Mythology, GODs, Goddesses, Muses, Monsters - Infoplease, A Particular Culture,

Mythology, the body of myths of a particular culture, and the study and interpretation of such myths. A

myth may be broadly defined as a narrative that through many retellings has become an accepted tradition in

a society. By this definition, the term mythology might include all traditional tales, from the creation stories of ancient Egypt to the sagas of Icelandic literature to the American folktale of Paul Bunyan.

Myths are universal, occurring in almost all cultures. They typically date from a time before the introduction of writing, when they were passed orally from one generation to the next. Myths deal with basic questions about the nature of the world and human experience, and because of their all-encompassing nature, myths can illuminate many aspects of a culture.

II  WHAT ARE MYTHS?

Although it is difficult to draw rigid distinctions among various types of traditional tales, people who study mythology find it useful to categorize them. The three most common types of tales are sagas, legends, and folktales.

When a tale is based on a great historical (or supposedly historical) event, it is generally known as a saga. Despite a saga’s basis in very distant historical events, its dramatic structure and characters are the product of storytellers’ imaginations. Famous sagas include the Greek story of the Trojan War and the Germanic epic poem the Nibelungenlied (Song of the Nibelungs).

A legend is a fictional story associated with a historical person or place. For example, many early saints of the Christian church are historical figures whose lives have been embellished with legend (see Saint Denis; Saint George). Legends often provide examples of the virtues of honored figures in the history of a group or nation. The traditional American story about young George Washington and the cherry tree—in which he could not lie about chopping it down—is best described as a legend, because George Washington is a historical figure but the story about the cherry tree is recognized today as fictional.

Folktales, a third variety of traditional tale, are usually simple narratives of adventure built around elements of character and plot—for example, the young man who slays a monster and wins the hand of a princess. The Greek tale of Perseus is a good example of this theme. He saves the Ethiopian princess Andromeda from a sea monster and then marries her. Folktales may contain a moral or observation about life, but their chief purpose is entertainment.

Myths may include features of sagas, legends, and folktales. What makes one of these tales a myth is its serious purpose and its importance to the culture. Experts usually define a myth as a story that has compelling drama and deals with basic elements and assumptions of a culture. Myths explain, for example, how the world began; how humans and animals came into being; how certain customs, gestures, or forms of human activity originated; and how the divine and human worlds interact. Many myths take place at a time before the world as human beings know it came into being. Because myth-making often involves gods, other supernatural beings, and processes beyond human understanding, some scholars have viewed it as a dimension of religion. However, many myths address topics that are not typically considered religious—for example, why features of the landscape take a certain shape.

III  COMMON TYPES OF MYTHS,

No system of classification encompasses every type of myth, but in discussing myths it is helpful to group them into broad categories. This article concentrates on three major categories: cosmic myths, myths of gods, and hero myths.

A  Cosmic Myths,

Cosmic myths are concerned with the world and how it is ordered. They seek to explain the origin of the world, universal catastrophes such as fire or flood, and the afterlife. Nearly all mythologies have stories about creation, a type of story technically known as cosmogony, meaning “birth of the world.” Creation stories also include accounts of how human beings first came into existence and how death and suffering entered human experience.

The oldest cosmogonies known today are those of Egypt and the ancient Near East. An example is the creation epic of the Babylonians, Enuma elish (When on high), which dates back to at least the 12th century bc. According to Enuma elish, in the beginning of the world there was only a watery void in which fresh waters mingled with salt waters of the sea. The fresh waters were personified as Apsu, a male being, and the salt waters as Tiamat, a female. The myth describes a conflict between these earliest gods and a younger generation that sprang from them. Ultimately the younger gods won the war, led by Marduk, a god of thunder and lightning who resembles the Greek god Zeus and the Norse god Thor. Marduk defeated the army of the elder gods and killed Tiamat—represented as a dragon—in single combat. He then split her carcass in two, forming heaven and earth from the halves, and established the sun, moon, and constellations.

Enuma elish contains several themes common to many ancient Near Eastern creation stories: the ordering of the world out of chaos, the central role of waters in the creation of the world, the victory of a divine king over enemies who represent chaos, and the creation of matter from the corpse of a world-mother. A very different type of creation story appears in the Spider Woman myth of the Native American Hopi people. According to this narrative, in the beginning the only two beings in existence were Tawa, the sun god, and Spider Woman, an earth goddess who lived in a shadowy, cavelike underworld. Human beings were created from clay by Spider Woman and animated by the gaze of Tawa. Tawa used his light and heat to create dry land, and the world took shape. Spider Woman led the humans and other creatures up to the earth’s surface, and each species was assigned its proper residence and role in the world. This myth features the common Native American theme of emergence, in which creatures emerge from the earth as if from a mother’s womb.

Other types of creation myth occur in the cosmogony of the Maya people, with its many cycles of creation and destruction, and in the ancient Hebrew account of creation by a single, all-powerful deity.

B  Myths of the GODs,

Many myths do not directly concern human beings, but focus rather on the activities of the gods in their own realm. In many mythologies the gods form a divine family, or pantheon (from the Greek pan, meaning “all,” and theos, “god”). The story of a power struggle within a pantheon is common to a large number of world mythologies—for example, the Babylonian Enuma elish centers on Marduk’s struggle for supremacy and his eventual victory over Tiamat. Greek mythology features a similar story of struggle between generations. In Greek mythology, the earliest gods were Gaea (Earth) and Uranus (Heaven), and their children were called the Titans. The eldest of the Titans, Cronus, overthrew his father and was eventually overthrown by his own son, Zeus, who became the new master of the universe. Similarly, the Aesir–the pantheon of the Norse gods—had to overcome an older group called the Vanir before gaining power. Unlike the Greek and Babylonian accounts, the Norse myth features a reconciliation between the two sides.

 In Greek mythology, Hermes (best known as the messenger of the gods) was a famous trickster.

Across cultures, mythologies tend to describe similar characters. A common character is the trickster. The trickster is recklessly bold and even immoral, but through his inventiveness he often helps human beings. In Greek mythology, Hermes (best known as the messenger of the gods) was a famous trickster. In one version of a characteristic tale, Hermes, while still an infant, stole the cattle of his half-brother Apollo. To avoid leaving a trail that could be followed, Hermes made shoes from the bark of a tree and used grass to tie them to the cattle’s hooves. When Apollo nonetheless discovered that Hermes had stolen his cattle, he was furious. In the end, Apollo was so enchanted with the music of a lyre that Hermes had made that he allowed Hermes to keep the cattle in exchange for the lyre. Other tricksters of mythology are the West African god Eshu, who tricked the supreme god Olodumare into abandoning the earth to dwell in heaven; the Indian god Krishna, whose trickery often aims at a higher moral purpose; and the Native American Coyote, who scattered the once-orderly stars in the sky and strewed the plants on earth.

Myths about the gods are as numerous as the cultures that produce them. Other types that occur across various cultures include myths about the Great Mother (for example, the Mesopotamian Ishtar, who journeys to the underworld to rescue her lost lover Tammuz); the Dying God (for example, the Egyptian Osiris, who is murdered and dismembered but ultimately resurrected); and the Savior God (for example, the Greek Prometheus, who helps humanity at the cost of incurring Zeus’s anger).

C Myths of Heroes,

Nearly all cultures have produced myths about heroes. Some heroes, such as the Greek Achilles, have one mortal and one divine parent. Others are fully human but are blessed with godlike strength or beauty. Many myths about heroes concern significant phases of the hero’s career, such as the circumstances of the hero’s birth, a journey or quest, and the return home.

The birth and infancy of a mythological hero is often exceptional or even miraculous. In the ancient Near Eastern and Mediterranean world, the births of many heroes followed similar patterns. For example, the Hebrew prophet Moses, the Greek hero Oedipus, and the Roman heroes Romulus and Remus were all exposed to the elements at birth and left to die, but miraculously survived. Other heroes were immediately able to care for themselves. In early infancy, the Greek hero Hercules strangled a pair of enormous serpents sent to kill him. The Irish Cú Chulainn, who later became a great warrior, also performed astonishing feats of strength as a child.

