Microsoft Encarta

Jumat, 02 Januari 2009

All about cell

Cell (biology)

INTRODUCTION

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.

M.I. Walker/Science Source/Photo Researchers, Inc.

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 vary considerably in size. The smallest 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.

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.”

Prokaryotic Cells

Anatomy of a Simple Bacterium

Bacteria cells typically are surrounded by a rigid, protective cell wall. The cell membrane, also called the plasma membrane, regulates passage of materials into and out of the cytoplasm, the semi-fluid that fills the cell. The DNA, located in the nucleoid region, contains the genetic information for the cell. Ribosomes carry out protein synthesis. Many baceteria contain a pilus (plural pili), a structure that extends out of the cell to transfer DNA to another bacterium. The flagellum, found in numerous species, is used for locomotion. Some bacteria contain a plasmid, a small chromososme with extra genes. Others have a capsule, a sticky substance external to the cell wall that protects bacteria from attack by white blood cells. Mesosomes were formerly thought to be structures with unknown functions, but now are know to be artifacts created when cells are prepared for viewing with electron microscopes.

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.

Eukaryotic Animal Cells

Animal Cell

An animal cell typically contains several types of membrane-bound organs, or organelles. The nucleus directs activities of the cell and carries genetic information from generation to generation. The mitochondria generate energy for the cell. Proteins are manufactured by ribosomes, which are bound to the rough endoplasmic reticulum or float free in the cytoplasm. The Golgi apparatus modifies, packages, and distributes proteins while lysosomes store enzymes for digesting food. The entire cell is wrapped in a lipid membrane that selectively permits materials to pass in and out of the cytoplasm.

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.

Plasma Membrane

The plasma membrane that surrounds eukaryotic cells is a dynamic structure composed of two layers of phospholipid molecules interspersed with cholesterol and proteins. Phospholipids are composed of a hydrophilic, or water-loving, head and two tails, which are hydrophobic, or water-hating. The two phospholipid layers face each other in the membrane, with the heads directed outward and the tails pointing inward. The water-attracting heads anchor the membrane to the cytoplasm, the watery fluid inside the cell, and also to the water surrounding the cell. The water-hating tails block large water-soluble molecules from passing through the membrane while permitting fat-soluble molecules, including medications such as tranquilizers and sleeping pills, to freely cross the membrane. Proteins embedded in the plasma membrane carry out a variety of functions, including transport of large water soluble molecules such as sugars and certain amino acids. Glycoproteins, proteins bonded to carbohydrates, serve in part to identify the cell as belonging to a unique organism, enabling the immune system to detect foreign cells, such as invading bacteria, which carry different glycoproteins. Cholesterol molecules in the plasma membrane act as stabilizers that limit the movement of the two slippery phospholipids layers, which slide back and forth in the membrane. Tiny gaps in the membrane enable small molecules such as oxygen (upper right) to diffuse readily into and out of the cell. Since cells constantly use up oxygen, decreasing its concentration within the cell, the higher concentration of oxygen outside the cell causes a net flow of oxygen into the cell. The steady stream of oxygen into the cell enables it to carry out aerobic respiration continually, a process that provides the cell with the energy needed to carry out its functions.

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.

Nucleus of a Cell

The nucleus, present in eukaryotic cells, is a discrete structure containing chromosomes, which hold the genetic information for the cell. Separated from the cytoplasm of the cell by a double-layered membrane called the nuclear envelope, the nucleus contains a cellular material called nucleoplasm. Nuclear pores, present around the circumference of the nuclear membrane, allow the exchange of cellular materials between the nucleoplasm and the cytoplasm.

Don W. Fawcett/Science Source/Photo Researchers, Inc.

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.

Golgi Apparatus

The Golgi apparatus, a minute cellular inclusion in the cytoplasm, is a series of smooth, stacked membranous sacs. The Golgi apparatus modifies proteins after they are produced by the ribosomes.

David M. Phillips/The Pop. Council/Science Source/Photo Researchers, Inc.

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.

Cytoskeleton

The cytoskeleton, a network of protein fibers, crisscrosses the cytoplasm of eukaryotic cells, providing shape and mechanical support. The cytoskeleton also functions as a monorail to transport substances around the cell. A cell such as an amoeba changes shape by dismantling parts of the cytoskeleton and reassembling them in other locations.

Don Fawcett/Science Source/Photo Researchers, Inc.

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.

Eukaryotic Plant Cells

Plant Cell

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 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.

Movement

Plant Cell

III

CELL FUNCTIONS

Movement

Euglena, Showing Whiplike Flagellum

The euglena is a single-celled alga with two or several flagella (depending on the species) located at one end for locomotion. Other algae, vertebrate sperm cells, and some protozoans and bacteria possess a single flagellum for movement.

Biophoto Associates/Science Source/Photo Researchers, Inc.

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.

Nutrition

Euglena, Showing Whiplike Flagellum

Biophoto Associates/Science Source/Photo Researchers, Inc.

Nutrition

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.

Don Fawcett-Keith Porter/Photo Researchers, Inc.

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.

Protein Synthesis

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.

