Fungus

Fungi (singular, fungus) are everywhere in the environment—in the soil; in lakes, rivers, and the seas; in the air (some are so tiny that they are carried by currents of wind or on the bodies of insects); and in and on plants and animals, including humans. Along with bacteria, fungi are responsible for the decay of organic matter and the release into the atmosphere of carbon, oxygen, nitrogen, and phosphorus. Many of them are eaten at the dinner table.

One of the most beneficial uses of a fungus came with the virtually accidental discovery of the antibiotic penicillin by Sir Alexander Fleming in 1928. Antibiotics, many of them derived from fungi, helped revolutionize the practice of medicine in the 20th century Not all fungi are beneficial, however. Some can cause serious diseases in plants and wreak havoc on whole segments of an agricultural economy. One of the best-known instances of fungus devastation in the 20th century was the destruction of elm trees in Europe and the United States by Dutch elm disease. The fungus responsible, Ceratocystis ulmi, probably arrived in Europe from Asia about the time of World War I. By the 1930s it had spread throughout Europe and Great Britain and killed thousands of trees. It appeared in the United States in 1930 and has since destroyed millions of elm trees. Overland spread of the disease normally occurs through transmission by elm bark beetles.
The word fungus (plural, fungi) is Latin for mushroom, and indeed, mushrooms are among the most commonly known fungi. For many years, most people—including scientists—considered fungi to be plants, mainly because, like plants, fungi do not move. Fungi also lack such complex plant structures as roots, stems, leaves, and flowers, though these traits are shared by some of the more primitive plants, such as mosses and liverworts. However, fungi lack some of the most important characteristics of plants. For example, fungi do not have chlorophyll and thus cannot undergo photosynthesis, which is a key trait all plants have in common. This factor, along with several other characteristics, led scientists to place the fungi into their own kingdom. Included within the kingdom Fungi, along with mushrooms, are molds, mildews, rusts, smuts, truffles, and yeasts. A general scientific term for fungi is mycota, from the Greek word for mushroom, mykes, and the study of these organisms is called mycology. Scientists estimate there are probably 1.5 million species of fungus worldwide, though only about 70,000 species have been described.

The Structure of Fungi

Although fungi are not uniform in appearance—a mushroom, for example, has a cap and stem while common bread mold grows in a thick mat—all fungi have similar structural elements. In most fungal species, the organism's cells are joined in long strands, or filaments, which are called hyphae (singular, hypha). The system of hyphae produced by an individual fungus may be extensive, with the hyphae accumulating in thick mats. The latter are called mycelia (singular, mycelium). What we see as mold on a piece of fruit or bread is actually the mycelium of the fungus that has colonized the food. The mycelium is considered the thallus, or body, of the fungus.
The yeasts are an exception to this, however. Although they are members of the fungi kingdom, yeasts do not share the same structural elements as most other fungi. Yeast cells do not adjoin one another to form hyphae but rather exist as individual unicellular organisms. Because of this difference, the yeasts are classified in a separate taxon, or group, within the fungi kingdom, while the remaining taxa are sometimes referred to as the “true” fungi.
In order to grow, the fungal mycelium uses the organic matter, either living or dead, in its environment. As the mycelium matures, it forms spores. These are seedlike reproductive bodies, each normally consisting of one cell, that become detached from the parent fungus and start new organisms. As the spore grows, it develops into a hypha that branches out and eventually forms the mycelium of a new fungus. In some fungi the spores may be produced directly by any portion of the mycelium; in others, such as the mushroom, they are formed in a special fruiting section, such as the mushroom cap. This section, normally the only visible or most visible section of the fungus, is called the sporophore.

Where Fungi Live

Fungi are very widely distributed throughout the world, particularly in the temperate and tropical regions where there is sufficient moisture for them to grow. They are less likely to be found in dry areas. Some few types of fungi have been reported in Arctic and Antarctic areas (some molds, after all, thrive on refrigerated food).
Fungi live both on land and in the water. Only a small portion of terrestrial fungi is normally seen above the ground. Most of the fungus consists of the complex network of hyphae, which grows just beneath the surface of the ground. The visible parts of fungi vary greatly in size. Some are so tiny that they cannot be seen without the aid of magnification. Others are quite large. Some mushrooms reach diameters of 8 to 10 inches (20 to 25 centimeters) and heights of 10 to 12 inches (25 to 30 centimeters). Bracket fungi that are 15 inches (38 centimeters) in diameter are fairly common; and mushrooms called puffballs have been known to grow to 60 inches (152 centimeters) in diameter. In 1992 scientists announced the discovery of a giant underground fungus, Armillaria ostoyae, that covered 1,500 acres (600 hectares) in Washington State. The only visible signs of its existence were aboveground mushrooms and a rot deadly to trees. In 2000 a larger Armillaria was found. This individual, which was discovered in the Blue Mountains of Oregon, covers 2,200 acres—about the size of 1,665 football fields. Experts estimate that it is at least 2,400 years old, making it not only the largest but also the oldest living organism on Earth.
Although fungi are distributed worldwide, the distribution of a specific species is limited by the temperature and moisture conditions of an area coupled with the available food supply. The best temperature for most fungi to thrive is from 68° to 86° F (20° to 30° C). Some types of fungi, however, do perfectly well at temperatures as high as 120° F (48° C), while a fairly large number of them do well at freezing temperatures, 32° F (0° C) or below.