Most heroes set off on a quest or a journey of some kind. One of the earliest tales of a hero’s journey is the Babylonian story known as the Gilgamesh epic, written in cuneiform on 12 clay tablets in about 2000 bc. The hero, Gilgamesh, embarks on a quest for immortality. A goddess named Siduri guides him, and in the course of his adventures he must do combat with monsters and visit the world of the dead. At the end of the quest, Gilgamesh must accept mortality, which the gods allotted to human beings when they created them. In Greek and Roman mythology the stories of Jason (who sailed in quest of the Golden Fleece) and of Aeneas (who traveled from Troy to Italy to found Rome) likewise describe journeys or quests. Other narratives that may be interpreted as heroic journeys include the biblical story of the Hebrew prophet Moses, who led his people on a 40-year journey through the wilderness, and the Celtic tale of King Arthur and the quest for the Holy Grail (see Arthurian Legend).

The most famous tale of a hero’s return home is probably the ancient Greek story of Odysseus, recounted in the Odyssey by the poet Homer. When the story opens, Odysseus has been away for nearly 20 years, fighting in the Trojan War and then kept captive by the sea nymph Calypso. Back in his kingdom of Ithaca, suitors who want to marry his wife Penelope are devouring and wasting his property and plotting against his son. Zeus persuades Calypso to let Odysseus leave and return home, but the god Poseidon is angry with Odysseus and is determined to kill him. In the course of his journey, Odysseus is shipwrecked, held captive by Calypso, and nearly devoured by monsters; all his companions are killed. When he finally returns to Ithaca, penniless and without allies, he must plot the destruction of the suitors and persuade Penelope that he really is who he claims to be. Of course, he succeeds brilliantly.

Native Americans of North America (History),

Native Americans of North America, indigenous peoples of North America. Native Americans had lived
throughout the continent for thousands of years before Europeans began exploring the “New World” in the 15th century.

Most scientists agree that the human history of North America began when the ancient ancestors of modern Native Americans made their way across a land bridge that once spanned the Bering Sea and connected northeastern Asia to North America. Scientists believe these people first migrated to the Americas more than 10,000 years ago, before the end of the last ice age (see Migration to the Americas). However, some Native Americans believe their ancestors originated in the Americas, citing gaps in the archaeological record and oral accounts of their origins that have been passed down through generations.

Native Americans excelled at using natural resources and adapting to the climates and terrains in which they lived. Over thousands of years distinct culture areas developed across North America. In the Northeast, for example, Native Americans used wood from the forests to build houses, canoes, and tools. Dense populations in the Pacific Northwest exploited the abundance of sea mammals and fish along the Pacific Coast. In the deserts of the Southwest, Native Americans grew corn and built multilevel, apartment-style dwellings from adobe, a sun-dried brick. In the Arctic, inhabitants adapted remarkably well to the harsh environment, becoming accomplished fishers and hunters.

Among the several hundred Native American groups that settled across North America, there existed, and still exists, many different ways of life and world views. Each group had distinctive social and political systems, clothing styles, shelters, foods, art forms, musical styles, languages, educational practices, and spiritual and philosophical beliefs. Nevertheless, Native American cultures share certain traits that are common to many indigenous peoples around the world, including strong ties to the land on which they live.

When European explorers and settlers began to arrive in the Americas in the 15th century, Native Americans found themselves faced with a new set of challenges. Some Native Americans learned to coexist with Europeans, setting up trade networks and adopting European technologies. Many more faced generations of upheaval and disruption as Europeans, and later Americans and Canadians, took Native American lands and tried to destroy their ways of life. During the 20th century, however, Native American populations and cultures experienced a resurgence. Today, Native Americans are working to reassert more control over their governments, economies, and cultures.

The indigenous peoples of North America are known by many terms. Most tribal peoples prefer to be identified by their tribal affiliation, such as Hopi, Onondaga, Mohawk, or Cherokee. The most common collective terms are Native American or American Indian. For many years, Indian was the most prevalent term. When Christopher Columbus and other European explorers arrived in the Americas, they thought they were in Asia, which the Spanish referred to as “the Indies.” They called the native peoples indios, as in the people of the Indies, later translated to Indian. However, some scholars believe the Europeans were not calling native peoples indios, but rather In Dios, meaning “Of God.”

The term Native American became popular in the United States in the 1960s, although some people believe it is too broad because it can refer to anyone born in the Americas, including Hawaiians and descendants of immigrants. In Canada, aboriginal people is a commonly used collective term. It refers to Indians, Métis (people of mixed indigenous and European ancestry), and Inuit. In the 1970s many Indians in Canada began calling their bands First Nations. When referring to the original inhabitants of the United States, this article uses Native Americans, American Indians, Indians, and native peoples interchangeably. When referring to the original inhabitants of Canada, the article generally uses aboriginal peoples, indigenous peoples, and native peoples.

This article divides its discussion of Native Americans into four main parts. The Culture Areas section examines Native American ways of life in ten different geographic regions. Traditional Way of Life looks at specific aspects of Native American life, such as food, clothing, and music. The History section describes the history of Native Americans in North America from the earliest times to the present day. Native Americans Today discusses contemporary life for indigenous peoples in the United States and Canada.

For a discussion of the indigenous peoples of Middle and South America, see Native Americans of Middle and South America. Other major articles on Native Americans in North America include Indian Treaties in Canada, Migration to the Americas, Native American Architecture, Native American Art, Native American Languages, Native American Literature, Native American Policy, and Native American Religions.

Rocket, Self Propelled Device As a FireWork,

Rocket, self-propelled device that carries its own fuel, as well as the oxygen, or other chemical agent,

A rocket can be as simple and small as a firework, which has a small amount of thrust, or as complex and powerful as the Saturn V rocket, which took humans to the Moon. British Congreve war rockets, which were used in the War of 1812, are referred to in a line of the United States national anthem: “And the rockets red glare…” Rockets have many applications both on Earth and in space. The most common and well-known use of rockets is for missiles—weapons that deliver explosive warheads through the air to specified targets (see Guided Missile). Rockets also have numerous peaceful purposes. Upper atmospheric research rockets, or sounding rockets, carry scientific instruments to high altitudes, helping scientists carry out astronomical research and learn more about the nature of the atmosphere. Jet-Assisted-Take-Off (JATO) rockets help lift heavily loaded planes from runways. Lifesaving rockets carry lifeline ropes to ships stranded offshore. Ships in distress can launch signal rockets to signal for help. Rocket ejection seats safely boost pilots out of jet planes during emergencies. Fireworks have provided entertainment for centuries, and model rockets form the basis of a popular hobby.

II  ROCKET USES,

People use all kinds of rockets for the same basic purpose: to carry objects through air and space. Missiles carry explosive devices to targets, while sounding rockets carry scientific instruments into the upper atmosphere. Launch vehicles boost satellites and other spacecraft into space, and smaller thruster rockets steer or stabilize spacecraft in space.

A  Missiles,

The term missile actually means any object thrown at an enemy and includes arrows, bullets, and other weapons. In modern military usage, however, missile usually means an explosive device propelled through the air by a rocket or an air-breathing engine. (Air-breathing engines differ from rockets in that rockets carry their own oxygen, while air-breathing engines get their oxygen from the air as they fly through it.)

Missiles can be launched from the ground, from airplanes, and even from submarines. Some missiles are designed to hit targets in the air, while others are built to hit targets on the ground. Some missiles, called guided missiles, have steering systems that guide them to their target.

B  Sounding Rockets,

Scientists use sounding rockets to carry scientific instruments into the upper atmosphere to take measurements of air quality, radiation from space, and other data. Many countries use sounding rockets to monitor weather and pollution. Engineers enable a rocket to reach its target altitude by shutting down the rocket at a specific height. The rocket then coasts upward until air friction and gravity stop its upward movement and cause it to fall back to Earth. The instruments usually include a radio transmitter that sends measurements back to Earth. Some sounding rockets carry parachutes that allow their controllers to recover the rocket and the instruments, but some fall back to Earth without a parachute. Engineers design a sounding rocket’s flight path so that the rocket will fall into the ocean or into an uninhabited area in order to avoid damaging property or hurting people.

C  Launch Vehicles,

Launch vehicles send satellites and other spacecraft into space. These vehicles must be far more powerful than other types of rockets, because they carry more cargo farther and faster than other rockets. To place an object into orbit around Earth, the launch vehicle must reach a velocity of about 30,000 km/h (about 18,500 mph). To escape Earth’s gravitational pull entirely and head into deep space, these rockets must attain a velocity, called an escape velocity, of about 40,000 km/h (about 25,000 mph). Engineers have found that the most efficient way for launch vehicles to reach these speeds is to use staged rockets, or rockets divided into different stages, one atop another.

