresponding to the seven types of smell receptors in the olfactory-cell hairs. Olfactory research also indicates that substances with similar odors have molecules of similar shape. Recent studies suggest that the shape of an odor-causing chemical molecule determines the nature of the odor of that molecule or substance. These molecules are believed to combine with specific cells in the nose or with chemicals within those cells. This process is the first step in a series that continues with the transmission of impulses by the olfactory nerve and ends with the perception of odor by the brain.

Sunday, November 25, 2012



The tongue serves as an organ of taste, with taste buds scattered over its surface and concentrated toward the back of the tongue. In chewing, the tongue holds the food against the teeth; in swallowing, it moves the food back into the pharynx, and then into the esophagus when the pressure of the tongue closes the opening of the trachea, or windpipe. It also acts, together with the lips, teeth, and hard palate, to form word sounds.

Taste is one of the five special senses, in humans and other animals, by which four gustatory qualities (sweetness, sourness, saltiness, and bitterness) of a substance are distinguished. Taste is determined by receptors, called taste buds, the number and shape of which may vary greatly between one person and another. In general, women have more taste buds than men. A greater number of taste buds appear to endow a greater sensitivity to sweetness, sourness, saltiness, and bitterness. In humans, the taste buds are located on the surface and sides of the tongue, the roof of the mouth, and the entrance to the pharynx. The mucous membrane lining these areas is invested with tiny projections of papillae, each of which in turn is invested with 200 to 300 taste buds. The papillae located at the back of the tongue, and called circumvallated, are arranged to form a V with the angle pointing backward; they transmit the sensation of bitterness. Those at the tip of the tongue transmit sweetness, whereas saltiness and sourness are transmitted from the papillae on the sides of the tongue. Each flask-shaped taste bud contains an opening at its base through which nerve fibers enter. These fibers transmit impulses directly to the brain. In order for a substance to stimulate these impulses, however, it must be in solution, moistened by the salivary glands. Sensations of taste have been determined to be strongly interrelated with sensations of smell.

Tuesday, July 31, 2012



Skin is outer body covering of an animal. The term skin is commonly used to describe the body covering of any animal but technically refers only to the body covering of vertebrates (animals that have a backbone). The skin has the same basic structure in all vertebrates, including fish, reptiles, birds, and humans and other mammals. This article focuses primarily on human skin.
The skin is essential to a person’s survival. It forms a barrier that helps prevent harmful micro organisms and chemicals from entering the body, and it also prevents the loss of life-sustaining body fluids. It protects the vital structures inside the body from injury and from the potentially damaging ultraviolet rays of the sun. The skin also helps regulate body temperature, excretes some waste products, and is an important sensory organ. It contains various types of specialized nerve cells responsible for the sense of touch.
The skin is the body’s largest organ—that of an average adult male weighs 4.5 to 5 kg (10 to 11 lb) and measures about 2 sq m (22 sq ft) in area. It covers the surface of the body at a thickness of just 1.4 to 4.0 mm (0.06 to 0.16 in). The skin is thickest on areas of the body that regularly rub against objects, such as the palms of the hands and the soles of the feet. Both delicate and resilient, the skin constantly renews itself and has a remarkable ability to repair itself after injury.
The skin is made up of two layers, the epidermis and the dermis. The epidermis, the upper or outer layer of the skin, is a tough, waterproof, protective layer. The dermis, or inner layer, is thicker than the epidermis and gives the skin its strength and elasticity. The two layers of the skin are anchored to one another by a thin but complex layer of tissue, known as the basement membrane. This tissue is composed of a series of elaborately interconnecting molecules that act as ropes and grappling hooks to hold the skin together. Below the dermis is the subcutaneous layer, a layer of tissue composed of protein fibers and adipose tissue (fat). Although not part of the skin itself, the subcutaneous layer contains glands and other skin structures, as well as sensory receptors involved in the sense of touch.

Hair is a distinguishing characteristic of mammals, a group of vertebrates that includes humans. A thick coat of body hair known as fur protects many mammals from the cold and from the sun’s ultraviolet rays. In humans, a species whose body hair is relatively sparse, this protective function is probably minimal, limited chiefly to the hair on the scalp.

Nails on the fingers and toes are made of hard, keratin-filled epidermal cells. They protect the ends of the digits from injury, help us grasp small objects, and enable us to scratch. The part of the nail that is visible is called the nail body, and the portion of the nail body that extends past the end of the digit is called the free edge. Most of the nail body appears pink because of blood flowing in the tissue underneath, but at the base of the body is a pale, semicircular area called the lunula. This area appears white due to an underlying thick layer of epidermis that does not contain blood vessels. The part of the nail that is buried under the skin is called the root. Nails grow as epidermal cells below the nail root and transform into hard nail cells that accumulate at the base of the nail, pushing the rest of the nail forward. Fingernails typically grow 1 mm (0.04 in) per week. Toenails generally grow more slowly.

An adult human has between 1.6 million and 4 million glands, or sweat glands. Most are of a type known as sweat glands, which are found almost all over the surface of the body and are most numerous on the palms and soles. Sweat glands begin deep in the dermis and connect to the surface of the skin by a coiled duct. Cells at the base of the gland secrete sweat, a mixture of water, salt, and small amounts of metabolic waste products. As the sweat moves along the duct, much of the salt is reabsorbed, preventing excessive loss of this vital substance. When sweat reaches the outer surface of the skin, it evaporates, helping to cool the body in hot environments or during physical exertion. In addition, nerve fibers that encircle the sweat glands stimulate the glands in response to fear, excitement, or anxiety. The sweat glands can secrete up to 10 liters (2.6 gallons) of fluid per day, far more than any other type of gland in the body.