Science

Eukaryotic

2013-01-17T09:40:00.000-08:00

Home

Eukaryotes

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.

Prokaryotes

2013-01-15T11:00:00.001-08:00

HOME

Prokaryotes

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.

Nutrition

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.

KINGDOM

2013-01-14T09:33:00.004-08:00

HOME

Kingdom Monera OR Prokaryote

Examples

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.

Examples

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.

Example

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.

NOSE

2012-12-02T10:46:00.003-08:00

HOME

NOSE

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.

SENSE OF SMELL

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.

TONGUE

2012-11-25T09:21:00.001-08:00

HOME

Tongue

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 BIDS

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.

HOME

2012-08-30T11:10:00.000-07:00

HOME

SCIENCE TOPICS.

SCIENCE

BIOLOGY

CELL

CELL DIVISION

CELL STRUCTURE

SCIENTIFIC METHOD

BOTANY

ZOOLOGY

DIGESTIVE SYSTEM

RESPIRATORY SYSTEM

CIRCULATORY SYSTEM

URINARY SYSTEM

ENDOCRINE SYSTEM

NERVOUS SYSTEM

SKELATAL SYSTEM

IMMUNE SYSTEM

MUSCULAR SYSTEM

MALE REPRODUCTIVE SYSTEM

FEMALE REPRODUCTIVE SYSTEM

Respiratory System

HUMAN EYE

EAR

SKIN

TONGUE

SENSE OF SMELL

KINGDOMS

PROKARYOTES

EUKARYOTES

HUMAN SKIN

2012-07-31T11:07:00.001-07:00

HOME

SKIN

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.

STRUCTURE OF SKIN

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

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

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.

GLANDS

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.

HUMAN EAR

2012-07-29T03:50:00.000-07:00

HOME

EAR

Ear is organ of hearing and balance. Only vertebrates, or animals with backbones, have ears. Invertebrate animals, such as jellyfish and insects, lack ears, but have other structures or organs that serve similar functions. The most complex and highly developed ears are those of mammals.

STRUCTURE OF HUMAN EYE

The human ear consists of three sections: the outer ear, the middle ear, and the inner ear. The outer ear includes the auricle (pinna), the visible part of the ear that is attached to the side of the head, and the waxy, dirt-trapping auditory canal. The tympanic membrane (eardrum) separates the external ear from the middle ear, an air-filled cavity. Bridging this cavity are three small bones—the malleus (hammer), the incus (anvil), and the stapes (stirrup). The cochlea and semicircular canals make up the inner ear.

A. OUTER EAR

The outer ear is made up of the auricle, or pinna, and the outer auditory canal. The auricle is the curved part of the ear attached to the side of the head by small ligaments and muscles. It consists largely of elastic cartilage, and its shape helps collect sound waves from the air. The earlobe, or lobule, which hangs from the lower part of the auricle, contains mostly fatty tissue.

The outer auditory canal, which measures about 3 cm (about 1.25 in) in length, is a tubular passageway lined with delicate hairs and small glands that produce a wax-like secretion called cerumen. The canal leads from the auricle to a thin taut membrane called the eardrum or tympanic membrane, which is nearly round in shape and about 10 mm (0.4 in) wide. It is the vibration of the eardrum that sends sound waves deeper into the ear, where they can be processed by complex organs and prepared for transmission to the brain. The cerumen in the outer auditory canal traps and retains dust and dirt that might otherwise end up on the eardrum, impairing its ability to vibrate.

The inner two-thirds of the outer auditory canal is housed by the temporal bone, which also surrounds the middle and inner ear. The temporal bone protects these fragile areas of the ear.

B. MIDDLE EAR

The eardrum separates the outer ear from the middle ear. A narrow passageway called the eustachian tube connects the middle ear to the throat and the back of the nose. The eustachian tube helps keep the eardrum intact by equalizing the pressure between the middle and outer ear. For example, if a person travels from sea level to a mountaintop, where air pressure is lower, the eardrums may cause pain because the air pressure in the middle ear becomes greater than the air pressure in the outer ear. When the person yawns or swallows, the eustachian tube opens, and some of the air in the middle ear passes into the throat, adjusting the pressure in the middle ear to match the pressure in the outer ear. This equalizing of pressure on both sides of the eardrum prevents it from rupturing.

C. INNER EAR

The chain of bones in the middle ear leads into the convoluted structures of the inner ear, or labyrinth, which contains organs of both hearing and balance. The three main structures of the inner ear are the cochlea, the vestibule, and the three semicircular canals.

The cochlea is a coiled tube that bears a close resemblance to the shell of a snail, which is what the word means in Greek. Along its length the cochlea is divided into three fluid-filled canals: the vestibular canal, the cochlear canal, and the tympanic canal. The partition between the cochlear canal and the tympanic canal is called the basilar membrane. Embedded in the basilar membrane is the spiral-shaped organ of Corti. The sensory cells in the organ of Corti have thousands of hairlike projections that receive sound vibrations from the middle ear and send them on to the brain via the auditory nerve. In the brain they are recognized and interpreted as specific sounds.

For example, when the head is upright, the gelatin and mineral particles press down on all the hairlike cells equally. When the head is tilted straight forward by dropping the chin, the gelatin and mineral particles pull on all the hairlike cells equally. If the head is tilted to one side or the other, the cells receive unequal stimulation, varying with the direction and amount of tilt. If the utriculus of both ears is destroyed by injury or disease, the head will hang down limply unless its position can be judged with the eyes. The utriculus is also used to detect the body’s starting or stopping. If a person stops suddenly, the gelatin and mineral particles continue to move, exerting a forward pull on the hairlike cells. The cells then send a specific pattern of nerve impulses to the brain.

THE HUMAN EYE

2012-07-28T14:50:00.003-07:00

HOME

THE HUMAN EYE

The entire eye, often called the eyeball, is a spherical structure approximately 2.5 cm (about 1 in) in diameter with a pronounced bulge on its forward surface. The outer part of the eye is composed of three layers of tissue. The outside layer is the sclera, a protective coating. It covers about five-sixths of the surface of the eye. At the front of the eyeball, it is continuous with the bulging, transparent cornea. The middle layer of the coating of the eye is the choroid, a vascular layer lining the posterior three-fifths of the eyeball. The choroid is continuous with the ciliary body and with the iris, which lies at the front of the eye. The innermost layer is the light-sensitive retina.