D  Thrusters,

Many spacecraft use small rockets called thrusters to move around in space. Thrusters can change the speed and direction of a spacecraft. They allow a spacecraft to steer in space, to jump to a higher orbit, or to fall back to Earth.

III  HOW ROCKETS WORK,

All rockets—whether small or large, simple or complex—work by the basic principle of action and reaction, which was formulated by English scientist Sir Isaac Newton in 1687. Newton’s third law of motion states, “For every action there is an equal and opposite reaction.” In the case of the rocket, the expulsion of exhaust gases from the rear is the action, and the forward movement of the rocket is the reaction.



needed to burn its fuel. Most rockets move by burning their fuel and expelling the hot exhaust gases that result. The force of these hot gases shooting out in one direction causes the rocket to move in the opposite direction. A rocket engine is the most powerful engine for its weight. Other forms of propulsion, such as jet-powered and propeller-driven engines, cannot match its power. Rockets can operate in space, because they carry their own oxygen for burning their fuel. Rockets are presently the only vehicles that can launch into and move around in space.

Representative House Of The United States,

House of Representatives, United States, larger of the two legislative chambers that make up the Congress of the United States. Along with the Senate of the United States, it drafts and passes laws that, if
signed by the president, govern the United States and its citizens. Usually called simply “the House,” it consists of 435 members chosen for two-year terms from districts of about equal population.

 CONSTITUTIONAL ORIGINS,

The bicameral (two-house) Congress emerged from a compromise between delegates from large and small states at the Constitutional Convention, which convened in Philadelphia in 1787 to revise the Articles of Confederation, the first constitution of the United States. All of the delegates at the convention agreed that the national government needed more power. The Articles of Confederation, which had governed the country since 1783, left the national government powerless to resolve trade disputes with other countries and to prevent ruinous economic competition between the states. The delegates worried, however, that giving too much authority to the national government would result in the kinds of abuses of power that had led the colonies to break away from Great Britain.

To prevent such problems, the framers of the Constitution gave most political power to the Congress, rather than to a single leader such as a king or president. The convention delegates disagreed over how to select members of Congress, however. The more populous states, such as Virginia and Pennsylvania, wanted power in the legislature that reflected their population and wealth. They favored a system that assigned congressional seats based on the number of residents in each state. Smaller states, such as New Jersey and Connecticut, feared that their interests would be ignored if they did not have equal representation in Congress.

The delegates compromised, deciding that seats in the House of Representatives would be distributed according to population and that seats in the Senate would be distributed equally among the states. The small states could rely on the Senate to defend their interests if the House passed legislation that threatened their rights. The framers of the Constitution specified that House members would be elected by popular vote (the vote of citizens) every two years, so members of the House would be constantly in touch with the citizens that they represent. “Here, sir, the people rule,” declared American statesman Alexander Hamilton in reference to the House. In contrast, the framers decided to shield the Senate from popular pressures by giving senators six-year terms, and by having state legislatures choose them. In 1913 popular pressure for reform led to adoption of the 17th Amendment to the Constitution, which required that senators be elected.

HOUSE MEMBERSHIP,
House members are usually called representatives; they are also referred to as congressmen or congresswomen, although technically these titles apply to both House and Senate members. Representatives must be at least 25 years old, U.S. citizens for at least seven years, and residents of the state from which they are elected. Only an amendment to the Constitution can change these three requirements. Many states enacted laws in the early 1990s to limit the number of terms their representatives (and senators) could serve, but in 1995 the Supreme Court of the United States ruled that these laws violated the Constitution.

House members are elected by voters in congressional districts. When the first Congress met in 1789, it had 65 members who represented districts of no more than 30,000 people. The House added members throughout the 19th century as the country’s population grew and new states were admitted. A 1911 law fixed the size at 435 members. Today each House member represents about 650,000 people.

About Cell Biology, The Basic Unit Of Life,

Introduction: 
Cell (biology), basic unit of life. Cells are the smallest structures capable of basic life processes, such as taking in nutrients, expelling waste, and reproducing. All living things are composed of cells. Some microscopic organisms, such as bacteria and protozoa, are unicellular, meaning they consist of a single cell. Plants, animals, and fungi are multicellular; that is, they are composed of a great many cells working in concert. But whether it makes up an entire bacterium or is just one of trillions in a human being, the cell is a marvel of design and efficiency. Cells carry out thousands of biochemical reactions each minute and reproduce new cells that perpetuate life.

 Cells: The word cell refers to several types of organisms. Cells such as paramecia, dinoflagellates, diatoms, and spirochetes are self-maintaining organisms; cells such as lymphocytes, erythrocytes, muscle cells, nerve cells, cardiac muscle, and chloroplasts are more specialized cells that are a part of higher multicellular organisms. Regardless of size or whether the cell is a complete organism or just part of an organism, all cells have certain structural components in common. All cells have some type of outer cell boundary that permits some materials to leave and enter the cell and a cell interior composed of a water-rich, fluid material called cytoplasm that contains hereditary material in the form of deoxyribonucleic acid (DNA).
Paramecium: The paramecium is a single-celled organism that propels itself by minute, hairlike projections called cilia. Cilia also create currents that sweep food particles toward the paramecium’s gullet for ingestion.
cell, a type of bacterium known as a mycoplasma, measures 0.0001 mm (0.000004 in) in diameter; 10,000 mycoplasmas in a row are only as wide as the diameter of a human hair. Among the largest cells are the nerve cells that run down a giraffe’s neck; these cells can exceed 3 m (9.7 ft) in length. Human cells also display a variety of sizes, from small red blood cells that measure 0.00076 mm (0.00003 in) to liver cells that may be ten times larger. About 10,000 average-sized human cells can fit on the head of a pin.
Along with their differences in size, cells present an array of shapes. Some, such as the bacterium Escherichia coli, resemble rods. The paramecium, a type of protozoan, is slipper shaped; and the amoeba, another protozoan, has an irregular form that changes shape as it moves around. Plant cells typically resemble boxes or cubes. In humans, the outermost layers of skin cells are flat, while muscle cells are long and thin. Some nerve cells, with their elongated, tentacle-like extensions, suggest an octopus.
In multicellular organisms, shape is typically tailored to the cell’s job. For example, flat skin cells pack tightly into a layer that protects the underlying tissues from invasion by bacteria. Long, thin muscle cells contract readily to move bones. The numerous extensions from a nerve cell enable it to connect to several other nerve cells in order to send and receive messages rapidly and efficiently.
By itself, each cell is a model of independence and self-containment. Like some miniature, walled city in perpetual rush hour, the cell constantly bustles with traffic, shuttling essential molecules from place to place to carry out the business of living. Despite their individuality, however, cells also display a remarkable ability to join, communicate, and coordinate with other cells. The human body, for example, consists of an estimated 20 to 30 trillion cells. Dozens of different kinds of cells are organized into specialized groups called tissues. Tendons and bones, for example, are composed of connective tissue, whereas skin and mucous membranes are built from epithelial tissue. Different tissue types are assembled into organs, which are structures specialized to perform particular functions. Examples of organs include the heart, stomach, and brain. Organs, in turn, are organized into systems such as the circulatory, digestive, or nervous systems. All together, these assembled organ systems form the human body.
The components of cells are molecules, nonliving structures formed by the union of atoms. Small molecules serve as building blocks for larger molecules. Proteins, nucleic acids, carbohydrates, and lipids, which include fats and oils, are the four major molecules that underlie cell structure and also participate in cell functions. For example, a tightly organized arrangement of lipids, proteins, and protein-sugar compounds forms the plasma membrane, or outer boundary, of certain cells. The organelles, membrane-bound compartments in cells, are built largely from proteins. Biochemical reactions in cells are guided by enzymes, specialized proteins that speed up chemical reactions. The nucleic acid deoxyribonucleic acid (DNA) contains the hereditary information for cells, and another nucleic acid, ribonucleic acid(RNA), works with DNA to build the thousands of proteins the cell needs.
II  CELL STRUCTURE:
Cells fall into one of two categories: prokaryotic or eukaryotic (see Prokaryote). In a prokaryotic cell, found only in bacteria and archaebacteria, all the components, including the DNA, mingle freely in the cell’s interior, a single compartment. Eukaryotic cells, which make up plants, animals, fungi, and all other life forms, contain numerous compartments, or organelles, within each cell. The DNA in eukaryotic cells is enclosed in a special organelle called the nucleus, which serves as the cell’s command center and information library. The term prokaryote comes from Greek words that mean “before nucleus” or “prenucleus,” while eukaryote means “true nucleus.”
A  Prokaryotic Cells:
Prokaryotic cells are among the tiniest of all cells, ranging in size from 0.0001 to 0.003 mm (0.000004 to 0.0001 in) in diameter. About a hundred typical prokaryotic cells lined up in a row would match the thickness of a book page. These cells, which can be rodlike, spherical, or spiral in shape, are surrounded by a protective cell wall. Like most cells, prokaryotic cells live in a watery environment, whether it is soil moisture, a pond, or the fluid surrounding cells in the human body. Tiny pores in the cell wall enable water and the substances dissolved in it, such as oxygen, to flow into the cell; these pores also allow wastes to flow out.
Pushed up against the inner surface of the prokaryotic cell wall is a thin membrane called the plasma membrane. The plasma membrane, composed of two layers of flexible lipid molecules and interspersed with durable proteins, is both supple and strong. Unlike the cell wall, whose open pores allow the unregulated traffic of materials in and out of the cell, the plasma membrane is selectively permeable, meaning it allows only certain substances to pass through. Thus, the plasma membrane actively separates the cell’s contents from its surrounding fluids.
While small molecules such as water, oxygen, and carbon dioxide diffuse freely across the plasma membrane, the passage of many larger molecules, including amino acids (the building blocks of proteins) and sugars, is carefully regulated. Specialized transport proteins accomplish this task. The transport proteins span the plasma membrane, forming an intricate system of pumps and channels through which traffic is conducted. Some substances swirling in the fluid around the cell can enter it only if they bind to and are escorted in by specific transport proteins. In this way, the cell fine-tunes its internal environment.
The plasma membrane encloses the cytoplasm, the semifluid that fills the cell. Composed of about 65 percent water, the cytoplasm is packed with up to a billion molecules per cell, a rich storehouse that includes enzymes and dissolved nutrients, such as sugars and amino acids. The water provides a favorable environment for the thousands of biochemical reactions that take place in the cell.
Within the cytoplasm of all prokaryotes is deoxyribonucleic acid (DNA), a complex molecule in the form of a double helix, a shape similar to a spiral staircase. The DNA is about 1,000 times the length of the cell, and to fit inside, it repeatedly twists and folds to form a compact structure called a chromosome. The chromosome in prokaryotes is circular, and is located in a region of the cell called the nucleoid. Often, smaller chromosomes called plasmids are located in the cytoplasm. The DNA is divided into units called genes, just like a long train is divided into separate cars. Depending on the species, the DNA contains several hundred or even thousands of genes. Typically, one gene contains coded instructions for building all or part of a single protein. Enzymes, which are specialized proteins, determine virtually all the biochemical reactions that support and sustain the cell.
Also immersed in the cytoplasm are the only organelles in prokaryotic cells—tiny bead-like structures called ribosomes. These are the cell’s protein factories. Following the instructions encoded in the DNA, ribosomes churn out proteins by the hundreds every minute, providing needed enzymes, the replacements for worn-out transport proteins, or other proteins required by the cell.
While relatively simple in construction, prokaryotic cells display extremely complex activity. They have a greater range of biochemical reactions than those found in their larger relatives, the eukaryotic cells. The extraordinary biochemical diversity of prokaryotic cells is manifested in the wide-ranging lifestyles of the archaebacteria and the bacteria, whose habitats include polar ice, deserts, and hydrothermal vents—deep regions of the ocean under great pressure where hot water geysers erupt from cracks in the ocean floor.