How Fungi Reproduce

The reproduction of fungi can be either sexual or asexual. Sexual reproduction, as with other organisms, involves the fusion of two nuclei when two sex cells unite. This joining produces spores that can grow into new organisms. However, the majority of fungi reproduce asexually. The simplest asexual process is direct fragmentation, or breaking up, of the fungus body, or thallus. Each of the fragments develops into a new individual organism if environmental conditions are favorable. Such fragmentation usually is the result of outside natural forces. Some yeasts reproduce by simple cell division, wherein a yeast cell divides into two new yeast cells. Other yeast species reproduce by a method called budding: a bud develops on the surface of the yeast cell, after which the nucleus of the cell divides into two. After one of the nuclei moves into the bud, it is capable of starting a life of its own. Some species of the true fungi also reproduce via this method.

How Fungi Obtain Food

Because they lack chlorophyll, fungi are unable to manufacture food out of the raw materials around them as plants do. Thus fungi are categorized as heterotrophic—they must get nutrition from other organisms. Some fungal species get their food from living organisms, a process that may harm the host or benefit it. The vast majority of fungi obtain their nutrients from dead plant or animal matter. By doing this, fungi are among the organisms that serve as decomposers—an essential role in the natural cycle of ecosystems
To obtain nutrients, a fungus secretes enzymes into the living or dead organism on which it grows. The enzymes digest the material, which is then absorbed through the walls of the hyphae. A common example of this action is the rotting of fruits such as peaches or apples. The brown, softened area on the fruit has been subjected to the enzyme secretions of the hyphae. Some fungi, more specialized in their food-absorbing techniques, produce tiny hyphae called rhizoids. These rootlike structures anchor the fungus to its food source and probably also absorb food. Other fungi produce absorptive structures called haustoria, another type of hypha outgrowth.

Saprophytic Fungi

Because they attack only dead organic matter, saprophytes are in good measure responsible for the decomposition of much plant and animal residue in natural ecosystems. In a forest, saprophytic fungi may be found growing on matter as varied as fallen trees, animal droppings, and dead leaves. These fungi play a vital role in the natural community—without them, the forest floor would accumulate enormous piles of dead matter.
In addition to the saprophytes living in nature, there are many such species that strike much closer to home, including those that attack foodstuffs such as bread, processed meat and cheese, and picked fruits and vegetables. Some saprophytes are responsible for the destruction of timber, textiles, paper, and leather. Most saprophytes need oxygen in order to survive and feed. Some few, such as those that cause fermentation, can survive without it.

Parasitic Fungi

Fungi classified as parasites attack living organisms in order to obtain nutrients, and in doing so cause illness or death to the organism being attacked . The fungi in this group are a leading cause of disease in plants. Some of the most common include the downy mildew found on grapes, onions, and tobacco, and the powdery mildew that infests grapes, apples, cherries, lilacs, peaches, and roses. Cereal grains such as corn and wheat are often plagued with smut, while rust can devastate crops from wheat, oats, and beans to asparagus and flowers.
A number of parasitic fungi cause diseases in animals, including humans. Some of these illnesses, such as ringworm and athlete's foot, are fairly benign and self-limiting. A number of fungal organisms, such as Aspergillis and the yeast Candida, cause illnesses ranging from mild to severe depending upon what organ or part of the body is infected. For example, Candida can cause the mild infection commonly known as thrush when it colonizes tongue and throat tissue. If the same organism gets into the bloodstream, however, it can be carried to the joints or to vital organs such as the liver or spleen, where the resulting illness can be severe and even be fatal in some circumstances. Among the most dreaded fungal diseases in humans and animals are the systemic mycoses. These diseases are caused by fungi that initially colonize the lungs but can soon spread to other organs. The organisms responsible for this type of illness are Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides immitis, and Cryptococcus neoformans.
One serious fungus-caused disease that may attack people and animals is ergotism. The fungus ergot develops on grasses, especially on rye. It contains a number of poisons called alkaloids. If the grain is harvested and the ergot is not removed, it will get into bread made from the rye and cause ergotism—also known as St. Anthony's fire—for which there is no known cure. The disease may infect cattle that eat the rye grains left in a field. Ergot also contains lysergic acid, the principle active agent in the drug LSD (lysergic acid diethylamide). The fungus does have some positive uses, however. It has been used to develop medicines that induce labor in pregnant women and curtail hemorrhaging after birth.