The cornea is a tough, five-layered membrane through which light is admitted to the interior of the eye. Behind the cornea is a chamber filled with clear, watery fluid, the aqueous humor, which separates the cornea from the crystalline lens. The lens itself is a flattened sphere constructed of a large number of transparent fibers arranged in layers. It is connected by ligaments to a ringlike muscle, called the ciliary muscle, which surrounds it. The ciliary muscle and its surrounding tissues form the ciliary body. This muscle, by flattening the lens or making it more nearly spherical, changes its focal length.

THE FUNCTION OF EYE

Focusing the eye, as mentioned above, is accomplished by a flattening or thickening (rounding) of the lens. The process is known as accommodation. In the normal eye accommodation is not necessary for seeing distant objects. The lens, when flattened by the suspensory ligament, brings such objects to focus on the retina. For nearer objects the lens is increasingly rounded by ciliary muscle contraction, which relaxes the suspensory ligament. A young child can see clearly at a distance as close as 6.3 cm (2.5 in), but with increasing age the lens gradually hardens, so that the limits of close seeing are approximately 15 cm (about 6 in) at the age of 30 and 40 cm (16 in) at the age of 50. In the later years of life most people lose the ability to accommodate their eyes to distances within reading or close working range. This condition, known as presbyopia, can be corrected by the use of special convex lenses for the near range.

EYE DISEASE

hordeolum

Ptosis

The most common disease of the eyelids is hordeolum, known commonly as a sty, which is an infection of the follicles of the eyelashes, usually caused by infection by staphylococci. Internal sties that occur inside the eyelid and not on its edge are similar infections of the lubricating Meibomian glands. Abscesses of the eyelids are sometimes the result of penetrating wounds. Several congenital defects of the eyelids occasionally occur, including coloboma, or cleft eyelid, and ptosis, a drooping of the upper lid. Among acquired defects are symblepharon, an adhesion of the inner surface of the eyelid to the eyeball, which is most frequently the result of burns. Entropion, the turning of the eyelid inward toward the cornea, and ectropion, the turning of the eyelid outward, can be caused by scars or by spasmodic muscular contractions resulting from chronic irritation. The eyelids also are subject to several diseases of the skin such as eczema and acne, and to both benign and malignant tumors. Another eye disease is infection of the conjunctiva, the mucous membranes covering the inside of the eyelids and the outside of the eyeball.

Respiratory System

2012-07-28T13:42:00.000-07:00

HOME

Respiratory System

Respiratory System, in anatomy and physiology, organs that deliver oxygen to the circulatory system for transport to all body cells. Oxygen is essential for cells, which use this vital substance to liberate the energy needed for cellular activities. In addition to supplying oxygen, the respiratory system aids in removing of carbon dioxide, preventing the lethal buildup of this waste product in body tissues. Day-in and day-out, without the prompt of conscious thought, the respiratory system carries out its life-sustaining activities. If the respiratory system’s tasks are interrupted for more than a few minutes, serious, irreversible damage to tissues occurs, followed by the failure of all body systems, and ultimately, death.

While the intake of oxygen and removal of carbon dioxide are the primary functions of the respiratory system, it plays other important roles in the body. The respiratory system helps regulate the balance of acid and base in tissues, a process crucial for the normal functioning of cells. It protects the body against disease-causing organisms and toxic substances inhaled with air. The respiratory system also houses the cells that detect smell, and assists in the production of sounds for speech.

STRUCTURE

The organs of the respiratory system extend from the nose to the lungs and are divided into the upper and lower respiratory tracts. The upper respiratory tract consists of the nose and the pharynx, or throat. The lower respiratory tract includes the larynx, or voice box; the trachea, or windpipe, which splits into two main branches called bronchi; tiny branches of the bronchi called bronchioles; and the lungs, a pair of saclike, spongy organs. The nose, pharynx, larynx, trachea, bronchi, and bronchioles conduct air to and from the lungs. The lungs interact with the circulatory system to deliver oxygen and remove carbon dioxide.

Nasal Passage

The flow of air from outside of the body to the lungs begins with the nose, which is divided into the left and right nasal passages. The nasal passages are lined with a membrane composed primarily of one layer of flat, closely packed cells called epithelial cells. Each epithelial cell is densely fringed with thousands of microscopic cilia, fingerlike extensions of the cells. Interspersed among the epithelial cells are goblet cells, specialized cells that produce mucus, a sticky, thick, moist fluid that coats the epithelial cells and the cilia. Numerous tiny blood vessels called capillaries lie just under the mucous membrane, near the surface of the nasal passages. While transporting air to the pharynx, the nasal passages play two critical roles: they filter the air to remove potentially disease-causing particles; and they moisten and warm the air to protect the structures in the respiratory system.

Pharynx

Air leaves the nasal passages and flows to the pharynx, a short, funnel-shaped tube about 13 cm (5 in) long that transports air to the larynx. Like the nasal passages, the pharynx is lined with a protective mucous membrane and ciliated cells that remove impurities from the air. In addition to serving as an air passage, the pharynx houses the tonsils, lymphatic tissues that contain white blood cells. The white blood cells attack any disease-causing organisms that escape the hairs, cilia, and mucus of the nasal passages and pharynx. The tonsils are strategically located to prevent these organisms from moving further into the body. One tonsil, called the adenoids, is found high in the rear wall of the pharynx. A pair of tonsils, the palatine tonsils, is located at the back of the pharynx on either side of the tongue. Another pair, the lingual tonsils, is found deep in the pharynx at the base of the tongue. In their battles with disease-causing organisms, the tonsils sometimes become swollen with infection. When the adenoids are swollen, they block the flow of air from the nasal passages to the pharynx, and a person must breathe through the mouth.

Air moves from the pharynx to the larynx, a structure about 5 cm (2 in) long located approximately in the middle of the neck. Several layers of cartilage, a tough and flexible tissue, comprise most of the larynx. A protrusion in the cartilage called the Adam’s apple sometimes enlarges in males during puberty, creating a prominent bulge visible on the neck.