B  Eukaryotic Animal Cells:


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.
The eukaryotic cell cytoplasm is similar to that of the prokaryote cell except for one major difference: Eukaryotic cells house a nucleus and numerous other membrane-enclosed organelles. Like separate rooms of a house, these organelles enable specialized functions to be carried out efficiently. The building of proteins and lipids, for example, takes place in separate organelles where specialized enzymes geared for each job are located.

The nucleus is the largest organelle in an animal cell. It contains numerous strands of DNA, the length of each strand being many times the diameter of the cell. Unlike the circular prokaryotic DNA, long sections of eukaryotic DNA pack into the nucleus by wrapping around proteins. As a cell begins to divide, each DNA strand folds over onto itself several times, forming a rod-shaped chromosome.
The nucleus is surrounded by a double-layered membrane that protects the DNA from potentially damaging chemical reactions that occur in the cytoplasm. Messages pass between the cytoplasm and the nucleus through nuclear pores, which are holes in the membrane of the nucleus. In each nuclear pore, molecular signals flash back and forth as often as ten times per second. For example, a signal to activate a specific gene comes in to the nucleus and instructions for production of the necessary protein go out to the cytoplasm.


Attached to the nuclear membrane is an elongated membranous sac called the endoplasmic reticulum. This organelle tunnels through the cytoplasm, folding back and forth on itself to form a series of membranous stacks. Endoplasmic reticulum takes two forms: rough and smooth. Rough endoplasmic reticulum (RER) is so called because it appears bumpy under a microscope. The bumps are actually thousands of ribosomes attached to the membrane’s surface. The ribosomes in eukaryotic cells have the same function as those in prokaryotic cells—protein synthesis—but they differ slightly in structure. Eukaryote ribosomes bound to the endoplasmic reticulum help assemble proteins that typically are exported from the cell. The ribosomes work with other molecules to link amino acids to partially completed proteins. These incomplete proteins then travel to the inner chamber of the endoplasmic reticulum, where chemical modifications, such as the addition of a sugar, are carried out. Chemical modifications of lipids are also carried out in the endoplasmic reticulum.
The endoplasmic reticulum and its bound ribosomes are particularly dense in cells that produce many proteins for export, such as the white blood cells of the immune system, which produce and secrete antibodies. Some ribosomes that manufacture proteins are not attached to the endoplasmic reticulum. These so-called free ribosomes are dispersed in the cytoplasm and typically make proteins—many of them enzymes—that remain in the cell.

The second form of endoplasmic reticulum, the smooth endoplasmic reticulum (SER), lacks ribosomes and has an even surface. Within the winding channels of the smooth endoplasmic reticulum are the enzymes needed for the construction of molecules such as carbohydrates and lipids. The smooth endoplasmic reticulum is prominent in liver cells, where it also serves to detoxify substances such as alcohol, drugs, and other poisons.
Proteins are transported from free and bound ribosomes to the Golgi apparatus, an organelle that resembles a stack of deflated balloons. It is packed with enzymes that complete the processing of proteins. These enzymes add sulfur or phosphorus atoms to certain regions of the protein, for example, or chop off tiny pieces from the ends of the proteins. The completed protein then leaves the Golgi apparatus for its final destination inside or outside the cell. During its assembly on the ribosome, each protein has acquired a group of from 4 to 100 amino acids called a signal. The signal works as a molecular shipping label to direct the protein to its proper location.

Lysosomes are small, often spherical organelles that function as the cell’s recycling center and garbage disposal. Powerful digestive enzymes concentrated in the lysosome break down worn-out organelles and ship their building blocks to the cytoplasm where they are used to construct new organelles. Lysosomes also dismantle and recycle proteins, lipids, and other molecules.
The mitochondria are the powerhouses of the cell. Within these long, slender organelles, which can appear oval or bean shaped under the electron microscope, enzymes convert the sugar glucose and other nutrients into adenosine triphosphate (ATP). This molecule, in turn, serves as an energy battery for countless cellular processes, including the shuttling of substances across the plasma membrane, the building and transport of proteins and lipids, the recycling of molecules and organelles, and the dividing of cells. Muscle and liver cells are particularly active and require dozens and sometimes up to a hundred mitochondria per cell to meet their energy needs. Mitochondria are unusual in that they contain their own DNA in the form of a prokaryote-like circular chromosome; have their own ribosomes, which resemble prokaryotic ribosomes; and divide independently of the cell.
Unlike the tiny prokaryotic cell, the relatively large eukaryotic cell requires structural support. The cytoskeleton, a dynamic network of protein tubes, filaments, and fibers, crisscrosses the cytoplasm, anchoring the organelles in place and providing shape and structure to the cell. Many components of the cytoskeleton are assembled and disassembled by the cell as needed. During cell division, for example, a special structure called a spindle is built to move chromosomes around. After cell division, the spindle, no longer needed, is dismantled. Some components of the cytoskeleton serve as microscopic tracks along which proteins and other molecules travel like miniature trains. Recent research suggests that the cytoskeleton also may be a mechanical communication structure that converses with the nucleus to help organize events in the cell.