Don W. Fawcett/Science Source/Photo Researchers, Inc.

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.

Cell Division

First Cell Divisions

As a fertilized egg goes through its first divisions, the daughter cells become progressively smaller. When there are a hundred or more cells, they form a hollow ball of cells, called a blastula, surrounding a fluid-filled cavity. Later divisions produce three layers of cells—endoderm (inner), mesoderm (middle), and ectoderm (outer)—from which the principal features of the animal will differentiate.

BBC Worldwide Americas, Inc.

Mitosis

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.

ORIGIN OF CELLS

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.

James L. Amos/Photo Researchers, Inc.

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).

THE DISCOVERY AND STUDY OF CELLS

Hooke’s Microscope

English scientist Robert Hooke built this microscope in the 17th century and used it to conduct pioneering research. He discovered the cell structure of plants by observing a thin slice of cork under his microscope.

Cecil Fox/Science Source/Photo Researchers, Inc.

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.

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.

Culver Pictures

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.

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.

Photo Researchers, Inc.

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.

Scanning Electron Microscope

This scanning electron microscope (SEM) at the University of California, Berkeley is located to the left with the computer images of the specimen on the computer screens to the right. Although a SEM cannot resolve objects as small as a transmission electron microscope, the images produced by the SEM are more useful for seeing the three-dimensional aspect of the surface structure of small objects.

Lawrence Migdale/Photo Researchers, Inc.

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.

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.

Contributed By:
Christopher King

All about coal

Coal

INTRODUCTION

Coal Miners

A coal train leaves a mine in Scranton, Pennsylvania, in 1919. United States President William Howard Taft created a federal Bureau of Mines with the objective of reducing mine accidents and fatalities.

CORBIS-BETTMANN

Coal, a combustible organic rock composed primarily of carbon, hydrogen, and oxygen. Coal is burned to produce energy and is used to manufacture steel. It is also an important source of chemicals used to make medicine, fertilizers, pesticides, and other products. Coal comes from ancient plants buried over millions of years in Earth’s crust, its outermost layer. Coal, petroleum, natural gas, and oil shale are all known as fossil fuels because they come from the remains of ancient life buried deep in the crust.

Coal is rich in hydrocarbons (compounds made up of the elements hydrogen and carbon). All life forms contain hydrocarbons, and in general, material that contains hydrocarbons is called organic material. Coal originally formed from ancient plants that died, decomposed, and were buried under layers of sediment during the Carboniferous Period, about 360 million to 290 million years ago. As more and more layers of sediment formed over this decomposed plant material, the overburden exerted increasing heat and weight on the organic matter. Over millions of years, these physical conditions caused coal to form from the carbon, hydrogen, oxygen, nitrogen, sulfur, and inorganic mineral compounds in the plant matter. The coal formed in layers known as seams.

Plant matter changes into coal in stages. In each successive stage, higher pressure and heat from the accumulating overburden increase the carbon content of the plant matter and drive out more of its moisture content. Scientists classify coal according to its fixed carbon content, or the amount of carbon the coal produces when heated under controlled conditions. Higher grades of coal have a higher fixed carbon content.

MODERN USES OF COAL

Eighty-six percent of the coal used in the United States is burned by electric power plants to produce electricity. When burned, coal generates energy in the form of heat. In a power plant that uses coal as fuel, this heat converts water into steam, which is pressurized to spin the shaft of a turbine. This spinning shaft drives a generator that converts the mechanical energy of the rotation into electric power (see Electric Motors and Generators; Steam Engine).

Coal is also used in the steel industry. The steel industry uses coal by first heating it and converting it into coke, a hard substance consisting of nearly pure carbon. The coke is combined with iron ore and limestone. Then the mixture is heated to produce iron (see Iron and Steel Industry). Other industries use different coal gases given off during the coke-forming process to make fertilizers, solvents, medicine, pesticides, and other products.

Fuel companies convert coal into easily transportable gas or liquid fuels (see Synthetic Fuels). Coal-based vapor fuels are produced through the process of gasification. Gasification may be accomplished either at the site of the coalmine or in processing plants. In processing plants, the coal is heated in the presence of steam and oxygen to produce synthesis gas, a mixture of carbon monoxide, hydrogen, and methane used directly as fuel or refined into cleaner-burning gas.

On-site gasification is accomplished by controlled, incomplete burning of an underground coal bed while adding air and steam. To do this, workers ignite the coal bed, pump air and steam underground into the burning coal, and then pump the resulting gases from the ground. Once the gases are withdrawn, they may be burned to produce heat or generate electricity. Or they may be used in synthetic gases to produce chemicals or to help create liquid fuels.

Liquefaction processes convert coal into a liquid fuel that has a composition similar to that of crude petroleum. Coal can be liquefied either by direct or indirect processes. However, because coal is a hydrogen-deficient hydrocarbon, any process used to convert coal to liquid or other alternative fuels must add hydrogen. Four general methods are used for liquefaction: (1) pyrolysis and hydrocarbonization, in which coal is heated in the absence of air or in a stream of hydrogen; (2) solvent extraction, in which coal hydrocarbons are selectively dissolved and hydrogen is added to produce the desired liquids; (3) catalytic liquefaction, in which hydrogenation takes place in the presence of a catalyst; and (4) indirect liquefaction, in which carbon monoxide and hydrogen are combined in the presence of a catalyst.