While the primary role of the larynx is to transport air to the trachea, it also serves other functions. It plays a primary role in producing sound; it prevents food and fluid from entering the air passage to cause choking; and its mucous membranes and cilia-bearing cells help filter air. The cilia in the larynx waft airborne particles up toward the pharynx to be swallowed.

Food and fluids from the pharynx usually are prevented from entering the larynx by the epiglottis, a thin, leaflike tissue. The “stem” of the leaf attaches to the front and top of the larynx. When a person is breathing, the epiglottis is held in a vertical position, like an open trap door. When a person swallows, however, a reflex causes the larynx and the epiglottis to move toward each other, forming a protective seal, and food and fluids are routed to the esophagus. If a person is eating or drinking too rapidly, or laughs while swallowing, the swallowing reflex may not work, and food or fluid can enter the larynx. Food, fluid, or other substances in the larynx initiate a cough reflex as the body attempts to clear the larynx of the obstruction. If the cough reflex does not work, a person can choke, a life-threatening situation. The Heimlich maneuver is a technique used to clear a blocked larynx (see First Aid). A surgical procedure called a tracheotomy is used to bypass the larynx and get air to the trachea in extreme cases of choking.

FEMALE REPRODUCTIVE SYSTEM

2012-07-28T13:31:00.000-07:00

HOME

FEMALE REPRODUCTIVE SYSTEM

The female reproductive system is more complex than that of the male reproductive system. In addition to producing female sex cells, known as ova (eggs), the female body also protects and nurtures a developing baby for about nine months while it grows within the woman’s uterus (womb). A woman’s breasts can provide nourishment for newborns.

BREST

During pregnancy, breast size increases as milk-producing glands known as lobules develop in preparation for breast-feeding. Breast milk can provide all the nourishment a newborn needs. As a newborn breast-feeds, the sucking action stimulates the release of oxytocin, a hormone that promotes milk flow.

OVARIES

The primary reproductive organs of a female are the ovaries, a pair of almond- shaped glands. At puberty, ovaries produce about 400,000 eggs. Each month an egg is released from the ovary and travels down one of the fallopian tubes. If the egg is released around the time of sexual intercourse and it meets and fuses with a male’s sperm, it becomes fertilized. If an egg is not fertilized, it moves from the fallopian tube to the uterus and then passes out of the body in the next menstrual cycle.

UTERUS

The uterus is a hollow, muscular organ located on top of the bladder. During pregnancy, a fertilized egg travels to the uterus, embeds within the uterine wall, and forms a fetus . Around 40 weeks after fertilization, the fetus is born. During birth, the baby leaves the uterus and travels through the mother’s vagina, a tube that extends from the uterus to the outside of the body.

Reproductive System (Male)

2011-11-30T04:18:00.001-08:00

HOME

Reproductive System (Male)

The primary function of the male reproductive system is to form sperm, the male reproductive cells, and deliver them to the female. Unlike the reproductive structures of females, the male reproductive structures are located outside of the pelvis. This external location keeps the temperature of sperm slightly below body temperature, which is necessary for their healthy development and survival.

Testis

Beginning at puberty, sperm is produced in the testes (testicles), a pair of glands located in a pouch called the scrotum. From each testis, sperm migrate to a long, coiled tube known as the epididymis, where they are stored for one to three weeks until they mature.

Penis

When a man becomes sexually aroused, spongy tissue in the penis fills with blood and the organ becomes erect. Sperm leave the scrotum and travel along a long vessel known as the vas deferens. Fluid produced by the seminal vesicles and the prostate gland mixes with sperm to produce semen. During ejaculation, semen containing 200 million to 300 million sperm propels from the body through the urethra in the penis.

MUSCULAR SYSTEM

2011-08-14T02:51:00.000-07:00

HOME

Muscular System

Muscle, tissue or organ of the animal body characterized by the ability to contract, usually in response to a stimulus from the nervous system. The basic unit of all muscle is the myofibril, a minute, threadlike structure composed of complex proteins. Each muscle cell, or fiber, contains several myofibrils, which are composed of regularly arranged myofilaments of two types, thick and thin. Each thick myofilament contains several hundred molecules of the protein myosin. Thin filaments contain two strands of the protein actin. The myofibrils are made up of alternating rows of thick and thin myofilaments with their ends interleaved. During muscular contractions, these interdigitated rows of filaments slide along each other by means of cross bridges that act as ratchets. The energy for this motion is generated by densely packed mitochondria that surround the myofibrils.

Three types of muscular tissue are recognized: smooth, skeletal, and cardiac.

Anterior Muscles

This illustration shows the body’s complex network of skeletal muscles. Skeletal muscles attach to bones of the skeleton and control all voluntary movements. A skeletal muscle crosses a joint between two bones and is attached to each bone either directly or by means of a tendon or a fibrous sheet or band called a fascia. Bones move when muscles contract, or shorten, across the joint.

The size of a muscle depends on the function it performs. Where dexterity is required, as in the fingers, the muscles are typically very small. Where strength is needed, as in the thigh, the muscles are large.

Posterior Muscles

IMMUNE SYSTEM

2011-08-14T02:00:00.000-07:00

HOME

Immune system

The immune system defends the body from invading organisms that may cause disease. One part of the immune system uses barriers to protect the body from foreign substances. These barriers include the skin and the mucous membranes, which line all body cavities; and protective chemicals, such as enzymes in saliva and tears that destroy bacteria.

Another part of the immune system uses lymphocytes, specialized white blood cells that respond to specific types of foreign invaders. B lymphocytes produce proteins called antibodies, which circulate in the blood and attack specific disease-causing organisms. T lymphocytes attack invading organisms directly.

Tonsils

Tonsils are masses of lymphoid tissue forming a ring around the walls of the pharynx, or throat. The lymphoid cells in the tonsils help protect the pharynx from invasion by disease-producing bacteria.

Thymus gland

Some lymphocytes form in the bone marrow and then travel to the thymus gland, where they mature into T lymphocytes.

Lymph nodes

Lymph nodes are masses of tissue that attract lymphocytes and deploy them to areas of the body under attack by infectious agents.

Spleen

The spleen is one of the lymphoid organs. Mature lymphocytes constantly travel through the blood to the lymphoid organs and then back to the blood again. This recirculation ensures that the body is continuously monitored for invading substances. Among its many functions, the spleen produces antibodies against various disease organisms and removes worn-out red blood cells from the bloodstream.