C-  Eukaryotic Plant Cells:
 Plant cells contain a variety of membrane-bound structures called organelles. These include a nucleus that carries genetic material; mitochondria that generate energy; ribosomes that manufacture proteins; smooth endoplasmic reticulum that manufactures lipids used for making membranes and storing energy; and a thin lipid membrane that surrounds the cell. Plant cells also contain chloroplasts that capture energy from sunlight and a single fluid-filled vacuole that stores compounds and helps in plant growth. Plant cells are plant growth. Plant cells are surrounded by a rigid cell wall that protects the cell and maintains its shape.


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 ribosomes, which manufacture the proteins that the chloroplasts typically need.
The central vacuole of a mature plant cell typically takes up most of the room in the cell. The vacuole, a membranous bag, crowds the cytoplasm and organelles to the edges of the cell. The central vacuole stores water, salts, sugars, proteins, and other nutrients. In addition, it stores the blue, red, and purple pigments that give certain flowers their colors. The central vacuole also contains plant wastes that taste bitter to certain insects, thus discouraging the insects from feasting on the plant.
In plant cells, a sturdy cell wall surrounds and protects the plasma membrane. Its pores enable materials to pass freely into and out of the cell. The strength of the wall also enables a cell to absorb water into the central vacuole and swell without bursting. The resulting pressure in the cells provides plants with rigidity and support for stems, leaves, and flowers. Without sufficient water pressure, the cells collapse and the plant wilts.
III  CELL FUNCTIONS
To stay alive, cells must be able to carry out a variety of functions. Some cells must be able to move, and most cells must be able to divide. All cells must maintain the right concentration of chemicals in their cytoplasm, ingest food and use it for energy, recycle molecules, expel wastes, and construct proteins. Cells must also be able to respond to changes in their environment.
A-  Movement:

Bacterium Showing Flagella Although many forms of bacteria are not capable of independent movement, species such as the Salmonella bacterium pictured here can move by means of fine threadlike projections called flagella. The arrangement of flagella across the surface of the bacterium differs from species to species; they can be present at the ends of the bacterium or all across the body surface. Forward movement is accomplished either by a tumbling motion or in a forward manner without tumbling.

Many unicellular organisms swim, glide, thrash, or crawl to search for food and escape enemies. Swimming organisms often move by means of a flagellum, a long tail-like structure made of protein. Many bacteria, for example, have one, two, or many flagella that rotate like propellers to drive the organism along. Some single-celled eukaryotic organisms, such as euglena, also have a flagellum, but it is longer and thicker than the prokaryotic flagellum. The eukaryotic flagellum works by waving up and down like a whip. In higher animals, the sperm cell uses a flagellum to swim toward the female egg for fertilization.
Movement in eukaryotes is also accomplished with cilia, short, hairlike proteins built by centrioles, which are barrel-shaped structures located in the cytoplasm that assemble and break down protein filaments. Typically, thousands of cilia extend through the plasma membrane and cover the surface of the cell, giving it a dense, hairy appearance. By beating its cilia as if they were oars, an organism such as the paramecium propels itself through its watery environment. In cells that do not move, cilia are used for other purposes. In the respiratory tract of humans, for example, millions of ciliated cells prevent inhaled dust, smog, and microorganisms from entering the lungs by sweeping them up on a current of mucus into the throat, where they are swallowed. Eukaryotic flagella and cilia are formed from basal bodies, small protein structures located just inside the plasma membrane. Basal bodies also help to anchor flagella and cilia.
Still other eukaryotic cells, such as amoebas and white blood cells, move by amoeboid motion, or crawling. They extrude their cytoplasm to form temporary pseudopodia, or false feet, which actually are placed in front of the cell, rather like extended arms. They then drag the trailing end of their cytoplasm up to the pseudopodia. A cell using amoeboid motion would lose a race to a euglena or paramecium. But while it is slow, amoeboid motion is strong enough to move cells against a current, enabling water-dwelling organisms to pursue and devour prey, for example, or white blood cells roaming the blood stream to stalk and engulf a bacterium or virus.
B-  Nutrition:
All cells require nutrients for energy, and they display a variety of methods for ingesting them. Simple nutrients dissolved in pond water, for example, can be carried through the plasma membrane of pond-dwelling organisms via a series of molecular pumps. In humans, the cavity of the small intestine contains the nutrients from digested food, and cells that form the walls of the intestine use similar pumps to pull amino acids and other nutrients from the cavity into the bloodstream. Certain unicellular organisms, such as amoebas, are also capable of reaching out and grabbing food. They use a process known as endocytosis, in which the plasma membrane surrounds and engulfs the food particle, enclosing it in a sac, called a vesicle, that is within the amoeba’s interior.
C  Energy:
 Cells require energy for a variety of functions, including moving, building up and breaking down molecules, and transporting substances across the plasma membrane. Nutrients contains energy, but cells must convert the energy locked in nutrients to another form—specifically, the ATP molecule, the cell’s energy battery—before it is useful. In single-celled eukaryotic organisms, such as the paramecium, and in multicellular eukaryotic organisms, such as plants, animals, and fungi, mitochondria are responsible for this task. The interior of each mitochondrion consists of an inner membrane that is folded into a mazelike arrangement of separate compartments called cristae. Within the cristae, enzymes form an assembly line where the energy in glucose and other energy-rich nutrients is harnessed to build ATP; thousands of ATP molecules are constructed each second in a typical cell. In most eukaryotic cells, this process requires oxygen and is known as aerobic respiration.
Some prokaryotic organisms also carry out aerobic respiration. They lack mitochondria, however, and carry out aerobic respiration in the cytoplasm with the help of enzymes sequestered there. Many prokaryote species live in environments where there is little or no oxygen, environments such as mud, stagnant ponds, or within the intestines of animals. Some of these organisms produce ATP without oxygen in a process known as anaerobic respiration, where sulfur or other substances take the place of oxygen. Still other prokaryotes, and yeast, a single-celled eukaryote, build ATP without oxygen in a process known as fermentation.
Almost all organisms rely on the sugar glucose to produce ATP. Glucose is made by the process of photosynthesis, in which light energy is transformed to the chemical energy of glucose. Animals and fungi cannot carry out photosynthesis and depend on plants and other photosynthetic organisms for this task. In plants, as we have seen, photosynthesis takes place in organelles called chloroplasts. Chloroplasts contain numerous internal compartments called thylakoids where enzymes aid in the energy conversion process. A single leaf cell contains 40 to 50 chloroplasts. With sufficient sunlight, one large tree is capable of producing upwards of two tons of sugar in a single day. Photosynthesis in prokaryotic organisms—typically aquatic bacteria—is carried out with enzymes clustered in plasma membrane folds called chromatophores. Aquatic bacteria produce the food consumed by tiny organisms living in ponds, rivers, lakes, and seas.

Mitochondria Mitochondria, minute sausage-shaped structures found in the clear cytoplasm of the cell, are responsible for energy production. Mitochondria contain enzymes that help convert food material into adenosine triphosphate (ATP), which can be used directly by the cell as an energy source. Mitochondria tend to be concentrated near cellular structures that require large inputs of energy, such as the flagellum, which is responsible for movement in sperm cells and single-celled plants and animals.

Amoeba Engulfing a Paramecium An amoeba, a single-celled organism lacking internal organs, is shown approaching a much smaller paramecium, which it begins to engulf with large outflowings of its cytoplasm, called pseudopodia. Once the paramecium is completely engulfed, a primitive digestive cavity, called a vacuole, forms around it. In the vacuole, acids break the paramecium down into chemicals that the amoeba can diffuse back into its cytoplasm for nourishment.