III

COAL FORMATION

How Coal Forms

The coal we find today formed from generations of plants that died in ancient tropical swamps and accumulated on the swamp bottoms. The plant material first formed a compact organic material called peat. As layers of sediment gradually accumulated over the peat, the pressure and heat exerted by the thickening layers gradually drove out the moisture and increased the carbon content of the peat, forming coal.

Coal is a sedimentary rock formed from plants that flourished millions of years ago when tropical swamps covered large areas of the world. Lush vegetation, such as early club mosses, horsetails, and enormous ferns, thrived in these swamps. Generations of this vegetation died and settled to the swamp bottom, and over time the organic material lost oxygen and hydrogen, leaving the material with a high percentage of carbon. Layers of mud and sand accumulated over the decomposed plant matter, compressing and hardening the organic material as the sediments deepened. Over millions of years, deepening sediment layers, known as overburden, exerted tremendous heat and pressure on the underlying plant matter, which eventually became coal.

Cutting Peat

A worker cuts peat from lush peatland in Ireland. Peat is the first stage in the transformation of vegetation into coal. For hundreds of years, people have cut, dried, and burned it for heating and cooking. This compact, dark-brown material contains about one third less heating value than coal.

Farrell Grenan/Photo Researchers, Inc.

Before decayed plant material forms coal, the plant material forms a dark brown, compact organic material known as peat. Although peat will burn when dried, it has a low carbon and high moisture content relative to coal. Most of coal’s heating value comes from carbon, whereas inorganic materials, such as moisture and minerals, detract from its heating value. For this reason, peat is a less efficient fuel source than coal. Over time, as layers of sediment accumulate over the peat, this organic material forms lignite, the lowest grade of coal. As the thickening geologic overburden gradually drives moisture from the coal and increases its fixed carbon content, coal evolves from lignite into successively higher-graded coals: subbituminous coal, bituminous coal, and anthracite. Anthracite, the highest rank of coal, has nearly twice the heating value of lignite (see Heat).

Coal formation began during the Carboniferous Period (known as the first coal age), which spanned 360 million to 290 million years ago. Coal formation continued throughout the Permian, Triassic, Jurassic, Cretaceous, and Tertiary Periods (known collectively as the second coal age), which spanned 290 million to 1.6 million years ago. Coals formed during the first coal age are older, so they are generally located deeper in Earth’s crust. The greater heat and pressures at these depths produce higher-grade coals such as anthracite and bituminous coals. Conversely, coals formed during the second coal age under less intense heat and pressure are generally located at shallower depths. Consequently, these coals tend to be lower-grade subbituminous and lignite coals.

COMPONENTS OF COAL

Coal contains organic (carbon-containing) compounds transformed from ancient plant material. The original plant material was composed of cellulose, the reinforcing material in plant cell walls; lignin, the substance that cements plant cells together; tannins, a class of compounds in leaves and stems; and other organic compounds, such as fats and waxes. In addition to carbon, these organic compounds contain hydrogen, oxygen, nitrogen, and sulfur. After a plant dies and begins to decay on a swamp bottom, hydrogen and oxygen (and smaller amounts of other elements) gradually dissociate from the plant matter, increasing its relative carbon content.

Coal also contains inorganic components, known as ash. Ash includes minerals such as pyrite and marcasite formed from metals that accumulated in the living tissues of the ancient plants. Quartz, clay, and other minerals are also added to coal deposits by wind and groundwater. Ash lowers the fixed carbon content of coal, decreasing its heating value.

COAL DEPOSITS AND RESERVES

Coal Production and Consumption

A comparison of the top ten coal-producing countries and the top ten coal-consuming countries shows that China is both the leading producer and the leading consumer of coal. Coal is burned in power plants to produce electricity and in steel mills to make coke for the production of steel.

Although coal deposits exist in nearly every region of the world, commercially significant coal resources occur only in Europe, Asia, Australia, and North America. Commercially significant coal deposits occur in sedimentary rock basins, typically sandwiched as layers called beds or seams between layers of sandstone and shale.

When experts develop estimates of the world’s coal supply, they distinguish between coal reserves and resources. Reserves are coal deposits that can be mined profitably with existing technology—that is, with current equipment and methods. Resources are an estimate of the world’s total coal deposits, regardless of whether the deposits are commercially accessible. Exploration geologists have found and mapped the world’s most extensive coal beds. At the beginning of 2001, global coal reserves were estimated at 984.2 billion metric tons, in which 1 metric ton equals 1,016 kg (2,240 lb). These reserves occurred in the following regions by order of importance: the Asia Pacific, including Australia, 29.7 percent; North America, 26.1 percent; Russia and the countries of the former Union of Soviet Socialist Republics (USSR), 23.4 percent; Europe, excluding the former USSR, 12.4 percent; Africa and the Middle East, 6.2 percent; and South and Central America, 2.2 percent.