Mycorrhizal Fungi

The mycorrhizal fungi form a close, mutually beneficial relationship with certain trees and plants. The hyphae of these fungi grow in networks surrounding the plant and tree roots, allowing the plant to extract certain minerals and other nutrients from the fungus. The fungus in turn obtains moisture and carbohydrates from the tree. Although these tree and plant species can survive without the association, studies have shown that the presence of mycorrhizae helps the plants to grow better relative to similar plants without the benefit of a mycorrhizal partner. Associations such as these, where both partners benefit from the relationship, are called mutualisms

Other Plant and Animal Associations

There are several other examples of symbioses between fungi and plants or animals. Lichens, for example, are combinations of fungi and algae living in such close association that they seem to be a single plant form .
Certain scale insects embed themselves in the bark of trees and remain there sucking sap from the tree for the rest of their lives. A type of fungus will spread itself in a network over the bark of the tree, covering the insects and feeding off them, without killing them. This is a case of double parasitism: the insects live off the tree and the fungus off the insects, both to the disadvantage of the tree.
Types of fungi called sooty molds live on the surfaces of plants in association with scale insects. They do not live as parasites on either the insects or the plants, but they obtain nourishment from the secretions of the insects. The extensive growth of the mold's mycelium, however, may prevent light from reaching the plant's surface and thus cause it to die because photosynthesis is inhibited.
An interesting kind of reverse parasitism occurs in certain ant colonies. The so-called leaf-cutter ants, which are members of the genera Acromyrmex and Atta, cultivate fungi in their underground colonies by feeding the organisms tiny bits of leaves. The ants themselves feed entirely on the fungi.

Some Familiar Fungi

A casual walk through a forest is sufficient to bring one into contact with many familiar fungi, such as varieties of mushrooms and the familiar bracket fungus that grows on wood. Other types of fungi are brought to one's attention largely in association with food: the mold that covers stale bread, the yeast that is used for baking or brewing, and the mushrooms and truffles that are available in supermarkets or offered as delicacies in many restaurants.

Molds

The fuzzy substance we see growing on old bread or fruit is mold, a mass of mycelia sprouting fruiting bodies that are often not immediately visible to the naked eye. The molds we see on dead organic matter such as old food are saprophytic. Many of these molds belong to the genera Aspergillus, Penicillium, and Rhizopus.

Mushrooms

This group of fungi are conspicuous because of the umbrella-shaped fruiting body that grows above the ground. Many mushrooms are edible but some are not. Some people use the term toadstool for poisonous mushrooms, but botanists make no such distinction. Among the mushrooms are the puffballs and earthstars, which grow in soil or on rotting wood in forests and grassy areas. Many of these are edible while young. When they mature, they dry out and become powdery inside. The largest of the puffballs, Calavatia gigantea, may be as large as 4 feet (120 centimeters) or more across. The earthstars are so named because, in addition to the puffball effect, they have a leaflike expanded base that resembles a star. Since some mushrooms are poisonous, only an expert in mushroom identification should collect mushrooms that are intended for people or animals to eat.

Truffles

For centuries this fungus, which grows underground, has been prized as a food delicacy. Truffles are saprophytes and grow in association with the roots of trees, particularly oaks. They range in size from the size of a pea to as large as an orange. Three countries are famed for their truffles: France, Italy, and England. Mature French truffles are black with white veins; Italian truffles are white; and English truffles are either black or brown, depending on the species. Because truffles grow underground, they are not always easy to locate, so hunting them is usually carried out with the aid of pigs or dogs, both of which have a more developed sense of smell than humans.

Yeasts

The yeasts are a group of unicellular fungi. Some yeasts are commercially significant because they are used in baking, brewing, and fermentation. Yeast does its work primarily by interacting with the carbohydrates (sugar and starches) in either dough for bread or liquid for brewing and fermentation Brewers' yeast has long been considered nutritionally useful to humans because it contains a high quantity of B vitamins. Some yeasts cause decay in fruits and vegetables, both of which are a ready source of carbohydrate which the yeast can metabolize via fermentation. Because of their chromosome structure and the ease with which the organisms can be maintained in the laboratory, yeasts have also served as an important tool in genetic and medical research

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Mars

As it is visible from Earth without the aid of a telescope, the planet Mars has intrigued stargazers since ancient times. Babylonians mentioned Mars in records from about 3,000 years ago, associating the red planet with their god of death. The modern name Mars is that of the Roman god of war; the planet's two moons, Phobos (Fear) and Deimos (Terror), are named after the sons of Ares, the Greek god of war.

Mars passes relatively close to the Earth in its orbit. In addition, it generally has no obscuring layer of clouds, unlike the Earth's other neighbor, Venus. Thus Mars is a nearly ideal subject for telescopic observation. Over the centuries observers have noted various phenomena on the planet's surface, including a seasonal growing and shrinking of the polar caps and seasonal changes in the appearance of dark markings. The explanation of many early observations had to await the exploratory space missions by the United States and Soviet Union during the 1960s and 1970s.

