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A plant, Musa textilis, of the banana family that is valuable for its hard fiber (also known as Manila hemp) and native to Borneo and the Philippines. Abaca is one of the strongest of the hard fibers. Commercially, it is known as Manila hemp. The…

A family of adenosine triphosphate–binding cassette (ABC) transmembrane proteins that use energy to transport various molecules across extracellular and intracellular membranes (cytoplasmic membranes, endoplasmic reticulum, mitochondria, or…

One of the major plant hormones that plays an important role in plant growth and development. Abscisic acid (ABA) is ubiquitous in vascular plants; the highest levels are found in young leaves and in developing fruits and seeds. Abscisic acid is…

The net movement (transport) of water and solutes from outside an organism to its interior. The unidirectional flow of materials into an animal from the environment generally takes place across the alimentary tract, the lungs, or the skin, and in…

A naturally occurring quaternary ammonium cation ester, with the formula CH3(O)COC2H4N(CH3)3+, which plays a prominent role in nervous system function as a neurotransmitter. Acetylcholine (Fig. 1) is an organic compound released from certain…

An order of monocotyledonous angiosperms composed of a single genus, Acorus (sweet flag or sweet calamus), with two species widespread in the Northern Hemisphere. Formerly, Acorus was included in the aroid family, Araceae, but several lines of…

A viral disease of humans caused by the human immunodeficiency virus (HIV), which attacks and compromises the body's immune system. Individuals infected with the human immunodeficiency virus (HIV) proceed through a spectrum of stages that ultimately…

An induced state in which antigens originally regarded as foreign become regarded as self by the immune system. Tolerance can be induced (tolerization) in all of the cells of the immune system, including T cells (also known as T lymphocytes), the…

A genus of gram-negative, immotile and nonspore-forming, oval to rod-shaped, often pleomorphic bacteria which occur as parasites or pathogens in mammals (including humans), birds, and reptiles. They are facultatively aerobic, capable of fermenting…

A heterogeneous collection of bacteria with diverse ecologies and physiologies. The initial definition was based on their branching filamentous cellular morphology during a stage of the growth cycle. The name actinomycete means “ray fungus,” and for…

A characteristic of an organism that makes it fit for its environment or for its particular way of life. Adaptation is a key biological process in organisms and can take many forms. Typically, adaptation refers to two mechanisms: (1) an adjustment…

A type of defective virus particle in which part of the genetic material of papovavirus SV40 is encased within an adenovirus protein coat (capsid). Human adenoviruses require human cells for their propagation; however, papovavirus SV40 can serve as…

A family of double-stranded DNA viruses, collectively known as adenoviruses, which are icosahedrally symmetric, usually found in the respiratory tract of the host species, and often associated with respiratory diseases. Members of the Adenoviridae…

A complex endocrine organ in proximity to the kidney. Adrenal gland tissue is present in all vertebrates from cyclostomes to placental mammals. The adrenal consists of two functionally distinct tissues: steroidogenic cells of mesodermal origin and…

A bacterial genus in the family Vibrionaceae comprising oxidase-positive, facultatively anaerobic, monotrichously flagellated gram-negative rods. The mesophilic species are A. hydrophila, A. caviae, and A. sobria; the psychrophilic one is A.…

A reaction in which suspended particles are aggregated or clumped. It occurs upon the admixture of another type of particle, a change in the composition of the suspending fluid, or the addition of a soluble agent that acts as a bridge between two or…

A substance that will cause a clumping of particles such as bacteria or erythrocytes. Of major importance are the specific or immune agglutinins, which are antibodies that will agglutinate bacteria containing the corresponding antigens on their…

An age-contingent reduction in physiological function and activity, with a concomitant increase in mortality rate and a diminution in reproductive rate. Aging is the process of becoming older, which is influenced by genetic and environmental…

Aircraft specifically designed or adapted from a general utility airframe for use in agriculture and forestry and for control of insect vectors of human, animal, and plant diseases. Agricultural aircraft (Fig. 1) have become an indispensable tool…

A discipline concerned with solving the engineering problems of providing food and fiber for the people of the world. These problems include designing agricultural machinery to work the soil and harvest the crops, as well as developing water…