Bone marrow

The bone marrow is the soft substance found in the center of some bones. All lymphocytes originate in the bone marrow. Those that mature in the bone marrow develop into B lymphocytes.

Lymphatic vessels

Lymphocytes travel throughout the body in the blood, but they often migrate into lymphatic vessels, which are found in all parts of the body except the brain. Lymphocytes travel within these vessels in a pale,

fat-laden liquid known as lymph.

SKELETAL SYSTEM

2011-08-14T01:55:00.000-07:00

HOME

Skeletal System

The human skeleton is a strong, flexible framework of 206 bones that supports the body and protects internal organs. In addition, the bones of the skeleton store calcium, a mineral essential for the activity of nerve and muscle cells. The soft core of bone, the bone marrow, is where red blood cells, certain white blood cells, and blood platelets form.

Bones come in different shapes and sizes, each adapted to perform specific functions. The breastbone, for example, is a flat plate of bone that helps to protect the heart and lungs in the chest. The fused bones of the skull safely encase the brain. The short, delicate bones in the wrist and hand enhance dexterity, providing flexibility for small, precise motions. The long, heavy femur bone in the leg acts as a strong lever for powerful or speedy movement. Cartilage is flexible connective tissue that provides support to skeletal bones and allows joints to move without rubbing against each other.

Exoskeleton

A form of exoskeleton is the shell of calcium or silica secreted by certain protozoans known as foraminiferans. Commercial sponges have an exoskeleton consisting of spongin, which is a tough, elastic substance. Cnidarians secrete a wide variety of exoskeletal substances, ranging from the elastic covering of the jellyfish to the stony material deposited by coral. The familiar shells of most mollusks are composed of calcium carbonate and an organic ground substance known as conchiolin. Among insects, each of the three principal divisions of the body—the head, the thorax, and the abdomen—is enclosed in a framework of horny plates. The plates of each primary division are separated from those of the next division by elastic tissue that permits flexibility of motion. The appendages are enclosed by sheaths projecting from the exoskeleton; elastic tissue similar to that between the plates joins the segments of the appendages and attaches them to the body.

Endoskeleton

Vertebrates have a more or less rigid group of structures composed of cartilage or bone or of a combination of these two connective tissues. The most primitive of these structures is the notochord, which is a backbone of cartilage occurring in fishes. Animals higher on the evolutionary scale have an axial skeleton, consisting of the skull, spinal column, and ribs, and an appendicular skeleton, made up of the pelvic and pectoral girdles and the appendages.

In higher animals, the skeleton formed in the embryo is initially cartilaginous; bone and calcium are deposited as the organism matures. In humans, the process of bone hardening, or ossification, is completed at about the age of 25. The last bone to ossify is the breastbone.

The total number of bones in any animal varies with its age; many bones fuse together during the ossification process. The average number of distinct skeletal structures in a young human is 200, exclusive of the 6 ossicles found in the ears. The human skeleton is subject to a number of pathological conditions, most important of which are fracture and a deficiency disease that is known as rickets.

NERVOUS SYSTEM

2011-08-14T00:34:00.000-07:00

HOME

Nervous System

The human nervous system oversees the activity of all other body systems. The central nervous system is composed of the brain and spinal cord. The peripheral nervous system is made up of a network of nerves connecting the brain and spinal cord with the rest of the body. Together, the central and peripheral nervous systems control both voluntary and involuntary functions in the body.

Brain

The center of the nervous system is the brain, which is composed of the brainstem, the cerebellum, and the cerebrum. The brainstem controls basic functions such as breathing rate and heartbeat and it contains nerve centers involved in hearing and vision.

The cerebellum controls balance, equilibrium, and locomotion. The cerebrum regulates such mental functions as understanding, remembering, speaking, learning, reasoning, and feeling emotions.

Spinal cord

The spinal cord extends from the brain into the spine through a canal encased in bone. The spinal cord is a collection of nerve fibers and cells that carry sensory impulses from the outer parts of the body up to the brain. The brain responds by sending impulses down through other nerve fibers in the spinal cord to stimulate muscles that are under voluntary control.

Autonomic Nervous System

Among the motor fibers may be found groups that carry impulses to viscera. These fibers are designated by the special name of autonomic nervous system. That system consists of two divisions, more or less antagonistic in function, that emerge from the central nervous system at different points of origin. One division, the sympathetic, arises from the middle portion of the spinal cord, joins the sympathetic ganglionated chain, courses through the spinal nerves, and is widely distributed throughout the body. The other division, the parasympathetic, arises both above and below the sympathetic, that is, from the brain and from the lower part of the spinal cord. These two divisions control the functions of the respiratory, circulatory, digestive, and urogenital systems.

Nerves cell

Each nerve cell consists of a central portion containing the nucleus, known as the cell body, and one or more structures referred to as axons and dendrites. The dendrites are rather short extensions of the cell body and are involved in the reception of stimuli. The axon, by contrast, is usually a single elongated extension; it is especially important in the transmission of nerve impulses from the region of the cell body to other cells. See Neuro physiology.

Urinary System

2011-08-13T04:09:00.000-07:00

HOME

Urinary system

The urinary system eliminates waste products from the body and helps regulate the body’s water and chemical balance. The organs of the urinary system include two oval-shaped kidneys, each with a long, slender tube, called the ureter, that connects to the bladder. Another tubular vessel, the urethra, extends from the bladder to an opening out of the body.

Kidney

The kidneys are the major excretory organs of the body. As the body’s blood flows through the kidneys, microscopic tubules called nephrons remove urea, salts, and other poisonous waste products formed during metabolism. The kidneys produce urine to carry away these waste products.

Nephrons also selectively absorb proteins, salts, sugar, calcium, and other nutrients. The kidneys return these life-sustaining substances to the bloodstream to maintain a healthy blood composition. The kidneys also secrete hormones that regulate body functions such as blood pressure and the production of red blood cells.

Bladder

Urine is produced in the kidneys and travels through the ureters to the bladder, a muscular sac that stores the urine. When the bladder is full, nerves signal the bladder to contract and expel urine from the body through the urethra.