Planetary Data

The fourth planet from the sun, Mars is the outermost of the terrestrial, or Earth-like, planets, which are the dense, rocky worlds closest to the sun. Because Mars has an elliptical orbit, its distance from the sun varies, from about 129 million miles (207 million kilometers) at the closest point in its orbit, or perihelion, to some 155 million miles (249 million kilometers) at its farthest point, or aphelion. The planet completes one revolution around the sun in about 687 Earth days, almost twice the time it takes the Earth to complete its orbit of about 365 days. Its distance from the Earth varies considerably, from less than 35 million miles (56 million kilometers) to nearly 250 million miles (400 million kilometers). The best time to view Mars from Earth is when it is at its closest to both the sun and the Earth so that it appears both bright and large.
With a mean diameter of about 4,219 miles (6,790 kilometers), Mars is the third smallest planet in the solar system. It is about half the size of Earth and is much less dense. Its gravity is about a third of the Earth's, its surface area about a fourth, and its mass only about a 10th. Like the Earth, Mars is roughly spherical, with a slight bulging at its equator and flattening at its poles.
Mars rotates on its axis at roughly the same rate as Earth; a Martian day, called a sol, lasts about 24.7 hours. The red planet is also tilted on its rotational axis at an angle similar to that of the Earth. Consequently, like Earth, it is subject to seasonal variations in climate as first one hemisphere and then the other receives more sunlight during the planet's orbit around the sun. Because of its more elliptical orbit, the seasons on Mars are not as even as they are on Earth. The spring and summer in the north, for example, last about 382 days, or more than half the 687-day year. In the south the summer is shorter.

Atmosphere, Surface, and Interior

The Martian atmosphere is composed mostly of carbon dioxide and is very thin, exerting about ¹/₁₀₀ the surface pressure that the Earth's atmosphere exerts. The thin atmosphere on Mars does not insulate the planet as well as the thicker one does on Earth. The surface of Mars is thus colder than Earth's would be if the two planets were the same distance from the sun. The temperature at the Martian surface varies widely during the course of a day, from about −118 ° F (−83 ° C) just before dawn to about −28 ° F (−33 ° C) in the afternoon. The atmosphere also does not shield the surface from ultraviolet radiation from the sun. This intense radiation bombardment is one reason why scientists believe that no living things currently exist on the surface of Mars.The Martian surface is dry and dusty. The question of whether liquid water has existed on the surface of Mars is of particular interest to scientists trying to determine if life ever existed on the planet. Liquid water is a requirement for all known forms of life. (But of course water does not in itself indicate the presence of living things.) Water currently exists on Mars as ice deposited at the poles, as ice trapped below the surface, and as vapor in the atmosphere. Rivers, lakes, and seas may have been present on Mars in its remote past, when temperatures may have been warmer and the atmospheric pressure higher. Because of the current low temperatures and pressure, it has been thought that liquid water has not existed at or near the surface in modern times. Recent findings, however, have prompted a reassessment of this view. Photographs taken in 2000 by the orbiting spacecraft Mars Global Surveyor showed hundreds of gullies that seemed to have formed relatively recently. Some planetary scientists believe that the gullies were carved by flowing water. They theorize that, episodically, small amounts of liquid water have flowed on and just below the surface in geologically recent times and that liquid water may still be present in parts of the planet's subsurface. But this theory has been disputed.

 Also challenged by recent findings was the view of Mars as a geologically “dead” planet on which volcanic activity had last occurred a few billion years ago. Data from the Mars Global Surveyor indicates that volcanoes may have erupted some 40 million years ago, which is considered recent in geologic time. The planet has the largest volcano in the solar system, Olympus Mons. At a height of 17 miles (27 kilometers), the volcano is three times higher than Earth's Mount Everest and covers an area the size of the state of Arizona. It sits on the Tharsis Plateau, a broad, elevated plain dotted with large volcanoes and fractures. The largest fracture system is Valles Marineris, a huge valley about 2,500 miles (4,000 kilometers) long and varying from 2¹/₂ to 6 miles (4 to 10 kilometers) in depth. The Tharsis Plateau may have been formed by a rising plume of hot mantle material, possibly accompanied by plate tectonic activity.