The science that deals with the selection, breeding, nutrition, and management of domestic animals for economical production of meat, milk, eggs, wool, hides, and other animal products. The broad field of agricultural science focuses on two realms:…

The techniques and methods used in plowing, harrowing, planting, tilling, harvesting, drying, storing, and processing of agricultural crops. Agricultural soil and crop practices seek to keep the quality and fertility of soils at high levels so as to…

A model for the functioning of an agricultural system, with all inputs and outputs. An ecosystem may be as small as a set of microbial interactions that take place on the surface of roots, or as large as the globe. An agroecosystem may be at the…

The science and study of crops and soils. Agronomy is the umbrella term for a number of technical research and teaching activities (see illustration): crop physiology and management, soil science, plant breeding, and weed management frequently are…

A plasma protein produced by the liver that maintains fluid balance in the blood and transports fatty acids in the plasma and interstitial fluid. It is the most abundant protein in human serum, and one of the first discovered and earliest studied…

Any of the deciduous shrubs and trees of the genus Alnus in the birch family (Betulaceae). There are approximately 30 species of alder, with 10 in the United States. Alders are widespread in cool north temperate regions and are also found southward…

The world's most valuable forage legume, Medicago sativa, also known as lucerne, and less often as purple medic, medica, snail clover, median herb, Burgundy hay or clover, and Chilean clover. Alfalfa is often referred to as the queen of forages. It…

A fluid-filled sac- or sausagelike, extraembryonic membrane lying between the outer chorion and the inner amnion and yolk sac of the embryos of reptiles, birds, and mammals. The allantois eventually fills up the space of the extraembryonic coelom in…

Any of a number of alternative forms of a gene. Allele is a contraction of allelomorph, a term that William Bateson used to designate one of the alternative forms of a unit showing mendelian segregation. New alleles arise from existing ones by…

Altered reactivity in humans and other animals to allergens (substances foreign to the body that trigger an allergic response) induced by exposure through injection, inhalation, ingestion, or skin contact. An allergy (or allergic reaction; Fig. 1)…

The study of changes in the characteristics of organisms with body size. Characteristics such as body parts or timing of reproductive events do not necessarily change in direct proportion to body size, and the ways in which they change relative to…

The dried, unripe fruits (berries) of a small, tropical, evergreen tree, Pimenta dioica (P. officinalis), of the myrtle family (Myrtaceae). Allspice (see illustration), which is also known as Jamaica pepper, is a native of the West Indies and parts…

Prunus dulcis (also known as Prunus amygdalus or Amygdalus communis), a small deciduous tree of the order Rosales that is closely related to the peach and other stone fruit trees, and grown widely for its ellipsoidal edible seeds. The fruit of the…

Vicugna pacos, a member of the camel family (Camelidae), which has been domesticated for more than 2000 years and is economically important for its long, fine wool. The alpaca (Vicugna pacos; formerly, Lama pacos) [see illustration] belongs to the…

A glycoprotein that is normally present in significant amounts only in the serum of the fetus. It is produced in the yolk sac, the liver, and other tissues of the gastrointestinal tract. Its role is unknown, but alpha fetoprotein may function as a…

An annual plant (seldom perennial) of the genus Amaranthus (family Amaranthaceae) that is distributed worldwide in warm and humid regions. Amaranth plants are botanically distinguished by their small chaffy flowers (see illustration), which are…

Any of the organic compounds possessing one or more basic amino groups and one or more acidic carboxyl groups. Of the more than 80 amino acids that have been found in living organisms, approximately 20 serve as the building blocks for proteins.…

A thin, cellular, extraembryonic membrane forming a closed sac which surrounds the embryo in all reptiles, birds, and mammals and is present only in these forms; hence the collective term amniotes is applied to these animals. The amnion contains a…

A collective term for the classes Reptilia (reptiles), Aves (birds), and Mammalia (mammals) of the subphylum Vertebrata. The remaining vertebrates, including the several classes of fishes and the amphibians, are grouped together as the Anamnia.…