Coal deposits in the United Kingdom, which led the world in coal production until the 20th century, extend throughout parts of England, Wales, and southern Scotland. Coalfields in western Europe underlie the Saar and Ruhr valleys in Germany, the Alsace region of France, and areas of Belgium. Coalfields in central Europe extend throughout parts of Poland, the Czech Republic, and Hungary. The most extensive and valuable coalfield in eastern Europe is the Donets Basin, between the Dnieper and Don rivers (in parts of Russia and Ukraine). Large coal deposits in Russia are being mined in the Kuznetsk Basin in southern Siberia. Coalfields underlying northwestern China are among the largest in the world. Mining of these fields began in the 20th century.

United States coal reserves are located in six major regions, three of which produce the majority of domestically mined coal. The most productive region in the United States is the Appalachian Basin, covering parts of Pennsylvania, West Virginia, Kentucky, Tennessee, Ohio, and Alabama. Large quantities of coal have also been produced by both the Illinois Basin—extending through Illinois, Indiana, and Kentucky—and the Western Interior Region—extending through Missouri, Kansas, and Oklahoma. Other commercially important U.S. coal regions include the Powder River Basin, underlying parts of Montana and Wyoming; the Green River Basin in Wyoming; the Uinta Basin, covering areas of Utah and Colorado; and the San Juan Basin, underlying parts of Utah, New Mexico and Colorado.

In 2002 estimates of total U.S. coal reserves were approximately 246 billion metric tons. At the beginning of the 21st century production amounted to about 980 million metric tons each year.

COAL MINING

Coal mining is the removal of coal from the ground. The mining method employed to extract the coal depends on the following criteria: (1) seam thickness, (2) the overburden thickness, (3) the ease of removal of the overburden, (4) the ease with which a shaft can be sunk to reach the coal seam, (5) the amount of coal extracted relative to the amount that cannot be removed, and (6) the market demand for the coal.

The two types of mining methods are surface mining and underground mining. In surface mining the layers of rock or soil overlying a coal seam are first removed after which the coal is extracted from the exposed seam. In underground mining, a shaft is dug to reach the coal seam. Currently, underground mining accounts for approximately 60 percent of the world recovery of coal.

Surface Mining

Giant Coal Bucket

Strip miners often use extremely large buckets to level or move land. The buckets have sharp, slanted teeth on the front edge and are dragged along the surface of the ground by powerful machines. This bucket, used in the Big Muskie coal mine in Ohio, is the size of a small house.

UPI/THE BETTMANN ARCHIVE

Surface mining is used to reach coal reserves that are too shallow to be reached by other mining methods. Types of surface mining include open-pit mining, drift mining, slope mining, contour mining, and auger mining.

Open-pit Mining

In open-pit mining, or strip mining, earth-moving equipment is used to remove the rocky overburden and then huge mechanical shovels scoop coal up from the underlying deposit. The modern coal industry has developed some of the largest industrial equipment ever made, including shovels capable of holding 290 metric tons of coal.

To reach the coal, bulldozers clear the vegetation and soil. Depending on the hardness and depth of the exposed sedimentary rocks, these rocky layers may be shattered with explosives. To do this, workers drill blast holes into the overlying sedimentary rock, fill these holes with explosives, and then blast the overburden to fracture the rock. Once the broken rock is removed, coal is shoveled from the underlying deposit into giant earth-moving trucks for transport.

Drift Mining

Drift mining is used when a horizontal seam of coal emerges at the surface on the side of a hill or mountain, and the opening into the mine can be made directly into the coal seam. This type of mining is generally the easiest and most economical type because excavation through rock is not necessary. If coal is available in this manner, it is likely to be mined.

Slope Mining

Slope mining occurs when an inclined opening is used to tap the coal seam (or seams). A slope mine may follow the coal seam if the seam is inclined and exposed to the surface, or the slope may be driven through rock strata overlying the coal to reach a seam. Coal transportation from a slope mine can be accomplished by conveyor or by track haulage (using a trolley locomotive if the grade is not severe) or by pulling mine cars up the slope using an electric hoist and steel rope if the grade is steep. The most common practice is to use a belt conveyor.

Contour Mining

Contour mining occurs on hilly or mountainous terrain, where workers use excavation equipment to cut into the hillside along its contour to remove the overlying rock and then mine the coal. The depth to which workers must cut into the hillside depends on factors such as hill slope and coal bed thickness.

Auger Mining

Auger mining is frequently employed in open-pit mines where the thickness of the overburden is too great for open-pit mining to be cost-effective. Open-pit mining would require the lengthy and costly removal of the overburden, whereas auger mining is more efficient because it cuts through the overburden and removes the coal as it drills. In this technique, the miners drill a series of horizontal holes into the coal bed with a large auger (drill) powered by a diesel or gasoline engine. These augers are typically about 60 m (200 ft) long and 0.6 to 2.1 m (2 to 7 ft) in diameter. As these enormous drills bore into the coal seam, they discharge coal like a wood drill producing wood shavings. Additional auger lengths are added as the cutting head of the auger penetrates farther into the coal. Penetration continues until the cutting head drifts into the top or bottom of the coal seam, into a previous hole, or until the maximum torque (energy required to twist an object) of the auger is reached.