D - Protein Synthesis:A typical cell must have on hand about 30,000 proteins at any one time. Many of these proteins are enzymes needed to construct the major molecules used by cells—carbohydrates, lipids, proteins, and nucleic acids—or to aid in the breakdown of such molecules after they have worn out. Other proteins are part of the cell’s structure—the plasma membrane and ribosomes, for example. In animals, proteins also function as hormones and antibodies, and they function like delivery trucks to transport other molecules around the body. Hemoglobin, for example, is a protein that transports oxygen in red blood cells. The cell’s demand for proteins never ceases.
Before a protein can be made, however, the molecular directions to build it must be extracted from one or more genes. In humans, for example, one gene holds the information for the protein insulin, the hormone that cells need to import glucose from the bloodstream, while at least two genes hold the information for collagen, the protein that imparts strength to skin, tendons, and ligaments. The process of building proteins begins when enzymes, in response to a signal from the cell, bind to the gene that carries the code for the required protein, or part of the protein. The enzymes transfer the code to a new molecule called messenger RNA, which carries the code from the nucleus to the cytoplasm. This enables the original genetic code to remain safe in the nucleus, with messenger RNA delivering small bits and pieces of information from the DNA to the cytoplasm as needed. Depending on the cell type, hundreds or even thousands of molecules of messenger RNA are produced each minute.
Once in the cytoplasm, the messenger RNA molecule links up with a ribosome. The ribosome moves along the messenger RNA like a monorail car along a track, stimulating another form of RNA—transfer RNA—to gather and link the necessary amino acids, pooled in the cytoplasm, to form the specific protein, or section of protein. The protein is modified as necessary by the endoplasmic reticulum and Golgi apparatus before embarking on its mission. Cells teem with activity as they forge the numerous, diverse proteins that are indispensable for life. For a more detailed discussion about protein synthesis, see Genetics: The Genetic Code.

Ribosomes On the surface of the rough endoplasmic reticulum are numerous small, dark structures called ribosomes. Ribosomes, which are also found floating free in the cytoplasm, are the sites of protein synthesis.
E-  Cell Division:
 Most cells divide at some time during their life cycle, and some divide dozens of times before they die. Organisms rely on cell division for reproduction, growth, and repair and replacement of damaged or worn out cells. Three types of cell division occur: binary fission, mitosis, and meiosis. Binary fission, the method used by prokaryotes, produces two identical cells from one cell. The more complex process of mitosis, which also produces two genetically identical cells from a single cell, is used by many unicellular eukaryotic organisms for reproduction. Multicellular organisms use mitosis for growth, cell repair, and cell replacement. In the human body, for example, an estimated 25 million mitotic cell divisions occur every second in order to replace cells that have completed their normal life cycles. Cells of the liver, intestine, and skin may be replaced every few days. Recent research indicates that even brain cells, once thought to be incapable of mitosis, undergo cell division in the part of the brain associated with memory.

The type of cell division required for sexual reproduction is meiosis. Sexually reproducing organisms include seaweeds, fungi, plants, and animals—including, of course, human beings. Meiosis differs from mitosis in that cell division begins with a cell that has a full complement of chromosomes and ends with gamete cells, such as sperm and eggs, that have only half the complement of chromosomes. When a sperm and egg unite during fertilization, the cell resulting from the union, called a zygote, contains the full number of chromosomes.
IV - ORIGIN OF CELLS:
The story of how cells evolved remains an open and actively investigated question in science (see Life). The combined expertise of physicists, geologists, chemists, and evolutionary biologists has been required to shed light on the evolution of cells from the nonliving matter of early Earth. The planet formed about 4.5 billion years ago, and for millions of years, violent volcanic eruptions blasted substances such as carbon dioxide, nitrogen, water, and other small molecules into the air. These small molecules, bombarded by ultraviolet radiation and lightning from intense storms, collided to form the stable chemical bonds of larger molecules, such as amino acids and nucleotides—the building blocks of proteins and nucleic acids. Experiments indicate that these larger molecules form spontaneously under laboratory conditions that simulate the probable early environment of Earth.
Scientists speculate that rain may have carried these molecules into lakes to create a primordial soup—a breeding ground for the assembly of proteins, the nucleic acid RNA, and lipids. Some scientists postulate that these more complex molecules formed in hydrothermal vents rather than in lakes. Other scientists propose that these key substances may have reached Earth on meteorites from outer space. Regardless of the origin or environment, however, scientists do agree that proteins, nucleic acids, and lipids provided the raw materials for the first cells. In the laboratory, scientists have observed lipid molecules joining to form spheres that resemble a cell’s plasma membrane. As a result of these observations, scientists postulate that millions of years of molecular collisions resulted in lipid spheres enclosing RNA, the simplest molecule capable of self-replication. These primitive aggregations would have been the ancestors of the first prokaryotic cells.
Fossil studies indicate that cyanobacteria, bacteria capable of photosynthesis, were among the earliest bacteria to evolve, an estimated 3.4 billion to 3.5 billion years ago. In the environment of the early Earth, there was no oxygen, and cyanobacteria probably used fermentation to produce ATP. Over the eons, cyanobacteria performed photosynthesis, which produces oxygen as a byproduct; the result was the gradual accumulation of oxygen in the atmosphere. The presence of oxygen set the stage for the evolution of bacteria that used oxygen in aerobic respiration, a more efficient ATP-producing process than fermentation. Some molecular studies of the evolution of genes in archaebacteria suggest that these organisms may have evolved in the hot waters of hydrothermal vents or hot springs slightly earlier than cyanobacteria, around 3.5 billion years ago. Like cyanobacteria, archaebacteria probably relied on fermentation to synthesize ATP.
Eukaryotic cells may have evolved from primitive prokaryotes about 2 billion years ago. One hypothesis suggests that some prokaryotic cells lost their cell walls, permitting the cell’s plasma membrane to expand and fold. These folds, ultimately, may have given rise to separate compartments within the cell—the forerunners of the nucleus and other organelles now found in eukaryotic cells. Another key hypothesis is known as endosymbiosis. Molecular studies of the bacteria-like DNA and ribosomes in mitochondria and chloroplasts indicate that mitochondrion and chloroplast ancestors were once free-living bacteria. Scientists propose that these free-living bacteria were engulfed and maintained by other prokaryotic cells for their ability to produce ATP efficiently and to provide a steady supply of glucose. Over generations, eukaryotic cells complete with mitochondria—the ancestors of animals—or with both mitochondria and chloroplasts—the ancestors of plants—evolved (see Evolution).
Fossilized Trilobites, Scientists study fossils to trace the evolution of life from simple cells to more complex organisms. Shown here are fossils of trilobites, primitive arthropods that once dominated the seas but became extinct about 250 million years ago.
V - THE DISCOVERY AND STUDY OF CELLS:
The first observations of cells were made in 1665 by English scientist Robert Hooke, who used a crude microscope of his own invention to examine a variety of objects, including a thin piece of cork. Noting the rows of tiny boxes that made up the dead wood’s tissue, Hooke coined the term cell because the boxes reminded him of the small cells occupied by monks in a monastery. While Hooke was the first to observe and describe cells, he did not comprehend their significance. At about the same time, the Dutch maker of microscopes Antoni van Leeuwenhoek pioneered the invention of one of the best microscopes of the time. Using his invention, Leeuwenhoek was the first to observe, draw, and describe a variety of living organisms, including bacteria gliding in saliva, one-celled organisms cavorting in pond water, and sperm swimming in semen. Two centuries passed, however, before scientists grasped the true importance of cells.
Modern ideas about cells appeared in the 1800s, when improved light microscopes enabled scientists to observe more details of cells. Working together, German botanist Matthias Jakob Schleiden and German zoologist Theodor Schwann recognized the fundamental similarities between plant and animal cells. In 1839 they proposed the revolutionary idea that all living things are made up of cells. Their theory gave rise to modern biology: a whole new way of seeing and investigating the natural world.
By the late 1800s, as light microscopes improved still further, scientists were able to observe chromosomes within the cell. Their research was aided by new techniques for staining parts of the cell, which made possible the first detailed observations of cell division, including observations of the differences between mitosis and meiosis in the 1880s. In the first few decades of the 20th century, many scientists focused on the behavior of chromosomes during cell division. At that time, it was generally held that mitochondria transmitted the hereditary information. By 1920, however, scientists determined that chromosomes carry genes and that genes transmit hereditary information from generation to generation.
During the same period, scientists began to understand some of the chemical processes in cells. In the 1920s, the ultracentrifuge was developed. The ultracentrifuge is an instrument that spins cells or other substances in test tubes at high speeds, which causes the heavier parts of the substance to fall to the bottom of the test tube. This instrument enabled scientists to separate the relatively abundant and heavy mitochondria from the rest of the cell and study their chemical reactions. By the late 1940s, scientists were able to explain the role of mitochondria in the cell. Using refined techniques with the ultracentrifuge, scientists subsequently isolated the smaller organelles and gained an understanding of their functions.
While some scientists were studying the functions of cells, others were examining details of their structure. They were aided by a crucial technological development in the 1940s: the invention of the electron microscope, which uses high-energy electrons instead of light waves to view specimens. New generations of electron microscopes have provided resolution, or the differentiation of separate objects, thousands of times more powerful than that available in light microscopes. This powerful resolution revealed organelles such as the endoplasmic reticulum, lysosomes, the Golgi apparatus, and the cytoskeleton. The scientific fields of cell structure and function continue to complement each other as scientists explore the enormous complexity of cells.
The discovery of the structure of DNA in 1953 by American biochemist James D. Watson and British biophysicist Francis Crick ushered in the era of molecular biology. Today, investigation inside the world of cells—of genes and proteins at the molecular level—constitutes one of the largest and fastest moving areas in all of science. One particularly active field in recent years has been the investigation of cell signaling, the process by which molecular messages find their way into the cell via a series of complex protein pathways in the cell.
Another busy area in cell biology concerns programmed cell death, or apoptosis. Millions of times per second in the human body, cells commit suicide as an essential part of the normal cycle of cellular replacement. This also seems to be a check against disease: When mutations build up within a cell, the cell will usually self-destruct. If this fails to occur, the cell may divide and give rise to mutated daughter cells, which continue to divide and spread, gradually forming a growth called a tumor. This unregulated growth by rogue cells can be benign, or harmless, or cancerous, which may threaten healthy tissue. The study of apoptosis is one avenue that scientists explore in an effort to understand how cells become cancerous.
further reading These sources provide additional information on Cell (biology).
Scientists are also discovering exciting aspects of the physical forces within cells. Cells employ a form of architecture called tensegrity, which enables them to withstand battering by a variety of mechanical stresses, such as the pressure of blood flowing around cells or the movement of organelles within the cell. Tensegrity stabilizes cells by evenly distributing mechanical stresses to the cytoskeleton and other cell components. Tensegrity also may explain how a change in the cytoskeleton, where certain enzymes are anchored, initiates biochemical reactions within the cell, and can even influence the action of genes. The mechanical rules of tensegrity may also account for the assembly of molecules into the first cells. Such new insights—made some 300 years after the tiny universe of cells was first glimpsed—show that cells continue to yield fascinating new worlds of discovery.