An enzyme that is essential for maintenance of cellular energy balance. All living cells carry out many energy-requiring processes, including growth and division, movement, and uptake of nutrients from their environment. To achieve this, they…

A stimulating drug that affects the brain and the body. Amphetamine is a stimulant of the central nervous system. Chemically, amphetamine is a racemic mixture of the l and d isomers of α-methyl-β-phenethylamine (see illustration). The l isomer has…

A disorder characterized by the accumulation of an unusual extracellular fibrous protein (amyloid) in the connective tissue of the body. The deposition of amyloid may be widespread, involving major organs and leading to serious clinical…

A collective term, also referred to as Anamniota, for the vertebrate animals that lack an amnion in development, including the Agnatha, Chondrichthyes, Osteichthyes, and Amphibia. The amnion is a protective embryonic envelope that encloses the…

A generalized or localized tissue reaction occurring within minutes of an antigen-antibody reaction. Similar reactions elicited by nonimmunologic mechanisms are termed anaphylactoid reactions. The term anaphylaxis was introduced by C. R. Richet to…

Synthetic manufactured drugs derived from the male sex hormone testosterone. Androgenic-anabolic steroids, often termed simply as anabolic steroids, are artificially produced testosterone variants. They can provide a number of medical benefits when…

A class of steroid hormones produced in the testes and adrenal cortex that act to regulate masculine secondary sexual characteristics. The major naturally occurring androgens, such as testosterone, are C-19 steroids (that is, they each contain 19…

A medical condition resulting from a reduction in the total quantity of hemoglobin or of red blood cells in the circulation. Anemia is a disorder marked by significant decreases in hemoglobin concentration and in the number of circulating red blood…

The condition of exhibiting no immune response to an antigen. Anergy is a self-tolerance mechanism in which the immune system shows unresponsiveness following an encounter with receptor-recognized antigens. Typically, lymphocytes (the B and T cells…

The origin and development of blood vessels. Angiogenesis is the process through which new blood vessels are formed. Blood vessels (Fig. 1) are composed of two basic cell types: vascular endothelial cells and periendothelial cells (including…

The application of genetic principles for improving heredity for economically important traits in domestic animals. The science of animal breeding is based on fundamental genetic concepts. Through controlled mating and selection, which can involve…

A method used by animals to conceal themselves from an enemy by appearing to be a section of the natural background. Animal camouflage is a strategy that animal organisms use to avoid or deflect the attacks of predators by misleading the latter's…

The increase in size or weight of an animal. A number of processes are involved in growth. These include hyperplasia, or increase in cell number due to cell proliferation (cell division or mitosis) and recruitment from stem cell populations;…

The classification rank comprising the animals. Animals are taxonomically assigned to the classification kingdom known as Animalia. (The other kingdoms for classification of organisms include Plantae, Fungi, Protista, and Monera; in certain…

The formation of new individuals, which may occur by asexual or sexual means. All animals must reproduce asexually or sexually to create new individuals of their species. In asexual reproduction, a single parent can produce offspring that are…

The symmetrical disposition of organs and other constituent parts of the bodies of living animals with respect to imaginary axes. Animal symmetry relates the organization of parts in animal bodies to the geometrical design that each type suggests.…

The use of live nonhuman animals for scientific research and experimentation. Animal testing has been a cornerstone of biomedical research, having contributed greatly to the understanding of human biology, to the unraveling of the complex underlying…

An annual herb, Pimpinella anisum (family Apiaceae), whose fruit is used to provide food flavoring and to make a distilled oil. Anise is one of the earliest aromatics mentioned in literature. The anise plant (see illustration) is an annual herb that…

An acute, infectious worldwide zoonotic disease (transmissible to humans) caused by the spore-forming bacterium Bacillus anthracis to which most animals, especially grazing herbivores, are susceptible. Anthrax is a serious, though rare, infectious…

An ongoing current event in which a large number of living species are threatened with extinction or are going extinct because of environmentally destructive human activities. The Earth is presently in the midst of a mass extinction event. In the…

The observation, measurement, and explanation of human variability in time and space. Anthropology is a science concerned with the interrelations of the biological, cultural, geographical, and historical aspects of humankind, including modern human…