Satellite Aids to Surface Mining

In the late 1990s some coal mining enterprises used technologies such as the global positioning system (GPS) to help guide the positioning of mining equipment. Satellites operated by the United States Air Force Space Command and leased to companies for commercial use track the position of mining equipment against a map of a mine’s topography. This map uses colors to distinguish soil that should be excavated, soil that should remain in place, and areas that should be filled in. The equipment driver observes this visual information on a monitor while operating the equipment. Some coal mining enterprises have used GPS to increase mining efficiency up to 30 percent.

Underground Mining

Big Pit Mining Museum, Blaenafon

Coal mining was once the main industry of the Rhondda Valley in Great Britain. The Big Pit ("Pwll Mawr") coal mine ceased production in 1980 but its buildings and machinery were preserved and converted into a museum of mining. Visitors descend into the mine via the 90-m (300-ft) lift shaft to walk through underground roadways, air doors, stables (for pit ponies), and engine houses constructed by past generations of miners.

Spectrum Colour Library

Underground, or deep, mining occurs when coal is extracted from a seam without removal of the overlying strata. Miners build a shaft mine that enters the earth through a vertical opening and descends from the surface to the coal seam. In the mine, the coal is extracted from the seam by various methods, including conventional mining, continuous mining, longwall mining, and room-and-pillar mining.

Conventional Mining

Conventional mining, also called cyclic mining, involves a sequence of operations that proceed in the following order: (1) supporting the roof, (2) ventilation, (3) cutting, (4) drilling, (5) blasting, (6) coal removal, and (7) loading. First, miners make the roof above the seam safe and stable by timbering or by roof bolting, processes intended to prevent the roof from collapsing. At the same time, they create ventilation openings so that dangerous gases can escape and fresh air can reach the miners. Then one or more slots—a few centimeters wide and extending for several meters into the coal—are cut along the face of the coal seam, also known as the wall face, by a large, mobile cutting machine. The cut, or slot, provides easy access to the face and facilitates the breaking up of the coal, which is usually blasted from the seam by explosives known as permissible explosives. This type of explosive produces an almost flame-free explosion and markedly reduces the amount of noxious fumes in comparison with conventional explosives. The coal may then be transported by rubber-tired electric vehicles (shuttle cars) or by chain (or belt) conveyor systems.

Continuous Mining

Continuous mining involves the use of a single machine known as a continuous miner that breaks the coal mechanically and loads it for transport. This mobile machine has a series of metal-studded rotating drums that gouge coal from the face of the coal seam. One continuous miner can mechanically break apart about 1.8 metric tons of coal per hour. Roof support is then installed, ventilation is advanced, and the coalface is ready for the next cycle. The method used to transport the coal requires the installation of mobile belt conveyors.

Longwall Mining

The longwall mining system uses a remote-controlled self-advancing roof in which large blocks of coal are completely extracted in a continuous operation. Hydraulic or self-advancing jacks, known as chocks, support the roof at the immediate face as the coal is removed. As the face advances, the roof is allowed to collapse behind the remote-controlled, roof-building machinery. Miners then remove the fallen coal. Coal recovery is comparable to that attainable with the conventional or continuous mining systems.

Room-and-Pillar Mining

Room-and-pillar mining is a means of developing a coalface and, at the same time, retaining supports for the roof. With this technique, rooms are developed from large, parallel tunnels driven into the solid coal, and the intervening pillars of coal are used to support the roof. The percentage of coal recovered from a seam depends on the number and size of protective pillars of coal thought necessary to support the roof safely. Workers may remove some coal pillars just before closing the mine.

VII

COAL MINING SAFETY

Coalmines are hazardous operations. In the 20th century, more than 100,000 miners died working in coalmines. Many accidents were caused by the mine structure failing through roof collapse or rock bursts (coal pillars exploding from the weight of excessive overburden). Other dangers that miners face include toxic or explosive gases released as the coal is mined, dangerous coal dust, and fires. Fires result when flammable gases trapped in the coal, such as methane, are released during mining operations and accidentally ignited.

Provision of adequate ventilation is an essential safety feature of underground coal mining. Not all of this ventilation is required to enable miners to work in comfort. Most of it is required to dilute the harmful gases, frequently termed damps, produced during mining operations.

Mine Failure

In room-and-pillar mines deep underground, the extreme weight of the overlying rock can break the pillars down, either gradually or in a violent collapse. If the mine roof or floor consists of softer material such as clay, the massive weight of the overburden can slowly push the pillars into the floor or ceiling, endangering the stability of the mine. When the mine roof and floor consist of particularly hard rock, massive overhead weight can overload the pillars, sometimes causing a spontaneous collapse or rock burst.