Other regions on Mars include smooth plains, densely cratered areas, mesas, and rolling hills formed by various combinations of fracturing, volcanism, and atmospheric-related erosion and deposition. The planet's hemispheres show striking geologic differences, with ancient, heavily cratered highlands in the south and younger, flat lowlands in the north.
Dark markings have been observed on the Martian surface for hundreds of years. These markings cover about a third of the surface and change in a seasonal pattern in both extent and color. Once thought to be vast seas or vegetation, the dark areas are now known to result from the accumulation of dust, which shifts along with seasonal winds.
Dust storms occur frequently on the planet, especially in the southern hemisphere in spring and summer. About every two or three years, Mars is engulfed by global dust storms. Local temperature differences generate strong winds that lift the dust from the surface to form thick clouds. The clouds block the sunlight, gradually causing the surface temperatures to even out and the winds to subside. Some of the atmospheric dust is deposited in a snowfall of dust and ice in the polar regions.
Ice caps form over both the north and south poles according to seasonal changes. Each cap grows larger when its hemisphere experiences fall and shrinks during the spring. During summer, most of the ice cap melts. The south pole's ice cap is larger. Its permanent ice cap, the portion that survives the summer melting, is covered with mesas, holes, and troughs and appears to be formed of carbon dioxide ice. Smaller pits and cracks mark the flatter permanent cap at the north pole, which is made of water ice.Scientists do not have direct information about the Martian interior, but they have developed a model based on the planet's known characteristics, such as its size, mass, gravity signature, and surface elevations. Mars most likely has a metal-rich core with a diameter of about 2,100 miles (3,400 kilometers). Surrounding the core is a molten rocky mantle that is probably more dense than the Earth's mantle. The thickness of the Martian crust is thought to vary from about only 2 miles (3 kilometers) in some places to more than 60 miles (90 kilometers) in others.
Unlike most of the other planets (except for Venus and maybe Pluto), Mars has no global magnetic field, though there are indications that the planet once had a strong field. Ancient rocks in the crust of the southern hemisphere are highly magnetized in striped patterns that suggest that the planet's magnetic field may have reversed polarity several times, as has the Earth's. The rocks are about 10 times as magnetized as any on Earth.

Satellites

Mars has two small satellites, Phobos and Deimos, which may be captured asteroids. Both are so small that they do not have enough internal gravity to draw them into spherical shapes; instead, they are shaped more or less like potatoes. Phobos is about 17 miles (27 kilometers) long; Deimos is about 9¹/₂ miles (15 kilometers) long. Both have rotational periods equal to their orbital periods, so that they always point the same face toward Mars. The surface of Deimos appears smooth because its craters are almost buried in regolith, a layer of fine rubble generated by repeated impacts with other bodies. Phobos is also covered with regolith, but its surface is far more rugged and very heavily cratered.Phobos is very close to Mars, and its orbit is gradually decaying, so that it is drawing closer to the planet with each orbit. Astronomers estimate that Phobos may fall to the Martian surface sometime in the next 100 million years. Deimos is in a more distant orbit and is gradually moving away from the planet.
Both satellites are very dark and are probably made of a carbonaceous chondrite material. This is a primitive substance that includes many of the first materials to precipitate out of the solar nebula during the creation of the solar system. It is found on many satellites, asteroids, and meteorites.

Observation and Exploration

For centuries astronomers have considered the possibility that life might exist on Mars, the most Earth-like of the planets. In the 1600s astronomers began to observe the planet with the aid of newly developed telescopes. In 1877 the Italian astronomer Giovanni Schiaparelli described what he believed was a system of interconnecting, straight-edged channels on the planet. He called these features canali, meaning “channels,” or “canals.” U.S. astronomer Percival Lowell thought that these features were not naturally occurring waterways but structures that had been built by an advanced but dying Martian civilization. Most astronomers could see no canals, however, and many doubted their reality. The controversy was finally resolved only when pictures sent from the United States Mariner probes showed many craters but nothing resembling manufactured channels or canals.Four of the Mariner series of unmanned space probes launched by the National Aeronautics and Space Administration (NASA) investigated Mars. The first craft to successfully fly by Mars was Mariner 4, which photographed the planet as it passed it in 1965. It was followed by Mariners 6 and 7, which analyzed the atmosphere and captured images as they flew by in 1969. The first spacecraft to orbit a planet other than Earth was Mariner 9, which circled Mars for nearly a year in 1971–72. The Soviet Union also sent a series of unmanned space probes to Mars in the 1960s and 1970s. Its Mars 3 lander, the first craft to successfully soft-land on the planet, touched down in 1971 amid a global dust storm. Its communications systems failed after about 20 seconds.
The United States Viking probes, consisting of two orbiting spacecraft and two landers, were intended in part to search for evidence of past or present forms of life on Mars. The two landers touched down on the planet in 1976 and performed numerous experiments, including a detailed chemical analysis of the Martian atmosphere and soil. No trace of any organic material was found.The next United States probe, the Mars Observer, was launched in September 1992 and was programmed to land on Mars in August 1993. The 980-million-dollar spacecraft lost contact with Earth, however, and was presumed lost in September 1993.
A team of scientists announced in 1996 that a meteorite from Mars that fell to Earth 13,000 years ago contained organic molecules, minerals, and carbonate globules that are all associated with bacterial life. The team believed that this provided the first evidence of life on early Mars. Most scientists, however, were skeptical of this claim.
In 1996 the United States launched the Mars Global Surveyor, the first in a new series of unmanned spacecraft designed to explore the planet. The probe began to orbit Mars in September 1997. After a delay due to an equipment malfunction, in March 1999 the craft began mapping a variety of the planet's properties, including its gravity and magnetic fields and the topography and mineral composition of the surface. It also took more than 100,000 photographs of the surface.
In 1997 the next Martian mission—the Pathfinder—landed on Mars to study the geology and atmosphere of the planet. On board the Mars Pathfinder was a small roving vehicle called Sojourner that collected and analyzed samples of the Martian soil and beamed images back to Earth. Evidence collected during Pathfinder's 83 days of surface operations indicated that the surface of Mars bears signs of an ancient Earth-like atmosphere and geology. Water-worn rock conglomerates and other sand and surface features that could only have been created by flowing, liquid water mark the surface of the planet. A tremendous amount of differentiation—heating, cooling, and recycling of crust material—appeared to have taken place at some point in the planet's history.Mars exploration suffered two major setbacks in the late 1990s. NASA's unmanned Mars Climate Orbiter, launched in December 1998, was designed to transmit daily weather images and other atmospheric data for a full Martian year, or 687 days. However, confusion of English and metric units in key navigation figures sent the craft off course as it attempted to enter orbit around Mars in September 1999. Less than three months later, NASA lost contact with another craft, the Mars Polar Lander, as it approached touchdown on the Mars surface.
Another global mapping orbiter, NASA's 2001 Mars Odyssey, reached the planet in October 2001. In addition to mapping the chemical composition of the surface, the orbiter confirmed the presence of water ice in the subsurface and revealed its distribution. Along with the Mars Global Surveyor, Odyssey also acted as a communications relay for two robotic wheeled rovers, Spirit and Opportunity. NASA's twin rovers landed on Mars in January 2004 to collect geologic data to help determine whether the planet's environment was suitable for life in the past. Spirit landed in Gusev Crater, which some scientists believe may have been an ancient lake bed. Opportunity arrived at Meridiani Planum. In the gravel at its landing site, Opportunity confirmed the presence of gray hematite, a mineral that on Earth usually forms in association with water.
The European Space Agency (ESA) sent its first mission to Mars in 2003. The orbiter Mars Express, designed to map a variety of properties in the Martian atmosphere, surface, and subsurface and to photograph surface features using a high-resolution stereoscopic camera, arrived at the planet in December 2003. The orbiter carried on board the British lander Beagle 2. After being released at Mars, however, the lander failed to return communications signals and was declared lost.