The capability that a bacterium acquires to counteract an inhibitory chemical molecule or compound that was formerly effective in killing it or preventing its growth. Antibiotic resistance in bacteria (Fig. 1) is a hot topic in the media and in…

Glycoprotein or protein molecules synthesized by polar and north temperate fishes to enable them to survive in freezing seawater. Similar antifreezes are found in some insects, but relatively little is known about their structure and…

A reaction that occurs when an antigen combines with a corresponding antibody to produce an immune complex. A substance that induces the immune system to form a corresponding antibody is called an immunogen. All immunogens are also antigens because…

A chemical substance used to destroy or prevent the growth of infectious microorganisms on or in a human or other animal body. An antiseptic is an antimicrobial compound or drug preparation that inhibits the spread and development of harmful…

An antibody that will combine with and generally neutralize a particular toxin. When the manifestations of a disease are caused primarily by a microbial toxin, the corresponding antitoxin, if available in time, may have a pronounced prophylactic or…

The main vessel of the systemic arterial circulation arising from the heart. The aorta (see illustration) is the main artery leaving the heart in all vertebrates and some invertebrates. In mammals, reptiles, and birds, which possess two heart…

One of the cellular aggregations traditionally known as chemoreceptors which are generally found adjacent to the aortic arch and pulmonary artery and which often are adjacent to the origin of the subclavian arteries (see illustration). The term is…

Programmed cell death triggered by extracellular signals or genetically determined events and carried out by physiological processes within the cell. Apoptosis is one of the major forms of cell death in multicellular organisms (necrosis is the other…

A narrow, elongated tube closed at one end, extending from the cecum (a blind pocket off the first part of the large intestine). The appendix (see illustration), also known as the vermiform appendix, is found in only a few mammals. The size and…

A deciduous tree, Prunus armeniaca, in the order Rosales that produces a simple fleshy stone fruit. The apricot is thought to be native to China; then it was distributed throughout Asia and Europe, and eventually North America, South America, and…

The cultivation of freshwater and marine species for human purposes. Aquaculture, or aquatic farming, is concerned with the raising of animals and the harvesting of plants in freshwater and marine environments (Fig. 1). In other words, it is the…

A group of diverse microscopic prokaryotic organisms that constitute one of the three domains of life. Taxonomists recognize three domains of cellular life: Archaea, Bacteria, and Eukaryota (also termed Eukarya or Eucarya) [Fig. 1]. The members of…

A sesquiterpene lactone (C15H22O5), derived from the leaves of Artemisia annua (sweet wormwood or qinghao), which is known for its antimalarial activity. Artemisinin (C15H22O5; see illustration) is an important antimalarial agent. Around 2000 years…

A white, crystalline compound, 1-aspartyl-1-phenylalanine methyl ester (APM), with formula (1). It is slightly soluble in water. Its sweetening properties were discovered accidentally in 1965 when the compound, a dipeptide, was produced as an…

An allergic inflammatory disease of the pulmonary airways, marked by labored breathing, wheezing, and coughing. Asthma is a serious pulmonary disease that inflames and narrows the airways of the lungs (Fig. 1). The inflammation involves mast cells,…

The scientific discipline that studies the origin, evolution, distribution, and future of life in the universe. Astrobiology is an interdisciplinary science integrating contributions from biology, geology, astronomy, paleontology, and planetary…

An abundant type of glial cell of the central nervous system. Astrocytes (Fig. 1) are specialized glial cells (nonneuronal cells that surround neurons) of the central nervous system (CNS) that exert numerous essential functions in both health and…

A form of microscopy in which a sharp tip is scanned over the surface of a sample while sensing the interaction force between the tip and the sample. Due to its ability to observe and manipulate surfaces in aqueous solution, atomic force microscopy…

A neurodevelopmental condition that impairs the way that a person relates to and communicates with other people. Persons with autism have difficulties with social communication and interaction. They also can have unusual behaviors, such as…

The occurrence in an organism of an immune response to one of its own tissues, that is, a response to a self constituent. Autoimmunity is an immune state in which antibodies are formed against the person's own body tissues. Foreign substances to…

