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A type of connective tissue, especially in mammals, that is specialized for the storage of neutral fats (lipids). Adipose cells originate mainly from fibroblasts (connective tissue cells found in virtually all animal organs). In the mammalian…

A regulatory enzyme whose activity is modified by the binding of an activator molecule to an alternate (allosteric) site that is different from the active (catalytic) site. Allosteric enzymes (Fig. 1) are special enzymes involved in regulatory…

A disease of the nervous system characterized by a progressive dementia that leads to profound impairment in cognition and behavior. Dementia occurs in a number of brain diseases where the impairment in cognitive abilities represents a decline from…

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 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…

Any substance that causes the immune system to produce specific antibodies or T cells against it. An antigen may be a foreign substance from the environment (such as chemicals, bacterial or viral proteins, or pollen; Fig. 1) or formed within the…

Chemical compounds biosynthetically or synthetically produced that destroy or usefully suppress the growth or metabolism of a variety of microscopic or submicroscopic forms of life. On the basis of their primary activity, antimicrobial agents (see…

Any treatment or procedure that includes the in-vitro handling of oocytes (immature ova or egg cells) and sperm or embryos for the purpose of establishing a pregnancy. Assisted reproductive technology (ART) is undertaken as a treatment of…

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 processes of both the increase in number and the increase in mass of bacteria. The net growth in a population of bacteria (Fig. 1) depends on its ability to produce new biomass, to produce new cells incorporating that biomass, and to survive…

A fluid connective tissue that circulates in the vascular system (blood vessels) of the body. Blood circulates through the body via vascular blood vessels (arteries, veins, and capillaries). Blood consists of plasma (the fluid component) and cells…

A genus of pathogenic gram-negative bacteria that are coccobacilli and obligate aerobes, and fail to ferment carbohydrates. Bordetella bacteria are respiratory pathogens. Bordetella pertussis, B. parapertussis, and B. bronchiseptica share greater…

Cells within a tumor that possess the capacity to self-renew and cause the heterogeneous lineages of cancerous cells that comprise the tumor. Cancer stem cells (CSCs) typically represent a small percentage of all cells in a tumor (rarely more than…

The administration of chemical substances for the treatment of disease, especially cancer, or for the prevention of aberrant growth of cells or tissues (neoplasms). In general, chemotherapy is the use of chemicals to treat any disease, but the term…

The major pathway for the complete oxidation of acetyl coenzyme A (the thioester of acetic acid with coenzyme A) in aerobic cells from animal and certain other species; also known as the Krebs cycle or tricarboxylic acid cycle. The citric acid cycle…

The response of corals to environmental stress in which they eject the single-cell algae that live within the transparent coral tissue, making the white coral skeleton visible. Corals (Fig. 1) are animals that live in a partnership (symbiosis) with…

A genome-engineering technique that allows precise targeting of specific stretches of genetic code and editing of DNA at designated locations. The CRISPR/Cas9 system has been used as a general tool to engineer the genomes of many different…

The ability of an organism to resist the action of an inhibitory molecule or compound. Drug resistance is the decreased reactivity of living organisms to the injurious actions of certain drugs and chemicals. Examples of drug resistance include…

A system of chemical communication among cells in vertebrate animals. The classical vertebrate endocrine system (Fig. 1) consists of a group of discrete glands that secrete unique products (hormones) into the bloodstream. These products travel in…

The intentional production of new genes and alteration of genomes by the deletion, substitution, or addition of new genetic material. Genetic engineering involves the artificial recombination of nucleic acid molecules in the test tube; their…

Plants that have altered genetic material (DNA) through recombinant DNA (rDNA) manipulation. Genetic engineering of plants allows for very specific genetic changes to be made to a plant's deoxyribonucleic acid (DNA), as opposed to the random…

Commercially grown plants that have altered genetic material through artificial recombinant manipulation. Genetic modification is the latest scientific tool for developing improved crop varieties. Such genetically modified (GM) crops can help to…