Francis Crick and James Watson The deoxyribonucleic acid (DNA) molecule is the genetic blueprint for each cell and ultimately the blueprint that determines every characteristic of a living organism. In 1953 American biochemist James Watson, left, and British biophysicist Francis Crick, right, described the structure of the DNA molecule as a double helix, somewhat like a spiral staircase with many individual steps. Their work was aided by X-ray diffraction pictures of the DNA molecule taken by British biophysicist Maurice Wilkins and British physical chemist Rosalind Franklin.in 1962 Crick, Watson, and Wilkins received the Nobel Prize for their pioneering work on the structure of the DNA molecule.

Antoni van Leeuwenhoek Although lacking basic scientific training, Antoni van Leeuwenhoek is credited with inventing the precursor to the modern microscope. Leeuwenhoek was the first to document the structure of red blood corpuscles and the nature of the circulatory system. In addition to describing animalcules (protozoans and bacteria) Leeuwenhoek also accurately described the life cycles of many types of insects.
Choloroplast:
Chloroplast, structure in the cells of plants and algae where photosynthesis takes place. Chloroplasts are mostly disk-shaped organelles, 4 to 6 micrometers in diameter. They occur most abundantly in leaf cells, where they can apparently orient themselves to light. Perhaps 40 to 50 chloroplasts exist in one cell and 500,000 in each sq mm (0.06 sq in) of leaf surface. Each chloroplast is enclosed in a double membrane. Internally, it consists of a ground substance called the stroma, which is traversed by a complex network of interconnected disks called thylakoids. Many of the thylakoids are stacked like saucers; the stacks are called grana. Molecules of chlorophyll, which absorb light for photosynthesis, are attached to the thylakoids. The light energy captured by the chlorophyll is converted to adenosine triphosphate, or ATP, in a series of chemical reactions that take place in the grana. Chloroplasts also contain small starch grains that temporarily store the products of photosynthesis.
In plants, chloroplasts develop in the presence of light from small, colorless organelles called proplastids. As cells divide in the growing parts of a plant, the proplastids inside them divide by fission. Thus, the daughter cells have the ability to produce chloroplasts. In algae, chloroplasts divide directly, without developing from proplastids. The self-reproducing ability of chloroplasts, their bacteria-like DNA and ribosomes, and their close similarity regardless of the type of cell they inhabit, suggest that they were once independent organisms that come to exist in symbiosis with the plant cell as host.

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Polio Vaccine (IPV): When to Get Vaccinated -

Poliomyelitis, infectious viral disease that sometimes results in paralysis. The infection chiefly affects children

and young adults and is caused by any one of three related viruses called polioviruses. In more than 95 percent of cases, the infection may pass without creating severe or even noticeable illness because the body’s immune system neutralizes the invading virus and protects against future infection. In a small percentage of cases the virus penetrates to the central nervous system, infecting cells that control muscle function. Such infection can result in permanent paralysis of limbs. In its most perilous form, the infection attacks the brain, creating complications that sometimes result in death.
The term poliomyelitis derives from Greek words referring to inflammation (itis) of the gray (polios) matter of the spinal cord (myelos). The shortened term polio is commonly used by the public and medical professionals, and the disease has also been referred to as infantile paralysis.

According to figures from the World Health Organization (WHO), approximately 7,100 cases of polio were reported throughout the world in 1999. Experts suspect that misdiagnosis and underreporting may make these numbers inaccurate. Thus, the actual number of polio cases may be roughly five times the reported figure—perhaps more than 20,000 cases, occurring primarily in Africa, the Middle East, and South Asia.

While still a health threat in many parts of the world, polio also represents one of the greatest medical success stories of the 20th century. The number of polio cases found worldwide today constitutes a 90 percent reduction from the number of cases reported in the late 1980s. Most health experts credit this achievement to a mass vaccination program called the Global Polio Eradication Initiative that WHO initiated in 1988 to eradicate polio worldwide by the year 2005.

In the United States and Canada, polio has been effectively eliminated, with fewer than ten cases reported in each country per year. This statistic contrasts strikingly with those from the first half of the 20th century, when epidemic polio was one of the most dreaded and feared diseases in North America. The first large-scale outbreak in the United States occurred in 1916, when over 37,000 cases were reported in 26 states, resulting in 6,000 deaths. Each year thereafter the number of polio cases in the United States averaged about 38,000 per year, including about 21,000 paralytic cases each year. In Canada the epidemic peaked in 1953, resulting in one of the highest national polio rates ever recorded in the world.

Fearing infection during epidemics in the first half of the 20th century, some people avoided beaches, public pools, theaters, fairs—any place of public gathering. Occasionally schools were closed until epidemics subsided. In response to the public’s fear of this disease, in 1938 President Franklin Delano Roosevelt (who had himself lost the use of both legs to polio) founded the National Foundation for Infantile Paralysis. Now known as the March of Dimes Birth Defects Foundation, the organization raised millions of dollars for polio research and the support of victims.

The discovery in the 1950s of vaccines that protect against poliovirus infection eventually led to successful polio vaccination programs in North America, Latin America, and Europe. By the early 1990s, WHO declared the western hemisphere free of polio. But the struggle to eradicate polio continues in many developing nations. Furthermore, health officials stress that vaccination programs must be maintained in areas considered polio free. They warn that the virus could easily become dangerous again in populations of children who have not acquired immunity, the body’s ability to resist infection from the polio virus. This danger was chillingly demonstrated in 1979 when an outbreak of paralytic polio erupted among unvaccinated members of an Amish sect in Pennsylvania and Maryland.

HOW POLIO DEVELOPS,

The three types of poliovirus belong to the virus family known as picornaviruses. This name derives from a combination of the words pico, meaning "small," and RNA, indicating that the viruses contain a core of the genetic material known as ribonucleic acid, or RNA. Three types of poliovirus have been identified: Type 1 (also known as Brunhilde), Type 2 (Lansing), and Type 3 (Leon). Type 1 is the most common form and the one most closely associated with polio’s more severe, paralytic progression. A person who develops immunity from exposure to one poliovirus type will not have immunity against the other polioviruses.

Poliovirus typically enters the body through the mouth and multiplies in the tonsils and lymph nodes of the upper respiratory tract. Infection proceeds from the mouth into the gastrointestinal tract through the stomach to the intestines. The virus multiplies in the intestines and is shed from the body in feces, often resulting in further infections. For example, a parent can become infected by an infant during diaper changes, or improper waste disposal can lead to contamination of a water supply. These infections, in turn, will spread the virus more widely.