 

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Vitamin

All living things, plant or animal, need vitamins for health, growth, and reproduction. Yet vitamins are not a source of calories and do not contribute significantly to body mass. The plant or animal uses vitamins as tools in processes that regulate chemical activities in the organism and that use basic food elements—carbohydrates, fats, and proteins—to form tissues and to produce energy.

Vitamins can be used over and over, and only tiny amounts are needed to replace those that are lost. Nevertheless, most vitamins are essential in the diet because the body does not produce enough of them or, in many cases, does not produce them at all.
Thirteen different vitamins have been identified by nutritionists: A, eight B-complex vitamins, C, D, E, and K. Some substances, such as carnitine and choline, behave like vitamins but are made in adequate amounts in the human body.
Vitamins were originally placed in categories based on their function in the body and were given letter names. Later, as their chemical structures were revealed, they were also given chemical names. Today, both naming conventions are used.

Daily Requirements

With a few exceptions, the body is unable to make vitamins; they must be supplied in the daily diet or through supplements. One exception is vitamin D, which can be produced in the skin when the skin is exposed to sunlight. Another vitamin, vitamin K, is not made by the human body but is formed by microorganisms that normally flourish in the intestinal tract only when green, leafy vegetables and vegetable oils are ingested.
The body's vitamin requirements are expressed in terms of recommended dietary allowances, or RDA. These allowances are the amount of essential nutrients that, if acquired daily, are considered to be sufficient to meet the known nutritional needs of most healthy persons. In the United States, the RDA values are established by the Food and Nutrition Board of the National Academy of Sciences/National Research Council (NAS/NRC). In addition, two agencies of the United Nations—the Food and Agriculture Organization and the World Health Organization— develop RDA for different, worldwide population groups.
In the past, the strength of a vitamin or the amount of the vitamin necessary to produce a certain effect in the body was often expressed in terms of international units, abbreviated IU. The unit corresponds to a weight of the purified vitamin, and its value differs from one vitamin to another. Today, the strength of a vitamin is generally expressed directly in metric weights— micrograms or milligrams.