An organism that has had its genetic material manipulated or altered in a laboratory setting through the use of genetic or genomic engineering techniques. Genetically modified organisms (GMOs) are microorganisms, plants, or animals that have been…

The requirement of high salt (NaCl) concentrations for growth of microorganisms. Microorganisms (mainly bacteria) can be classified by their physiological tolerance to salt (Fig. 1). Most normal bacteria, such as Escherichia coli, and most…

The muscular organ that propels the flow of blood, hemolymph, or circulating fluid in invertebrates. As might be expected from the great diversity of invertebrate forms, the hearts of invertebrates and their associated circulatory systems are very…

Pathologies of the heart and its blood vessels. The heart is the muscular pumping organ of the cardiovascular system of vertebrates (Fig. 1). However, a diseased or defective heart can have serious consequences. Almost all cardiovascular or heart…

A type of human cancer cell maintained in tissue culture since 1951, originally excised from the cervical carcinoma of a patient named Henrietta Lacks. The first immortal human cells to grow successfully outside the human body were labeled HeLa…

Mature cells that have been reprogrammed genetically to an embryonic-like state that then can give rise to one of many different cell types. Induced pluripotent stem cells (iPS cells; Fig. 1) represent a major breakthrough in stem cell research.…

A relatively complex response pattern that is usually present in one or both sexes of a given animal species. Instinctive behavior (see illustration) comprises any species-typical pattern of responses that are not clearly acquired through training.…

The study of the establishment, spread, and ecological impact of species translocated from one region or continent to another outside their normal range, particularly when the translocation is the result of human activity (intentional or otherwise).…

A species translocated from one region or continent to another outside its normal range, particularly when the translocation is the result of human activity (intentional or otherwise). Many thousands of species, including animals, plants, fungi,…

The physiologic and metabolic processes involved in the assimilation of dietary lipids and in the synthesis and degradation of lipids. Lipids (Fig. 1) form a class of compounds composed of neutral fats (triglycerides), phospholipids, glycolipids,…

The study of bacteria that cause human disease. The field of medical bacteriology encompasses the detection and identification of bacterial pathogens (Fig. 1), the determination of the sensitivity and mechanisms of resistance of bacteria to…

A type of skin cancer caused by the uncontrolled proliferation of epidermal melanocytes (cells containing dark pigments). Melanoma (see illustration) is well known for its rapid progression, metastasis, and poor prognosis. Statistics from the United…

The mental ability to store and access information that has been acquired through experience. Memory (Fig. 1) is a critical component of practically all aspects of human thinking, including perception, learning, language, and problem solving.…

An experience of discomfort, primarily associated with tissue damage. Pain involves somesthetic sensations that are generally unpleasant and often cause suffering or distress. Pain (see illustration) is more complex than other sensory systems, such…

The treatment of foods or beverages with mild heat, irradiation, or chemical agents to improve keeping quality or to inactivate disease-causing microorganisms. Originally, Louis Pasteur observed that spoilage of wine and beer could be prevented by…

A substance that acts as a molecular messenger, transmitting information from one member of a species to another member of the same species. A pheromone is a chemical substance secreted by an individual that influences the behavior of other…

A property of biological materials that imparts coloration. Biological pigmentation results from organic compounds or substances (often termed biochromes) that provide a characteristic color to biological cells and tissues, including those of…

The interrelationships between two animals (most often of unlike species) whereby one acts as a predator that ensnares and eats the other (the prey). Predation occurs when one animal (the predator) kills and then eats another living animal (the…

The common name for a number of fish in the family Salmonidae that live in coastal waters of the North Atlantic and North Pacific and breed in rivers tributary to the oceans. Salmon (Fig. 1) are large, predatory fish belonging to the genera…

