One important property of glycerol or glycerin is that is not poisonous to humans. Therefore it is used in foods, syrups, ointments, medicines, and cosmetics. Glycerol also has special chemical properties that allow it to be used where oil would fail. Glycerol is a thick syrup that is used as the "body" to many syrups, for example, cough medicines and lotions used to treat ear infect…
A glycol is an aliphatic organic compound in which two hydroxyl (OH) groups are present. The most important glycols are those in which the hydroxyl groups are attached to adjacent carbon atoms, and the term glycol is often interpreted as applying only to such compounds. The latter are also called vicinal diols, or 1,2-diols. Compounds in which two hydroxyl groups are attached to the same carbon at…
Glycolysis, a series of enzymatic steps in which the six-carbon glucose molecule is degraded to yield two three-carbon pyruvate molecules, is a central catabolic pathway in plants, animals and many microorganisms. In a sequence of 10 enzymatic steps, energy released from glucose is conserved by glycolysis in the form of adenosine triphosphate (ATP). So central is glycolysis to life that its sequen…
The plurality of available sources, from the Nag Hammadi texts to the writings of those fathers of the church, the so-called heresiologists who fought against Gnosticism as a heresy, makes difficult a reconstruction both of its origins and its history. The use of the category "Gnosticism" has been criticized because it provides an overview that hides the complexity of ancient histori…
Goats belong to the order Artiodactyla (genus Capra), which is made up of a number of hoofed mammals having an even number of toes. Goats have existed on Earth for at least 35 million years and, during the course of evolution, have undergone an incredibly wide radiation, both in distribution and ecology. Although the taxonomy of this group is still unclear, eight species are generally recognized a…
The goatsuckers, nightjars, and nighthawks number 70 species of birds in the family Caprimulgidae. These birds have a relatively large head, with a wide beak, and a large mouth with a seemingly enormous gape. The mouth is fringed by long, stiff bristles, and is an adaptation for catching insects in flight. The unusually large mouth of goatsuckers was once believed to be useful for suckling milk at…
Goldenseal (Hydrastis canadensis L.) is a woodland plant belonging to the family Ranunculaceae. The plant is also known as eyebalm, eyeroot, hydrastis, orangeroot, tumeric root, and yellowroot. Mainly found in the wild, goldenseal grows to a height of about 1 ft (30 cm). It has an erect, hairy stem, and produces small, greenish-white flowers that bloom in early spring, and later turn into clusters…
Gophers are small rodents. Although the name is often used popularly to refer to a variety of animals, including snakes, in the United States gophers are the pocket gophers that live in the grasslands of western Canada, eastward to the Great Lakes, and down into northern South America. Pocket gophers (family Geomyidae) have fur-lined cheek pouches that let them carry food in large quantities. Thes…
Gorillas inhabit forests of Central Africa, and are the largest and most powerful of all primates. Adult males stand 6 ft (1.8 m) upright (although this is an unnatural position for a gorilla) and weigh up to 450 lb (200 kg), while females are much smaller. Gorillas live up to about 44 years. Mature males (older than 13 years), or silverbacks, are marked by a band of silver-gray hair on their back…
One of the major theoretical hurdles to a reachable synthesis of current theories of particles and force interactions into a grand unification theory (also known as Grand Unified Field Theory, Grand Unified Theory, or GUT) is the need to reconcile the evolving principles of quantum theory with the principles of general relativity advanced by German-American physicist Albert Einstein (1879-1955) ne…
Grapes are various species of woody vines in the genus Vitis, family Vitaceae. This family contains about 700 species most of which occur in tropical and subtropical climates, although some occur in temperate habitats. The genus Vitis has about 50 species. Grapes are ecologically important as food for wildlife. They are also cultivated by humans in large quantities, mostly for the production of ta…
Grasshoppers are plant-eating insects characterized by long hind legs designed for locomotion by jumping. Like all insects, the body of grasshoppers is divided into three main parts: head, thorax, and abdomen. On the head are two antennae for feeling and detecting scent, and two compound eyes comprised of many optical units called facets, each of which is like a miniature eye. The chewing mouthpar…
Grasslands are environments in which herbaceous species, especially grasses, make up the dominant vegetation. Natural grasslands, commonly called prairie, pampas, shrub steppe, palouse, and many other regional names, occur in regions where rainfall is sufficient for grasses and forbs but too sparse or too seasonal to support tree growth. Such conditions occur at both temperate and tropical latitud…
Gravitational lenses are accidental natural arrangements of gravity, light, and distant astronomical objects that create altered images of the those objects. Commonly, a lens is a piece of glass shaped so as to bend light passing through it. In the process, it alters the image of the light source as observed through the lens. A gravitational lens bends light using gravity rather than glass. Gravit…
Grebes are aquatic birds that make up the family Podicipedidae. This is the only family in the order Podicipitiformes, a rather unique group of birds that is not Western grebes (Aechmophorus occidentalis) displaying across the water. Photograph by Phil Dotson/The National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. closely related to other living orders, a…
To understand the greenhouse effect, Earth's energy budget must be known. An energy budget is an account of all of the energy coming into and leaving a system and of any energy that is stored in (or produced by) the system itself. Almost all of the energy coming to Earth from space has been radiated by the closest star, the Sun. The Sun emits electromagnetic energy at a rate and spectral qu…