How Vitamins Work

In the body, proteins, carbohydrates, and fats combine with other substances to yield energy and build tissues. These chemical reactions are catalyzed, or accelerated, by enzymes produced from specific vitamins, and they take place in specific parts of the body.
The vitamins needed by humans are divided into two categories: water-soluble vitamins (the B vitamins and vitamin C) and fat-soluble vitamins (A, D, E, and K). The water-soluble vitamins are absorbed by the intestine and carried by the circulatory system to the specific tissues where they will be put into use. The B vitamins act as coenzymes, compounds that unite with a protein component called an apoenzyme to form an active enzyme The enzyme then acts as a catalyst in the chemical reactions that transfer energy from the basic food elements to the body. It is not known whether vitamin C acts as a coenzyme.
When a person takes in more water-soluble vitamins than are needed, small amounts are stored in body tissue, but most of the excess is excreted in urine. Because water-soluble vitamins are not stored in the body in appreciable amounts, a daily supply is essential to prevent depletion.
Fat-soluble vitamins seem to have highly specialized functions. The intestine absorbs fat-soluble vitamins, and the lymph system carries these vitamins to the different parts of the body. Fat-soluble vitamins are involved in maintaining the structure of cell membranes. It is also believed that fat-soluble vitamins are responsible for the synthesis of certain enzymes.
The body can store larger amounts of fat-soluble vitamins than of water-soluble vitamins. The liver provides the chief storage tissue for vitamins A and D, while vitamin E is stored in body fat and to a lesser extent in reproductive organs. Relatively little vitamin K is stored. Excessive intake of fat-soluble vitamins, particularly vitamins A and D, can lead to toxic levels in the body.
Many vitamins work together to regulate several processes within the body. A lack of vitamins or a diet that does not provide adequate amounts of certain vitamins can upset the body's internal balance or block one or more metabolic reactions.

Sources of Vitamins

Vitamins, though they are available from a variety of sources, are unevenly distributed in natural sources. For example, some vitamins, such as vitamin D, are produced only by animals, whereas other vitamins are found only in plants. (For natural sources of vitamins, see table.) All vitamins can be synthesized, or produced commercially, from foods and other sources, and there is no evidence that natural vitamins are superior to those that are synthetically derived.
Some foods are fortified with vitamins—that is, vitamins that are not normally present in the food, or that have been removed during processing, are added to the food before it is sold. Milk, for example, is fortified with vitamin D, and vitamins that have been lost from flour during processing are often replaced.
Although vitamin supplementation is generally unnecessary for otherwise well-nourished persons, there are times when the body's vitamin requirements may increase and when vitamin supplementation may be essential. Those likely to require such supplements include pregnant women, the elderly, and the chronically ill. Excessive intakes of supplemental vitamins should be avoided, however, because of the possibility of toxicity.

Kinds of Vitamins

Vitamin A,

also called retinol, is a fat-soluble vitamin that is readily destroyed upon exposure to heat, light, or air. The vitamin has a direct role in vision and is a component of a pigment present in the retina of the eye. It is essential for the proper functioning of most body organs and also affects the functioning of the immune system.
Vitamin A deficiency results in various disorders that most commonly involve the eye and the epithelial tissues—the skin and the mucous membranes lining the internal body surfaces. An early symptom of vitamin A deficiency is the development of night blindness, and continued deficiency eventually results in loss of sight. If deficiency is prolonged, the skin may become dry and rough. Vitamin A deficiency may also result in defective bone and teeth formation.
Excessive intake of vitamin A causes a toxic condition. The symptoms may include nausea, coarsening and loss of hair, drying and scaling of the skin, bone pain, fatigue, and drowsiness. There may also be blurred vision and headache in adults, and growth failure, enlargement of the liver, and nervous irritability in children.

Vitamin B complex

consists of several vitamins that are grouped together because of the loose similarities in their properties, distribution in natural sources, and physiological functions. All the B vitamins are soluble in water. Most of the B vitamins have been recognized as coenzymes, and they all appear to be essential in facilitating the metabolic processes of all forms of animal life. The complex includes B₁ (thiamine), B₂ (riboflavin), niacin (nicotinic acid), B₆ (a group of related pyridines), B₁₂ (cyanocobalamin), folic acid, pantothenic acid, and biotin.
Vitamin B₁, or thiamine, helps the body convert carbohydrates into energy and helps in the metabolism of proteins and fats. Vitamin B₁ deficiency affects the functioning of gastrointestinal, cardiovascular, and peripheral nervous systems. Beriberi and Wernicke-Korsakoff syndrome (often seen in alcoholics) are the primary diseases related to thiamine deficiency. General symptoms of beriberi include loss of appetite and overall lassitude, digestive irregularities, and a feeling of numbness and weakness in the limbs and extremities.
Vitamin B₂, or riboflavin, is required to complete several reactions in the energy cycle. Reddening of the lips with cracks at the corners of the mouth, inflammation of the tongue, and a greasy, scaly inflammation of the skin are common symptoms of deficiency.
Niacin, or nicotinic acid, helps the metabolism of carbohydrates. Prolonged deprivation leads to pellagra, a disease characterized by skin lesions, gastrointestinal disturbance, and nervous symptoms.
A form of Vitamin B₆ is a coenzyme for several enzyme systems involved in the metabolism of proteins, carbohydrates, and fats. No human disease has been found to be caused by a deficiency of this vitamin. Chronic use of large doses of vitamin B₆ can create dependency and cause complications in the peripheral nervous system.
Vitamin B₁₂, or cyanocobalamin, is a complex crystalline compound that functions in all cells, but especially in those of the gastrointestinal tract, the nervous system, and the bone marrow. It is known to aid in the development of red blood cells in higher animals. Deficiency most commonly results in pernicious anemia
Folic acid is necessary for the synthesis of nucleic acids and the formation of red blood cells. Folic-acid deficiency most commonly causes folic-acid-deficiency anemia. Symptoms include gastrointestinal problems, such as sore tongue, cracks at the corners of the mouth, diarrhea, and ulceration of the stomach and intestines. Large doses of folic acid can cause convulsions and other nervous-system problems.
Pantothenic acid promotes a large number of metabolic reactions essential for the growth and well- being of animals. Deficiency in experimental animals leads to growth failure, skin lesions, and graying of the hair. A dietary deficiency severe enough to lead to clear-cut disease has not been described in humans.
Biotin plays a role in metabolic processes that lead to the formation of fats and the utilization of carbon dioxide. Biotin deficiency results in anorexia, nausea, vomiting, inflammation of the tongue, pallor, depression, and dermatitis.