Endocrine system

2011-08-13T02:42:00.000-07:00

HOME

Endocrine system

The endocrine system is made of many glands—groups of cells that release chemical substances called hormones into the bloodstream. Hormones influence and regulate diverse activities such as metabolism, growth, mental development, and emotional behaviour.

Pituitary gland

The pituitary gland is a pea-sized gland located in the centre of the skull. The pituitary gland acts as a master controlling gland, releasing a number of hormones that activate other glands.

Parathyroid gland

The parathyroid glands are four small glands located in the neck behind the thyroid gland. These glands secrete a hormone that regulates the body’s use of calcium and phosphorus to maintain healthy bones. Parathyroid hormone also affects muscle contraction and the conduction of nerve impulses.

Thyroid gland

The thyroid gland, located in the neck, secretes the hormone thyroxine. Thyroxine increases body metabolism, in which food is broken down and converted into heat and energy. Too little thyroxine in the blood produces lethargy and fatigue, while too much thyroxine results in overactivity, nervousness, and weight loss.

Adrenal gland

The adrenals are two small glands, each located on the top of one kidney. These two glands consist of an inner core, called the adrenal medulla, and an outer area, known as the adrenal cortex. The adrenal medulla releases the hormone epinephrine, which speeds up heart rate and increases blood pressure to help the body cope with emergencies. The adrenal cortex releases hormones that control the level of salts and water in the blood and help regulate the use of sugar. It also secretes small amounts of male sex hormones, or androgens, in both males and females. Of the two parts of the adrenal glands, only the adrenal cortex is under the control of the pituitary gland.

Pancreas

The pancreas is a long, narrow gland located in the abdomen behind the stomach and beneath the liver. The pancreas secretes insulin, a hormone that regulates the body’s use of sugar. When too little insulin is produced, the body tissues cannot use or store sugar, and a disease known as diabetes mellitus develops.

Ovaries

Females have sex glands called ovaries that release hormones called estrogens. These hormones contribute to the development of female sexual characteristics, including skin, hair, and breast development. Estrogens work with certain pituitary hormones to control the menstrual cycle.

Testes

Males have sex glands called testes that secrete androgens, male sex hormones. In addition to contributing to male sexual characteristics, androgens contribute to the production of sperm and the development of the prostate gland.

CIRCULATORY SYSTEM

2011-08-12T10:36:00.000-07:00

HOME

Circulatory System

Cardio vascular system, in humans, the combined function of the heart, blood, and blood vessels to transport oxygen and nutrients to organs and tissues throughout the body and carry away waste products. Among its vital functions, the circulatory system increases the flow of blood to meet increased energy demands during exercise and regulates body temperature. In addition, when foreign substances or organisms invade the body, the circulatory system swiftly conveys disease-fighting elements of the immune system, such as white blood cells and antibodies, to regions under attack. Also, in the case of injury or bleeding, the circulatory system sends clotting cells and proteins to the affected site, which quickly stop bleeding and promote healing.

Blood consists of three types of cells: oxygen-bearing red blood cells, disease-fighting white blood cells, and blood-clotting platelets, all of which are carried through blood vessels in a liquid called plasma. Plasma is yellowish and consists of water, salts, proteins, vitamins, minerals, hormones, dissolved gases, and fats.

Three types of blood vessels form a complex network of tubes throughout the body. Arteries carry blood away from the heart, and veins carry it toward the heart. Capillaries are the tiny links between the arteries and the veins where oxygen and nutrients diffuse to body tissues. The inner layer of blood vessels is lined with endothelial cells that create a smooth passage for the transit of blood. This inner layer is surrounded by connective tissue and smooth muscle that enable the blood vessel to expand or contract. Blood vessels expand during exercise to meet the increased demand for blood and to cool the body. Blood vessels contract after an injury to reduce bleeding and also to conserve body heat.

Heart

The human heart is a hollow, pear-shaped organ about the size of a fist. The heart is made of muscle that rhythmically contracts, or beats, pumping blood throughout the body. Oxygen-poor blood from the body enters the heart from two large blood vessels, the inferior vena cava and the superior vena cava, and collects in the right atrium. When the atrium fills, it contracts, and the blood passes through the tricuspid valve into the right ventricle. When the ventricle becomes full, it starts to contract, and the tricuspid valve closes to prevent blood from moving back into the atrium. As the right ventricle contracts, it forces blood into the pulmonary artery, which carries blood to the lungs to pick up fresh oxygen. When blood exits the right ventricle, the ventricle relaxes and the pulmonary valve shuts, preventing blood from passing back into the ventricle.

Heart Valve

Respiratory System

2011-08-11T09:59:00.000-07:00

HOME

RESPIRATORY SYSTEM

The respiratory system consists of the lungs, a pair of elastic organs housed in the chest cavity, and the air passages leading to them. The air inhaled into the lungs provides oxygen to cells throughout the body. Air forced out of the lungs removes carbon dioxide from the body.

Air enters the respiratory system through the nose or mouth. It then travels through the larynx (voice box) and into the trachea (windpipe). At about the middle of the chest, the trachea divides into two tubes, the right and left bronchi. The right bronchus carries air to the three lobes of the right lung. The left bronchus supplies air to the two lobes of the left lung.

LUNGS

Inside the lungs the bronchi divide into smaller branches called bronchioles, which eventually empty into thousands of minute sacs called alveoli. The alveoli are surrounded by thin- walled capillaries. The air in the alveoli passes through to the blood cells within the capillaries. At the same time, carbon dioxide from these blood cells passes into the alveoli. From the alveoli, air containing carbon dioxide travels out of the lungs and is exhaled through the nose or mouth.

Human Digestive System

2011-01-28T08:42:00.000-08:00

HOME

The Digestive System

The Human Digestive System consist of series of organs and structures that help break down the food and absorb nutrients fro through outthe body. Food enters th digestive system through mouth and passes to phyrnx to esophagus and then stomach where it half digested after it passes to Small Intestine here digetion completed and moves to large intestine and finaly expelled out through out he body. Other organs such as the liver, further aid in the break downof food, absortion of nutritions, and elimination of undigestible material from the body.