The groundhog or woodchuck (Marmota monax) is a husky, waddling rodent in the squirrel family Sciuridae, order Rodentia. The groundhog is a type of marmot (genus Marmota), and is also closely related to the ground squirrels and gophers. The natural habitat of the groundhog is forest edges and grasslands, ranging from the eastern United States and Canada through much of the Midwest, to parts of the…
Groundwater occupies the void space in a geological strata. It is one element in the continuous process of moisture circulation on Earth, termed the hydrologic cycle. Almost all groundwater originates as surface water. Some portion of rain hitting the earth runs off into streams and lakes, and another portion soaks into the soil, where it is available for use by plants and subject to evaporation b…
A group is a simple mathematical system, so basic that groups appear wherever one looks in mathematics. Despite the primitive nature of a group, mathematicians have developed a rich theory about them. Specifically, a group is a mathematical system consisting of a set G and a binary operation * which has the following properties: Note that commutativity is not required. That is, it need not be true…
Grouse (and ptarmigan) are medium-sized birds in the family Tetraonidae, order Galliformes. Grouse and ptarmigan are often hunted for food and sport, and are sometimes broadly referred to as upland gamebirds because they are not hunted in wetlands, as are ducks and geese. Grouse are ground-dwelling birds with a short, turned-down bill. They have long, heavy feet with a short elevated fourth toe be…
Growth and decay refers to a class of problems in mathematics that can be modeled or explained using increasing or decreasing sequences (also called series). A sequence is a series of numbers, or terms, in which each successive term is related to the one before it by precisely the same formula. There are many practical applications of sequences. One example is predicting the growth of human popula…
Several hormones play important roles in human growth. The major human growth hormone (hGH), or somatotropin, is a protein made up of 191 amino acids secreted by the anterior pituitary and coordinates normal growth and development. Human growth is characterized by two spurts, one at birth and the other at puberty. HGH plays an important role at both of these times. Normal individuals have measurab…
Guinea fowl are seven species of medium-sized terrestrial birds in the family Phasianidae, order Galliformes, which also includes other fowl-like birds, such as the grouse, ptarmigan, turkey, quail, peafowl, and pheasants. The natural range of guinea fowl is sub-Saharan Africa, the Arabian Peninsula, and Madagascar. However, these birds have been introduced to some other places, and are commonly k…
Guinea pigs, or cavies, are about 20 species of rodents in the family Caviidae. Guinea pigs are native to South America, occurring from Colombia and Venezuela in the north, to Brazil and northern Argentina. These animals occur in rocky habitats, savannas, forest edges, and swamps, and can be rather common within their preferred habitat. Guinea pigs have a stout body, with a relatively large head, …
Gulls are 43 species of seabirds, in the subfamily Larinae of the family Laridae, which also includes the terns. Gulls occur in a wide range of coastal habitats, ranging from inland lakes, rivers, and wetlands, to marine shores and estuaries. Their distribution is virtually world-wide, but most species occur in the Northern Hemisphere. Species of gulls range in body length from 8-32 in (20-81 cm).…
One of the most popular species of freshwater topical fish is the guppy. The first specimens were brought to the British Museum in London for description in 1859 by R. J. L. Guppy, a biologist from Trinidad (West Indies) after whom the fish is named. The species originally possessed the scientific name Lebistes reticulatus, but in 1963, the Latin name was changed to Poecilia reticulata and remains…
The Gutenberg discontinuity occurs within Earth's interior at a depth of about 1,800 mi (2,900 km) below the surface, where there is an abrupt change in the seismic waves (generated by earthquakes or explosions) that travel through Earth. At this depth, primary seismic waves (P waves) decrease in velocity while secondary seismic waves (S waves) disappear completely. S waves shear material, …
Gutta percha is a rubberlike gum obtained from the milky sap of trees of the Sapotaceae family, found in Indonesia and Malaysia. Once of great economic value, gutta percha is now being replaced by plastics in many items, although it is still used in some electrical insulation and dental work. The English natural historian John Tradescant (c. 1570-1638), introduced gutta percha to Europe in the 162…
Gymnosperms are one of the two major groups of plants that produce seeds; the other is the angiosperms. Gymnosperm literally means "naked seed," which refers to the development of seeds exposed on a flat structure, that is, not within an ovary as in the angiosperms. Gymnosperms became common about 290 million years ago and although many of the earlier types are now extinct, four kind…
Gynecology, from the Greek meaning "the study of women," is a medical specialty dealing with the health of a woman's genital tract. The genital tract is made up of the reproductive organs including the vagina, cervix, uterus, ovaries, fallopian tubes, and their supporting structures. Marked changes occur in a woman's reproductive organs upon her reaching menarche (the a…
A gyroscope is heavy disk placed on a spindle that is mounted within a system of circles such that it can turn freely. When the disk, called a flywheel, is made to spin, the gyroscope becomes extremely resistant to any change in its orientation in space. If it is mounted in gimbals, a set of pivot and frame mountings that allow it freedom of rotation about all three axes, a fast-spinning gyroscope…
The term habitat refers to the type of environment in which an organism or species occurs. For plants, habitat is mostly defined by its physical attributes (e.g., rainfall, temperature, topographic position, soil texture and moisture) and its chemical properties (e.g., soil acidity, concentrations of nutrients and toxins, oxidation reduction status). For terrestrial animals, the habitat is defined…