Gases

Because coal and natural gas form by similar natural processes, methane (a principal component of natural gas) is often trapped inside coal deposits. Gases trapped in deeper coal beds have a harder time escaping. Consequently, high-grade coals, which are typically buried deeper than low-grade coals, often contain more methane in the pores and fractures of the deposit. As coalminers saw or blast into a coal deposit, they can release these methane pockets, which may explode spontaneously, often with deadly results. Miners use a technique called methane drainage to reduce dangerous releases of methane. Before mining machinery cuts into the wall face, holes are drilled into the coal and methane is drawn out and piped to the surface.

Coalminers also risk being exposed to other deadly gases, including carbon monoxide, a poisonous by-product of partially burned coal. Carbon monoxide is deadly in quantities as little as 1 percent. It is especially prevalent in underground mines after a methane explosion. In the early 1800s, after a gas explosion, coalminers used canaries to test for carbon monoxide. If the canary died, the miners increased ventilation in the mine to remove the carbon monoxide. The miners then conducted the same test with another canary and repeated the process until a bird survived. Miners also tested for carbon monoxide and methane with a small flame. If the flame’s size increased, methane was present in the air; if the flame went out, carbon monoxide was present.

Other dangerous gases locked inside coal deposits include hydrogen sulfide, a poisonous, colorless gas with an odor of rotten eggs, and carbon dioxide, a colorless, odorless gas. To prevent injury from inhaling these gases, underground coalmines must be sufficiently ventilated. To do this fresh air is continuously pumped through ventilation holes to push out or dilute dangerous gases and provide air for miners to breathe.

Coal Dust

As coal is blasted, shredded, and hauled in a mine, large amounts of coal and silica dust are produced. Coal dust is extremely flammable, and if ignited it can be more violently explosive than methane. Miners can reduce the buildup of coal dust by injecting pressurized water into coal beds before the coal is blasted or cut, by spraying water at all points where dust is likely to be formed, and by installing dust extraction units at strategic points.

Miners who inhale coal dust over a prolonged period can damage their lung tissue. Often, these miners develop spots, lumps, or fibrous growths in their lungs, a condition known as black lung disease, a type of pneumoconiosis. Furthermore, black lung disease can develop into other, often fatal illnesses, including heart disease, emphysema, and cancer. To protect miners from black lung, many mines are equipped with coal-dust filtering units. Miners who operate drilling, cutting, or loading machinery should wear masks at all times.

Fires

Coalmine fires can be triggered during routine mining operations. For example, sparks generated by mining equipment can ignite explosive gases, coal dust, and even the coal bed itself. Because coal beds provide an almost inexhaustible fuel source, once a coal seam is ignited, it can be extremely difficult to extinguish. The intense heat generated by burning coal can rupture the overlying rock strata, sometimes causing the roof to collapse. Uncontrollable fires in some coal deposits have continued burning for years, posing a danger to local communities.

Regulations to prevent and control mine fires have been enacted in the United States and other countries. The U.S. regulations require mining companies to install fire-fighting equipment and automatic fire-suppression systems in mines. Internal-combustion engines, which power mining machinery, such as trucks, shuttles, tractors, and scoops, reach high temperatures that can pose a fire hazard. As a result, this equipment must be designed and operated according to the guidelines of the U.S. Mine Safety and Health Administration.

VIII

POSTMINING PROCESSES

Once the coal has been mined, it must be prepared for commercial use and then either stored or transported.

Coal Preparation

As-mined coal, also known as run-of-mine coal, often contains unwanted impurities such as rock and dirt and comes in a mixture of different-sized fragments. Thus, another sequence of processes is necessary to make the coal consistent in quality and suitable for selling. These processes are called coal preparation or coal cleaning. Effective preparation of coal prior to combustion improves the homogeneity of the coal supplied, reduces transport costs, improves the burning efficiency, and produces fewer pollutants.

Coal preparation is the stage in coal production when the run-of-mine coal is processed into a range of clean, graded, and uniform coal products suitable for the commercial market, mostly power plants. In some cases, the run-of-mine coal is of such quality that it meets the user’s specifications without the need for preparation, in which case the coal would merely be crushed and put through a large sievelike device to deliver the specified product.

A number of physical separation technologies are used in cleaning and preparing coal. After the raw run-of-mine coal is crushed, it is separated into various-sized fragments for optimum treatment. Larger material— lumps of coal about 10 to 150 mm (0.4 to 6 in) in length—is usually treated using a technology known as dense-medium separation. The dense medium is usually a liquid with a density just slightly greater than that of the coal. The coal can then be separated from other impurities, such as rock, by being floated in a tank containing the high-density liquid, which is usually a suspension of finely ground magnetite. Because the coal is lighter, it floats and is separated off, while heavier rock and other impurities sink and are removed as waste. Any magnetite mixed with the coal is separated using water sprays and is then recovered, using magnetic drums, and recycled.

The smaller-sized fragments are treated in a variety of ways. In the froth flotation method, coal particles are removed in a froth produced by blowing air into a water bath containing a chemical reagent (substance that takes part in a chemical reaction with another substance). The bubbles attract the coal but not the waste and are skimmed off to recover the smaller-sized fragments. After treatment, the smaller-sized fragments are screened and either dewatered or dried, and then recombined before going through final sampling and quality control procedures. Recombination also enables customers to selectively purchase different grades of coal. More expensive, higher quality supplies can be carefully mixed with lower quality coals to produce an average blend suited to the needs of commercial customers, such as power plants.