Mouth

The mouth plays a role in digestion, speech, and breathing. Digestion begins when food enters the mouth. Teeth break down food and the muscular tongue pushes food back toward the pharynx, or throat. Three salivary glands—the sublingual gland, the submandibular gland, and the parotid gland—secrete enzymes that partially digest food into a soft, moist, round lump. Muscles in the pharynx swallow the food, pushing it into the esophagus, a muscular tube that passes food into the stomach. The epiglottis prevents food from entering the trachea, or windpipe, during swallowing.

Pharynx

It is muscular tube located in the neck, lined with mucous membrane, that connects the nose and mouth with the trachea (windpipe) and esophagus and serves as a passageway for both air and food. About 13 cm (5 in) long in humans, it lies in the front of the spinal column. The pharynx contains the tonsils and, in children, the adenoids. Because it begins in the back of the nasal cavity, the upper part of the pharynx is called the nasopharynx. The lower part, or oropharynx, refers to the area in the back of the mouth. The pharynx ends at the epiglottis, a flap of cartilage that prevents food from entering the trachea but allows it to enter the esophagus.

Esophagus

The presence of food in the pharynx stimulates swallowing, which squeezes the food into the esophagus. The esophagus, a muscular tube about 25 cm (10 in) long, passes behind the trachea and heart and penetrates the diaphragm (muscular wall between the chest and abdomen) before reaching the stomach. Food advances through the alimentary canal by means of rhythmic muscle contractions (tightenings) known as peristalsis. The process begins when circular muscles in the esophagus wall contract and relax (widen) one after the other, squeezing food downward toward the stomach. Food travels the length of the esophagus in two to three seconds.

Stomach

Located on the left side of the body just below the diaphragm, the stomach is a muscular, elastic sac that connects the esophagus with the small intestine. Food enters the stomach through the esophagus. A muscle at the junction of the esophagus and the stomach, called the esophageal sphincter, prevents food from flowing back into the esophagus to cause heartburn. In the stomach three layers of stomach muscles (longitudinal, circular, and oblique) knead, toss, and churn the food, mixing it thoroughly with digestive juices secreted by the stomach lining. This process transforms the partially solid food mass into a semiliquid pulp that passes into the duodenum, the first part of the small intestine. Food is prevented from moving back into the stomach by a muscle known as the pyloric sphincter..

Small intestine

The small intestine is a coiled tube 6-m (20-ft) long that is located in the center of the abdominal cavity. It is the site where the majority of digestion occurs. Here secretions from the intestinal wall, bile duct, and pancreatic duct further break down food passed from the stomach so that nutrients can be removed and passed into the bloodstream. The lining of the intestinal wall is studded with millions of finger-like projections called villi that come into contact with liquefied food. Each villus is lined with absorptive cells that absorb nutrients from digested food. Beneath these cells lie a lymph vessel and a network of capillaries (tiny blood vessels) that carry nutrients from the small intestine to the cells of the body. Fatty nutrients pass into the lymph vessel, while sugars, amino acids, and other nutrients pass into the bloodstream and travel to the liver. Longitudinal and circular muscles encircle the intestinal tube and regularly contract to move food through the small intestine into the large intestine.

Large intestine

Liver

The liver is the largest internal organ in the human body, located at the top of the abdomen on the right side of the body. A dark red organ with a spongy texture, the liver is divided into right and left lobes by the falciform ligament. The liver performs more than 500 functions, including the production of a digestive liquid called bile that plays a role in the breakdown of fats in food. Bile from the liver passes through the hepatic duct into the gallbladder, where it is stored. During digestion bile passes from the gallbladder through bile ducts to the small intestine, where it breaks down fatty food so that it can be absorbed into the body. Nutrient-rich blood passes from the small intestine to the liver, where nutrients are further processed and stored. Deoxygenated blood leaves the liver via the hepatic vein to return to the heart.

Zoology

2010-12-17T09:53:00.000-08:00

HOME

Zoology

It is the branch of biology devoted to the study of the animal kingdom (Animalia). This article discusses the history and concerns of that study. For a discussion of animals and a description of animal groups.

History of zoology

The study of zoology can be viewed as a series of efforts to analyze and classify animals. Attempts at classification as early as 400 bc are known from documents in the Hippocratic Collection. Aristotle, however, was the first to devise a system of classifying animals that recognized a basic unity of plan among diverse organisms; he arranged groups of animals according to mode of reproduction and habitat. Observing the development of such animals as the dogfish, chick, and octopus, he noted that general structures appear before specialized ones, and he also distinguished between asexual and sexual reproduction. His Historia Animalium contains accurate descriptions of extant animals of Greece and Asia Minor. He was also interested in form and structure and concluded that different animals can have similar embryological origins and that different structures can have similar functions.

In Roman times Pliny the Elder compiled four volumes on zoology in his 37-volume treatise called Historia Naturalis. Although widely read during the Middle Ages, they are little more than a collection of folklore, myth, and superstition. One of the more influential figures in the history of physiology, the Greek physician Galen, dissected farm animals, monkeys, and other mammals and described many features accurately, although some were wrongly applied to the human body. His misconceptions, especially with regard to the movement of blood, remained virtually unchanged for hundreds of years. In the 17th century, the English physician William Harvey established the true mechanism of blood circulation.

Until the Middle Ages, zoology was a conglomeration of folklore, superstition, misconception, and descriptions of animals, but during the 12th century it began to emerge as a science. Perhaps the most important naturalist of the era was the German scholar St. Albertus Magnus, who denied many of the superstitions associated with biology and reintroduced the work of Aristotle. The anatomical studies of Leonardo da Vinci were far in advance of the age. His dissections and comparisons of the structure of humans and other animals led him to important conclusions. He noted, for example, that the arrangement of joints and bones in the leg are similar in both horses and humans, thus grasping the concept of homology (the similarity of corresponding parts in different kinds of animals, suggesting a common grouping). The value of his work in anatomy was not recognized in his time. Instead, the Belgian physician Andreas Vesalius is considered the father of anatomy; he circulated his writings and established the principles of comparative anatomy.

Classification dominated zoology throughout most of the 17th and 18th centuries. The Swedish botanist Carolus Linnaeus developed a system of nomenclature that is still used today—the binomial system of genus and species (see Classification) —and established taxonomy as a discipline. He followed the work of the English naturalist John Ray in relying upon the form of teeth and toes to differentiate mammals and upon beak shape to classify birds. Another leading systematist of this era was the French biologist Comte Georges Leclerc de Buffon. The study of comparative anatomy was extended by such men as Georges Cuvier, who devised a systematic organization of animals based on specimens sent to him from all over the world.