Large quantities of poliovirus multiply in intestinal tissue known as Peyer’s patches, where cells of the body’s lymphatic system are concentrated. Passage of the virus into the body’s lymphatic system stimulates the production of antibodies. These specialized immune-system defenders, in time, will destroy the viral intruder. From the lymphatic system, the virus typically invades the bloodstream.

Types of Polio Disease,

Once the virus enters the bloodstream, the virus may cause one of four types of illnesses marked by varying severity. Some infections result in abortive poliomyelitis, a mild form of the disease characterized by fever, headache, sore throat, fatigue, nausea, and vomiting. This short-lived form of the illness lasts only from hours to a few days. In more than 95 percent of cases, the disease gets no worse. Sometimes, however, the virus may invade the nervous system, causing more severe forms of the disease.

Some poliovirus infections of nerve cells, or neurons, result in nonparalytic poliomyelitis. In addition to the fever and other symptoms seen in abortive poliomyelitis, nonparalytic poliomyelitis causes pain and stiffness in the neck and back. This infection typically produces aseptic meningitis—an inflammation of the membranes that surround the brain and spinal cord. As with abortive poliomyelitis, however, symptoms from nonparalytic polio usually subside within a few days without causing permanent damage.

In perhaps 1 or 2 percent of cases a more disabling form of the disease occurs, called paralytic poliomyelitis. In this form viral infection of neurons in the spinal cord may cause temporary damage to these cells—or permanent destruction. The muscles activated by the involved neurons become painful, and muscular weakness in the arms and legs may develop, sometimes followed by paralysis. The muscles affected and the degree of damage that occurs depend on the area of the spinal cord that has been invaded and on the number of neurons involved. Any limb or combination of limbs may be affected—one leg, one arm, or both legs and both arms. In children under 5 years of age paralysis of one leg is most common. In adults paralysis of all four limbs—a condition known as quadriplegia—is more common. In some cases muscles that control breathing are affected, requiring that the patient be placed on artificial respiration, in which breathing is controlled mechanically. Roughly 2 to 5 percent of infants with paralytic polio die. For adults the disease is even deadlier, causing death in 15 to 30 percent of cases.

In the most serious cases of polio infection the virus attacks the brain, causing bulbar poliomyelitis. Various nerves in the head and face, including those that send signals to the ears, eyes, and the muscles controlling chewing and swallowing, may be affected. Sometimes the virus affects the part of the brain that controls breathing and heartbeat, resulting in death.

Abu Ali Al Hasan Ibn Al Haytham(Alhazen), The Father Of Science,

Alhazen (965-1040?), Arab scientist and natural philosopher, who made important contributions in optics, astronomy, and mathematics. His Arab name is Abu Ali al-Hasan ibn al-Haytham. His major work, Optics, included valuable analyses and explanations of light and vision.

Alhazen was born in Basra, in what is now Iraq. He was invited to Cairo by the Muslim ruler al-Hakim. After failing in an attempt to regulate the flow of the Nile, Alhazen feared that al-Hakim would punish him. To avoid punishment, he pretended to be insane until al-Hakim's death. He devoted the rest of his life to scientific study.

Alhazen's most important and original contributions were in optics. He developed a broad theory that explained vision, using geometry and anatomy. This theory says that each point on a lighted area or object radiates light rays in every direction, but only one ray from each point, which strikes the eye perpendicularly, can be seen. The other rays strike at different angles and are not seen.

In astronomy, Alhazen added to the theories of the 2nd-century astronomer Ptolemy. He also summarized or explained some of the difficult mathematical theorems of the ancient Greek mathematician Euclid.

South Pacific Regional Trade And Economic Cooperation Agreement,

Pacific Islands or Oceania, the more than 25,000 islands and islets of 25 nations and territories spread over the western and central Pacific Ocean. Although the Pacific Islands are scattered across millions of square kilometers, their total land area is just 1,261,456 sq km (487,051 sq mi)—slightly larger than South Africa, slightly smaller than Peru, and four-fifths the size of Alaska. The islands of New Guinea, New Zealand, and Hawaii constitute 93 percent of the land area, while the remaining thousands of islands have a total land area of 89,339 sq km (34,494 sq mi), slightly less than the American state of Indiana. New Guinea, shared by the Indonesian province of Papua (formerly Irian Jaya) and the nation of Papua New Guinea, is the second largest island in the world, after Greenland. New Zealand’s South Island and North Island, Oceania’s next largest islands, are the world’s 12th and 14th largest islands, respectively.


MELANESIA, MICRONESIA, AND POLYNESIA, 
The Pacific Islands are usually divided into three subregions: Melanesia (the prefix mela, meaning dark or black, refers to the dark complexion of many Melanesian people), Micronesia (the prefix micro, meaning small, refers to the small size of Micronesia’s islands and atolls), and Polynesia (the prefix poly, meaning many, refers to the many islands of Polynesia).

Melanesia stretches in a 5600-km (3500-mi) arc off the northern and eastern coast of Australia. From northwest to southeast, Melanesia includes New Guinea, lying just north of Australia; the Bismarck Archipelago, belonging to Papua New Guinea; smaller archipelagos of Papua New Guinea; the Solomon Islands, some of which belong to Papua New Guinea but most of which are part of the nation of Solomon Islands; the many islands of the nation Vanuatu; the islands of New Caledonia and Dependencies, a French territory; and the Fiji Islands (an island nation commonly known as Fiji).

The tiny islands and atolls of Micronesia are scattered widely across a large area north of Melanesia and east of Asia. Micronesia has four main island groups. The Caroline Islands lie north of the equator from New Guinea and belong mostly to the Federated States of Micronesia, a self-governing country in free association with the United States. A small portion of the Carolines belongs to Palau, also a self-governing country in free association with the United States. To the north of the Carolines are the Mariana Islands, which make up the Commonwealth of the Northern Mariana Islands, a self-governing U.S. commonwealth, and Guam, an unincorporated U.S. territory. To the east of the Marianas are the Marshall Islands, an island group and republic in free association with the United States. Southeast of the Marshalls is the nation of Kiribati, which straddles the equator. The tiny nation of Nauru, a single island west of Kiribati, is also counted as part of Micronesia. Micronesia’s islands are so small that their land area totals just 3240 sq km (1250 sq mi). Even among the smaller islands of Oceania—that is, Oceania excluding New Guinea, New Zealand, and Hawaii—Micronesia makes up just 3.6 percent of the total land mass.

Polynesia, lying in the central and southern Pacific, encompasses a vast triangle stretching east from Melanesia and Micronesia. Polynesia is larger than both Melanesia and Micronesia combined. The southwestern tip of the Polynesian triangle is the nation of New Zealand, lying southeast of Australia and far south of the tropic of Capricorn. The southeastern tip is Easter Island, part of Chile lying just south of the tropic of Capricorn three-fourths of the distance from Australia to South America. The triangle’s northwestern tip is Hawaii, straddling the tropic of Cancer halfway between North America and Asia. These three tips, however, are outliers: Most of Polynesia is clustered just east of Melanesia south of the equator. From north to south, the Polynesian islands immediately east of Melanesia form the nation of Tuvalu; Wallis and Futuna, a French territory north of Fiji; and the nation of Tonga. Farther east, from north to south, are Tokelau, a territory of New Zealand; the independent nation of Samoa (formerly Western Samoa); American Samoa, a U.S. territory; Niue, a self-governing island in free association with New Zealand; and the Cook Islands, a self-governing island group also in free association with New Zealand. Still farther east lie the five archipelagos of the French territory French Polynesia: the Austral Islands, the Society Islands (with well-known Tahiti and Bora-Bora), the Tuamotu Archipelago (including the Gambier Islands), and the Marquesas Islands. Beyond French Polynesia is Pitcairn Island, a dependency of the United Kingdom.

Oceania is sometimes defined to include Australia, but because of Australia’s continental size and its distinct geography, climate, and cultures it is more often considered a separate region of the world. Similarly, the Philippine, Indonesian, and Japanese archipelagos, which border Melanesia and Micronesia, bear a greater resemblance to the rest of Asia than the Pacific Islands do. Other, smaller island groups on the far northern and eastern edges of the Pacific (for example, the Aleutian Islands of Alaska and the Galápagos Islands of Ecuador) are usually classified with the nearby regions of the Western Hemisphere.