Transport and Storage

Once the coal has been extracted, it is moved from the mine to the power plant or other place of use or it is stored. Over short distances coal is usually transported by conveyor or truck. For long distances trains, barges, ships, or pipelines are used. Preventive measures are taken at every stage during transport and storage to reduce potential environmental impacts. Dust can be controlled by using water sprays, compacting the coal, and enclosing the stockpiles. Sealed systems can be used to move the coal from the stockpiles to the combustion plant. Well-designed coal-storage facilities can limit the problem of contaminated water run-off. All water is carefully treated before reuse or disposal.

ENVIRONMENTAL ISSUES

Because significant volumes of earth must be displaced to mine coal, coalmines and the resulting rock waste can harm the environment. Furthermore, burning coal releases environmentally harmful chemical compounds into the air.

Mining and Mining Waste

Coal Strip Mine

This excavated area, which borders farmland, has been strip mined for coal. Unless the strip mine is filled and revegetated, surface water runoff from the mined area can flush sediments and sulfur-bearing compounds (acid drainage) into nearby streams and rivers, endangering plant and wildlife communities.

Charles E. Rotkin/Corbis

Surface mining has resulted in a great deal of damage to the landscape. Many surface mines have removed acres of vegetation and altered topographic features, such as hills and valleys, leaving soil exposed for erosion. Longwall mining, which allows the mine to collapse, results in widespread land subsidence, or sinking. Coal and rock waste, often dumped indiscriminately during surface and underground mining processes, weathers rapidly, producing acid drainage. Acid drainage contains sulfur-bearing compounds that combine with oxygen in water vapor to form sulfuric acid. In addition, weathering of coalmine waste can produce alkaline compounds, heavy metals, and sediments. Acid drainage, alkaline compounds, heavy metals, and sediment leached from mine waste into groundwater or washed away by rainwater can pollute streams, rivers, and lakes.

Today, enterprises in many countries must secure government permits before mining for coal. In the United States, mining companies must submit plans detailing proposed methods for blasting, road construction, land reclamation, and waste disposal. New land reclamation methods, driven by stringent laws and regulations, require coal mining companies to restore strip-mined landscapes to nearly premined conditions.

Burning Coal

The burning of coal produces environmentally harmful emissions. Some gases produced from burning coal, such as carbon dioxide, are known as greenhouse gases because they trap the Earth’s heat like the roof of a greenhouse and may contribute to global warming. Other emissions from coal combustion can lead to air and water pollution.

Greenhouse Effect

Earth absorbs much of the heat energy radiated from the Sun. The planet then reradiates this heat back into the atmosphere. Carbon dioxide and some other gases that are naturally present in the atmosphere prevent much of the heat from escaping back into space, maintaining Earth at a temperature that can support life. These gases are known as greenhouse gases because they trap the Sun’s heat in much the same way as the glass roof of a botanical greenhouse (see Greenhouse Effect). However, the immense quantity of fossil fuels burned during the world’s rapid industrialization over the last 200 years has raised levels of carbon dioxide in the atmosphere by about 28 percent. This dramatic increase in atmospheric carbon dioxide, coupled with continuing depletion of the world’s forests, which absorb carbon dioxide, has led many scientists to predict a heating of the atmosphere on a global scale. Such a global warming could disrupt weather patterns, cause the polar ice caps to melt, and possibly lead to other environmental problems.

Today, many industrial countries are working to reduce emissions of greenhouse gases. One proposal is to establish a system requiring companies that create greenhouse gases to pay to emit carbon dioxide above a specified level. This payment could take several forms, including (1) purchasing the rights to pollute from a company with carbon dioxide emissions below the specified level; (2) purchasing forests, which absorb carbon dioxide, and keeping them from being developed; or (3) paying to upgrade a plant in a developing country, thus lowering that plant’s carbon dioxide emissions.

Acid Rain

Anatomy of an Air Scrubber

A venturi air scrubber removes polluting particles from gas emissions by spraying a scrubber liquid directly into the emissions. The scrubber liquid surrounds the dirty particles, which are carried with the gas emissions into the separator cylinder. As the gas cycles upward through the cylinder, the liquid-covered particles drop from the gas into the contaminated liquid reservoir.

The U.S. Clean Air Act, implemented in 1970 and revised in 1970 and 1990, is the federal law regulating air pollution in the United States. This legislation has significantly reduced emissions of sulfur oxides, known as acid gases (see Air Pollution). For example, the Clean Air Act requires facilities such as coal-burning power plants to burn low-sulfur coal. High-grade coals (coals with a higher heating value) generally contain more sulfur than low-grade coals such as lignite and subbituminous coal. Therefore, certain processes have been developed to remove sulfur-bearing compounds from high-grade coal prior to burning. The Clean Air Act also requires use of pollution-trapping equipment such as air scrubbers (devices installed inside plant smokestacks to remove sulfur dioxide from coal emissions). In addition, revisions to the Clean Air Act in 1990 established a system that allows coal-burning power plants to buy and sell sulfur emission permits with one another. This system tries to establish a financial incentive to lower sulfur emissions by rewarding power plants that reduce emissions below federal levels. Power plants that cut their sulfur emissions below the permitted levels can sell permits to burn coal to companies that exceed federal levels. Companies that reduce emissions reap financial rewards while polluters must pay an extra cost to pollute.