Current study of zoology

In the 20th century zoology has become more diversified and less confined to such traditional concerns as classification and anatomy. Broadening its range to include such studies as genetics, ecology, and biochemistry, zoology has become an interdisciplinary field applying a great variety of techniques to obtain knowledge of the animal kingdom.

Taxonomically oriented studies concentrate on the different divisions of animal life. Invertebrate zoology deals with multicellular animals without backbones; its subdivisions include Entomology (the study of insects) and Malacology (the study of mollusks). Vertebrate zoology, the study of animals with backbones, is divided into ichthyology (fish), herpetology (amphibians and reptiles), ornithology (birds), and mammalogy (mammals).

Malacology

Entomology

Paleontology, the study of fossils, is subdivided by taxonomic groups. In each of these fields, researchers investigate the classification, distribution, life cycle, and evolutionary history of the particular animal or group of animals under study. Most zoologists are also specialists in one or more of the process-oriented disciplines.

Morphology, the study of structure, includes gross morphology, which examines entire structures or systems, such as muscles or bones; histology, which examines body tissues; and cytology, which focuses on cells and their components. Many great advances made in cytology in recent years are attributable to the electron microscope and the scanning electron microscope. Special staining techniques and radioactive tracers have been used to differentiate structural detail at the molecular level. Methods have been developed for mapping neural connections between parts of the brain and for stimulating and recording impulses from specific brain sites and even individual nerve cells.

Physiology, the study of function, is closely associated with morphology. An important subdivision is cellular physiology, which is closely related to molecular biology. Another active field, physiological ecology, studies the physical responses of animals to their environment. Much of this work has been carried out on desert, arctic, and ocean animals that must survive extremes of temperature or pressure.

Animal behavioral studies developed along two lines. The first of these, animal psychology, is primarily concerned with physiological psychology and has traditionally concentrated on laboratory techniques such as conditioning. The second, the most important recent development in the field is the concentration on socio-biology, which is concerned with the behaviour, ecology, and evolution of social animals such as bees, ants, schooling fish, flocking birds, and humans.Sociobiology is still in its infancy and is quite controversial, chiefly because it has raised anew the old dispute about whether behaviour is genetically determined.

Embryology, the study of the development of individual animals, has investigated the way in which developing parts interact—for example, the interactions between the eyestalk and the epidermis during development of the lens of the eye. The emerging field of molecular development applies the techniques of molecular biology, including molecular genetics.

The study of the interactions between animals and their environment is known as Ecology. Primary attention is given to the complex pattern of interactions among the many species constituting a community. Ecology has been central to the development of conservation and environmental control during the past 20 years. It has revealed the deleterious effects of pesticides and industrial pollutants and has provided important insights into wiser management of agriculture, forestry, and fisheries.

Evolutionary Zoology, which draws on all of the fields just mentioned, is concerned with the mechanisms of evolutionary change—speciation and adaptation—and with the evolutionary history of animal groups. Particularly relevant to evolutionary studies are systematics, phylogenetics, paleontology, and zoogeography. Systematics deals with the delineation and description of animal species and with their arrangement into a classification. Phylogenetics is the study of the developmental history of groups of animals. Zoogeography, the study of the distribution of animals over the earth, is closely related to ecology and systematics. See Animal Distribution; Paleontology; Species and Speciation.

BOTANY

2010-12-16T00:18:00.000-08:00

HOME

Botany

Botany, branch of biology concerned with the study of plants (kingdom Plantae; see Plant). Plants are now defined as multicellular organisms that carry out photosynthesis. Organisms that had previously been called plants, however, such as bacteria, algae, and fungi, continue to be the province of botany, because of their historical connection with the discipline and their many similarities to true plants, and because of the practicality of not fragmenting the study of organisms into too many separate fields.

HISTORICAL DEVELOPMENT

The Greeks believed that plants derived their nourishment from the soil only. Not until the 17th century did the Belgian scientist Jan Baptista van Helmont show that, although only water was added to a potted willow, it gained nearly 75 kg (165 lb), whereas the soil it stood in lost only about 60 g (about 2 oz) of weight over a period of five years. This demonstrated that the soil contributes very little to the increase in the weight of plants. In the 18th century the English chemist Joseph Priestley demonstrated that growing plants “restore” air from which the oxygen has been removed (by the burning of candles or the breathing of animals), and the Dutch physiologist Jan Ingenhousz (1730-99) extended this observation by showing that light is required for plants to restore air. These and other discoveries formed the basis for modern plant physiology, that branch of botany dealing with basic plant functions.

That water moves upward through the wood and that solutes move downward through the stems of plants was discovered independently in the 17th century by Marcello Malpighi in Italy and Nehemiah Grew in England. These facts have now been known for some 300 years, but only in the last few years have acceptable theories explaining the movements of liquids in plants been developed, using a variety of refined analytical techniques.

CLASSICAL STUDIES

Gross observations and experiments on photosynthesis and the movement of water in plants can be made without knowledge of their structure, but explanations of these phenomena require knowledge of morphology—the study and interpretation of plant form, development, and life histories—and of anatomy—the study of plant tissues and their origin and relations to one another. The cellular nature of plants was first pointed out by the English scientist Robert Hooke in the 17th century, when he observed that cork bark consists of cells. In 1838 the German botanist Matthias Schleiden proposed that all plant tissues consist of cells; this implied a basic sameness of living things and laid the foundation for the development of cytology, the study of the structure and function of cells as individual units rather than as aggegrate tissue. The German pathologist Rudolf Virchow showed in 1858 that cells are derived from preexisting cells, and thus that a continuity exists between past and present living things.

BOTANY TODAY

Botany does not depend on the fossil record for information concerning evolution and classification as much as does zoology, because the record for plants is much less complete than that for animals. Nevertheless, paleobotany, the study of fossil plants, has contributed greatly to the overall understanding of the evolution of the major groups of plants and especially to understanding of the interrelationships among the classes of seed plants. But much remains to be learned before fundamental questions such as the origin of the flowering plants (see Angiosperm) can be answered.