The part of the nervous system that controls visceral (internal organ) functions of the body. The autonomic nervous system (see illustration) innervates smooth and cardiac muscles and the glands, and regulates visceral processes involuntarily,…

Destruction of the components of a cell by its own enzymes. Autophagy is a process of all eukaryotic cells. Various types of autophagy occur during normal development or as a response to various types of stress, including starvation. Defects in…

A photographic technique used to localize a radioactive substance within a solid specimen; also known as radioautography. Procedure A photographic emulsion is placed in contact with the object to be tested and is left for several hours, days, or…

The growth and maintenance of a single species in isolation, free from foreign or contaminating species. Isolation in axenic culture is achieved usually by growing the species in an environment that was sterilized previously and was thereby rid of…

A highly contagious intestinal disease, also known as shigellosis, caused by rod-shaped bacteria of the genus Shigella. Bacillary dysentery, which also is referred to as shigellosis, is a significant infection of children in the developing world,…

The ongoing lower level of species extinction intensity occurring between episodes of mass extinctions. Extinction is a normal part of the history of life. Out of every 100 species that have ever lived on Earth, 99 are no longer in existence.…

Extremely small, relatively simple microorganisms possessing the prokaryotic type of cell construction. Bacteria are very small prokaryotic microorganisms, usually measuring 0.3 to 2.0 micrometers in diameter (Fig. 1). They are found almost…

The study of gene structure and function in bacteria. Bacterial genetics is a scientific field concerned with the mechanisms of heredity in bacteria (Fig. 1). In general, genetics itself is concerned with determining the number, location, and…

The production of visible light (usually blue-green) by bacteria. The phenomenon of bacterial luminescence is seen in many species of several genera, including Vibrio, Photobacterium, Alteromonas, and Xenorhabdus. With very few exceptions, the light…

The biochemical reactions that together enable bacteria to live, grow, and reproduce. Strictly speaking, bacterial metabolism describes the total chemical reactions that take place in a bacterial cell, whereas bacterial physiology describes the role…

The classification, nomenclature, and identification of bacteria. Bacterial taxonomy is concerned with the naming of bacterial organisms and with organizing these names according to various criteria. Biologically, bacteria (Fig. 1) are prokaryotic…

Any of the viruses that infect bacterial cells. Bacteriophages, often simply called phages, are discrete particles, with dimensions ranging from approximately 20 to 200 nanometers. A given bacterial virus can infect only one or a few related species…

A fast-growing tree, Ochroma pyramidale (also known as O. lagopus), which is widely distributed in tropical America, especially in Ecuador. Balsa (Ochroma pyramidale; also, O. lagopus) belongs to the mallow family, Malvaceae, in the order Malvales.…

Raising of cattle for meat. Beef cattle production is a section of animal agriculture concerned with the rearing of cattle in order to produce meat for human or other animal consumption (Fig. 1). This is in contrast to dairy cattle production, which…

The management and maintenance of colonies of honeybees for human purposes. Beekeeping is an ancient art. Although the commonly known honeybee species (Fig. 1) is native to Europe and Africa, humans have transported them to other continents. The…

A major division of the animal kingdom comprising all forms with a bilaterally symmetrical body plan (that is, their left and right sides are mirror images, or nearly so) and organ systems that develop from three tissue layers. The animal kingdom,…

The predominant orange pigment of bile. It is the major metabolic breakdown product of heme, the prosthetic group of hemoglobin in red blood cells, and other chromoproteins such as myoglobin, cytochrome, and catalase. The breakdown of hemoglobin…

Impairments or conditions in which binocular visual skills are dysfunctional and the eyes are unable to align properly. Humans, like all mammals, have true binocular vision. This means that, in health, the two eyes of an individual normally move in…

The study of the role of sounds in the life history of animals. Almost all animals, from insects to large mammals, depend on sounds for their well-being and survival. Sounds are used by animals for a wide variety of reasons such as advertisement for…

A method for quantitatively determining the concentration, biological activity, or potency of a substance by its effect on the growth of a suitable animal, plant, or microorganism under controlled conditions. A bioassay (biological assay) is a…