Vitamin C,

or ascorbic acid, is water-soluble and easily destroyed. It is essential in wound healing and in the formation of collagen, a protein important in the formation of healthy skin, tendons, bones, and supportive tissues. Deficiency results in defective collagen formation and is marked by joint pains, irritability, growth retardation, anemia, shortness of breath, and increased susceptibility to infection. Scurvy is the classic disease related to deficiency. Symptoms peculiar to infantile scurvy include swelling of the lower extremities, pain upon flexing them, and bone lesions. Excessive ascorbic-acid intake can cause kidney stones, gastrointestinal disturbances, and red-blood-cell destruction.

Vitamin D

is a fat-soluble compound essential for calcium metabolism in animals and therefore important for normal mineralization of bone and cartilage. The skin forms vitamin D when exposed to sunlight, but in some circumstances sunlight may lack sufficient amounts of ultraviolet rays to bring about adequate production of the vitamin.
Deficiencies cause many biochemical and physiological imbalances. If uncorrected, faulty mineralization of bones and teeth causes rickets in growing children and osteomalacia (progressive loss of calcium and phosphorus from the bones) in adults. Common early symptoms of rickets include restlessness, profuse sweating, lack of muscle tone in the limbs and abdomen, and delay in learning to sit, crawl, and walk. Rickets may produce such conditions as bowlegs and knock-knees. Deficiency may also cause osteoporosis, a bone condition characterized by an increased tendency of the bones to fracture. Large doses of vitamin D are toxic, and symptoms include weakness, loss of appetite, nausea, vomiting, diarrhea, excessive thirst, and weight loss.

Vitamin E

is a fat-soluble compound. The metabolic roles of this vitamin are poorly understood. Its primary role appears to be as an inhibitor of oxidation processes in body tissues. Deficiency is rare but may impair neuromuscular function. Although serious toxicity has not been attributed to large doses of vitamin E, adverse effects have been reported.

Vitamin K

is fat-soluble and essential for the synthesis of certain proteins necessary for the clotting of blood. Deficiency, though relatively uncommon, results in impaired clotting of the blood and internal bleeding.

Vitamin-like substances

include a number of compounds that resemble vitamins in their activity but are normally synthesized in the human body in adequate amounts. They are often classified with the B vitamins because of similarities in function and distribution in foods. Their status as essential nutrients remains uncertain. Choline is found in all living cells and plays a role in nerve function and various metabolic processes. Myoinositol is a water-soluble compound; its significance in human nutrition is not established. Para-aminobenzoic acid is an integral part of folic acid but its role in human nutrition has not been documented. Carnitine has an essential role in the transport of fatty substances. Lipoic acid seems to have a coenzyme function similar to that of thiamine; however, because it is synthesized in the human liver and kidneys, it is not considered a vitamin. Bioflavinoids are a group of substances that affect the permeability of capillaries but do not normally have to be added to human diets.

History

The value of certain foods in maintaining health was recognized long before the first vitamins were actually identified. In the 18th century, for example, it had been demonstrated that the addition of citrus fruits to the diet would prevent the development of scurvy. In the 19th century it was shown that substituting unpolished for polished rice in a rice-based diet would prevent the development of beriberi.
In 1906 the British biochemist Frederick Hopkins demonstrated that foods contained necessary “accessory factors” in addition to proteins, carbohydrates, fats, minerals, and water. In 1911 the Polish chemist Casimir Funk discovered that the anti-beriberi substance in unpolished rice was an amine (a type of nitrogen-containing compound), so Funk proposed that it be named vitamine—for “vital amine.” This term soon came to be applied to the accessory factors in general. It was later discovered that many vitamins do not contain amines at all. Because of its widespread use, Funk's term continued to be applied, but the final letter e was dropped.
In 1912 Hopkins and Funk advanced the vitamin hypothesis of deficiency, a theory that postulates that the absence of sufficient amounts of a particular vitamin in a system may lead to certain diseases. During the early 1900s, through experiments in which animals were deprived of certain types of foods, scientists succeeded in isolating and identifying the various vitamins recognized today.

 

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