Fly Ash

The burning of coal releases ashes known as fly ash into the atmosphere. Fly ash contains toxic metals such as arsenic and cadmium. In the United States the Clean Air Act requires that fly ash be removed from coal emissions. As a result, antipollution devices such as air scrubbers, baghouses, and electrostatic precipitators are used to trap these pollutants. Baghouses work like giant vacuum cleaners, drawing coal emissions through giant fabric bags that trap the fly ash inside. Electrostatic precipitators use discharge electrodes (electrically charged parts of an electric circuit) to trap ash particles. In an electrostatic precipitator the electrodes are located between long, positively charged collection plates. As the fly ash passes between these collection plates, the discharge electrodes give each particle a negative charge. These negatively charged particles are then attracted to and held by the positively charged collection plates.

CLEAN COAL TECHNOLOGY

Since 1986 the United States government and private industry have been working together to develop cleaner and more efficient ways to harness the energy in coal. This joint effort, known as the Clean Coal Technology Demonstration Program, includes several technologies, such as fluidized bed coal combustion, furnace sorbent injection, and advanced flue-gas desulfurization.

Fluidized bed coal combustion burns coal in a limestone bed that transfers heat to water, generating steam. This steam is pressurized and used to turn a turbine shaft, which subsequently drives an electric generator. The limestone absorbs sulfur dioxide emitted by the coal, thus reducing the amount of acid gases released during combustion.

A process called furnace sorbent injection removes acid gas from coal emissions at less cost than expensive scrubbers. A sorbent is a highly absorbent material, such as powdered limestone. It is injected into furnaces, where the powdered limestone reacts with the acid gases emitted by the burning coal. The used powder is siphoned away through the furnace outtake and is captured (with fly ash) in a baghouse or electrostatic precipitator.

A process called advanced flue-gas desulfurization also removes acid gas from burning coal without expensive scrubbers. Emissions from burning coal are piped into a container called an absorber, where the acid gases react with an absorbing solution (such as a mixture of lime, water, and oxygen). This reaction forms gypsum, a soft white mineral valuable as an ingredient in cement.

HISTORY OF COAL USE

Early humans used wood, straw, and dried dung for fuel. One of the earliest known references to coal was made by Greek philosopher and scientist Aristotle, who referred to charcoallike rock found in Thrace (a region on the northeastern shore of the Aegean Sea) and in northeastern Italy. Although authentic records are unavailable, historians believe coal was first used commercially in China. Reports indicate the Fu-shun mine in northeastern China provided coal for smelting copper and for casting coins around 1000 bc.

Coal cinders found among Roman ruins in England suggest that the Romans harnessed energy from coal before ad 400. The written records of the monk Reinier of Liège from the early 13th century describe workers mining black earth in Europe. Blacksmiths used this black earth as fuel for metalworking. Other historical records contain numerous references to coal mining in England, in Scotland, and in continental Europe throughout the 13th century.

In the early 18th century the demand for coal escalated when English iron founders John Wilkinson and Abraham Darby used coal, in the form of coke, to manufacture iron. An almost constant demand for coal was created by metallurgical and engineering developments, most notably the invention of the coal-burning steam engine by Scottish mechanical engineer James Watt in 1769.

Until the American Revolution (1775-1783), most of the coal consumed by the American colonies was imported from England or Nova Scotia. Wartime shortages and the need to manufacture munitions spurred the formation of small American coal-mining companies that mined Virginia’s Appalachian bituminous field and other deposits. By the early 1830s U.S. mining companies had emerged throughout the Appalachian region and along the Ohio, Illinois, and Mississippi rivers. The construction of the first practical locomotive in 1804 in England by British engineer Richard Trevithick sparked a tremendous demand for coal. The growth of the railroad industry and the subsequent rise of the steel industry in the 1800s spurred enormous growth in the coal industry in the United States and Europe.

The widespread use of petroleum as a fuel before, during, and after World War I (1914-1918) eventually reduced the demand for coal. The change from coal to oil as fuel in warships (particularly in the United States and British navies) in the early 1900s, the switch in the railway industry to diesel-electric locomotive engines in the 1940s and 1950s, and increasing use of natural gas as a heating fuel all contributed to a decline in coal production. In the 1980s and 1990s, petroleum continued to supplant coal in industry and was increasingly used in oil-fired power plants. Still, electric utilities continued to burn large amounts of coal to produce electricity.

Contributed By:
James Speight

Microsoft Encarta

Jumat, 02 Januari 2009

All about cell

All about coal

About Me

Blog Archive

Labels

Friends

Followers

Shoutmix

Microsoft Encarta

Jumat, 02 Januari 2009

All about cell

All about coal

About Me

Blog Archive

Labels

Friends

Followers

Shoutmix

Feed