Botanists—those engaged in the study of plants—occupy themselves with a broad range of activities. Many botanists are in academic positions that involve both teaching and research duties. The latter may involve laboratory work or field studies. Strictly speaking, botany is a pure science concerned with investigating the basic nature of plants. Many aspects of botany, however, have direct importance to human welfare and advancement, and applied botany is an important field. Such fields as forestry and horticulture are closely tied to basic botanical studies, whereas those such as pharmacology and agronomy are not as closely related but still depend on basic botanical knowledge.

Scientific Method

2010-12-11T04:14:00.000-08:00

HOME

Scientific Method

Whatever the aim of their work, scientists use the same underlying steps to organize their research: (1) they make detailed observations about objects or processes, either as they occur in nature or as they take place during experiments; (2) they collect and analyze the information observed; and (3) they formulate a hypothesis that explains the behavior of the phenomena observed.

Observation and Experiments

A scientist begins an investigation by observing an object or an activity. Observation typically involves one or more of the human senses—hearing, sight, smell, taste, and touch. Scientists typically use tools to aid in their observations. Scientists typically apply their observation skills to an experiment. An experiment is any kind of trial that enables scientists to control and change at will the conditions under which events occur. It can be something extremely simple, such as heating a solid to see when it melts, or something highly complex, such as bouncing a radio signal off the surface of a distant planet. Scientists typically repeat experiments, sometimes many times, in order to be sure that the results were not affected by unforeseen factors.

Most experiments involve real objects in the physical world, such as electric circuits, chemical compounds, or living organisms. However, with the rapid progress in electronics, computer simulations can now carry out some experiments instead. If they are carefully constructed, these simulations or models can accurately predict how real objects will behave.

Data Collection and Analysis

During an experiment, scientists typically make measurements and collect results as they work. This information, known as data, can take many forms. Data may be a set of numbers, such as daily measurements of the temperature in a particular location or a description of side effects in an animal that has been given an experimental drug. Scientists typically use computers to arrange data in ways that make the information easier to understand and analyze. Data may be arranged into a diagram such as a graph that shows how one quantity (body temperature, for instance) varies in relation to another quantity (days since starting a drug treatment). A scientist flying in a helicopter may collect information about the location of a migrating herd of elephants in Africa during different seasons of a year. The data collected may be in the form of geographic coordinates that can be plotted on a map to provide the position of the elephant herd at any given time during a year.

Scientists use mathematics to analyze the data and help them interpret their results. The types of mathematics used include statistics, which is the analysis of numerical data, and probability, which calculates the likelihood that any particular event will occur.

Formulating the Hypothesis

Once an experiment has been carried out and data collected and analyzed, scientists look for whatever pattern their results produce and try to formulate a hypothesis that explains all the facts observed in an experiment. In developing a hypothesis, scientists employ methods of induction to generalize from the experiment’s results to predict future outcomes, and deduction to infer new facts from experimental results.

Formulating a hypothesis may be difficult for scientists because there may not be enough information provided by a single experiment, or the experiment’s conclusion may not fit old theories. Sometimes scientists do not have any prior idea of a hypothesis before they start their investigations, but often scientists start out with a working hypothesis that will be proved or disproved by the results of the experiment. Scientific hypotheses can be useful, just as hunches and intuition can be useful in everyday life. But they can also be problematic because they tempt scientists, either deliberately or unconsciously, to favor data that support their ideas. Scientists generally take great care to avoid bias, but it remains an ever-present threat. Throughout the history of science, numerous researchers have fallen into this trap, either in the hope of self-advancement or because they firmly believe their ideas to be true.

CELL STRUCTURE

2010-12-10T03:56:00.000-08:00

HOME

Cell Membrane

Most of the lipids in the plasma membrane are of a specific type known as phospholipids. A phospholipid molecule has a head region at one end that is hydrophilic—it can mix with water. At the other end are two long tails that are hydrophobic—they do not mix well with water. In the plasma membrane’s bilayer construction, phospholipid molecules are arranged so that their hydrophilic heads point outward on either side of the membrane, and their hydrophobic tails point toward each other in the middle of the membrane. This orientation keeps the hydrophobic tails away from the watery fluids that both fill and surround living cells. In fact, the plasma membrane stays intact precisely because the phospholipid molecules strongly resist any change in configuration that would expose their hydrophobic tails to the watery environment.

While the phospholipids are held in a bilayer, scientists believe the plasma membrane as a whole is a fluid structure because phospholipid molecules and some proteins can move sideways within the membrane. In one second, a single phospholipid molecule can travel the length of a large bacterial cell. Proteins drift more slowly through the membrane. With protein molecules scattered among the phospholipid molecules, the plasma membrane appears to be a mosaic of phospholipids and proteins. Some of the proteins are found on the inner or outer surface of the plasma membrane, while others span the membrane and protrude on either end. Scientists refer to this concept of the plasma membrane’s structure as the fluid mosaic model.

Function

The plasma membrane forms an extremely effective seal around the cell. Only a very few molecules can pass directly through the lipid bilayer to get from one side of the membrane to the other. Many substances that a cell needs in order to survive cannot cross the lipid bilayer on their own, including glucose (a sugar that cells burn for energy), amino acids (the building blocks of proteins), and ions, such as sodium and potassium. A cell uses two methods to move such substances from one side of the plasma membrane to another, known as passive transport and active transport. Both of these processes involve proteins in the plasma membrane.

Cytoplasm

The most extensive organelle in the cell is the cytoskeleton, a web of protein filaments that branches extensively throughout the cytoplasm and gives the cell its shape. The cytoskeleton proteins, as well as other proteins in the cell, are made by tiny spherical organelles known as ribosomes. Several other important organelles are found in the cells. Among them are the lysosomes, membranous sacs storing enzymes that digest and recycle worn out cell parts; and the mitochondria, sacs where the cell's energy is generated. The endoplasmic reticulum, another organelle, is an extensive network of membrane folds and tubes that serves in part as the cell's factory floor where large molecules, such as lipids, are manufactured. These large molecules are sent to another organelle, the Golgi apparatus, which consists of layers of membranes where the molecules are modified, sorted, and packaged for transport.

Nucleus