Unconsolidated granular material consisting of mineral, rock, or biological fragments between 63 micrometers and 2 millimeters in diameter. Finer material is referred to as silt and clay; coarser material is known as gravel. Sand is usually produced…

An inherited disorder of red blood cells characterized by lifelong anemia and recurrent painful episodes. Sickle cell disease, also termed sickle cell anemia, is a chronic, hereditary hemolytic and thrombotic disease in which hypoxia (oxygen…

Cells that have the ability to self-replicate and to give rise to mature cells. The concept of stem cells (Fig. 1) was originally based on renewing tissues. Many adult tissues, including the skin, blood, and intestines, consist of mostly mature and…

A general term used to describe environmental events of a challenging sort as well as the body's response to such events, in which a sense of control and ability to cope is an important factor in determining responses. In a general biological sense,…

A harmonious balance of human needs with resource consumption over an extended period of time. Sustainable development is a closely related term to sustainability, described in the 1987 Brundtland Report—issued by the United Nations (UN) World…

A zone of forest vegetation encircling the Northern Hemisphere between the arctic-subarctic tundras in the north and the steppes, hardwood forests, and prairies in the south. The taiga is the largest terrestrial biome (a major community of plants…

A potentially fatal infectious disease caused by protozoan parasites of the genus Trypanosoma. Trypanosomiasis, or sleeping sickness, is the term used to describe a number of diseases affecting vertebrate animals. The causal agents of…

Two babies born to a mother at one birth. There are two types of twins—monozygotic and dizygotic. Members of a twin pair are called co-twins (Fig. 1). Controversy surrounding the definition of a twin arose with the advent of reproductive…

An abnormal metabolic condition in which the pancreas produces little or no insulin and characterized by excessive levels of sugar glucose in the blood and tissues of the body. Type 1 diabetes (T1D; formerly termed insulin-dependent or juvenile…

A metabolic disorder characterized by the presence of excess glucose in the blood and tissues of the body, occurring as a result of resistance to insulin action coupled with an inadequate compensatory insulin secretory response. Type 2 diabetes…

The science and analysis of yeast classification. Yeasts (Fig. 1) are single-celled eukaryotic fungi that have a vegetative body, or thallus, under normal growth conditions. The polyphasic approach to yeast systematics is a universal consensus…

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 distinct phylum of helminths (parasitic worms), commonly known as the spiny- or thorny-headed worms. The phylum Acanthocephala comprises three classes: Archiacanthocephala, Palaeacanthocephala, and Eoacanthocephala (a suggested fourth class,…

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…

An approach to management of natural resources that emphasizes how little is known about the dynamics of ecosystems and that, as more is learned, management will evolve and improve. Adaptive management is a systematic strategy that seeks to improve…

Habituation to a specific practice, such as drinking alcoholic beverages or using drugs (for example, heroin, cocaine, or amphetamines), and disorders characterized by the chronic use of a drug, alcohol, or similar substance. Addiction and addictive…

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…

Malfunctions of the paired adrenal glands. The adrenal glands can have congenital defects and can be damaged by infections and destructive tumors. Anencephaly, that is, gross underdevelopment of the brain in fetal life, leads to hypoplasia of the…

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.…

The branch of meteorology that deals with atmospheric effects on the operation of vehicles in the atmosphere, including winged aircraft, lighter-than-air devices such as dirigibles, rockets, missiles, and projectiles. The air which supports flight…

Aeronomy concentrates on the regions above the tropopause or upper part of the atmosphere. The region of the atmosphere below the tropopause is the site of most of the weather phenomena that so directly affect all life on the planet; this region has…

Any of a group of secondary metabolites produced by the common molds Aspergillus flavus and A. parasiticus that cause a toxic response in vertebrates when introduced in low concentration by a natural route. The group constitutes a type of mycotoxin.…

A highly fatal insect-borne viral disease of horses and mules, and a mild, subclinical disease in donkeys and zebras. African horse sickness occurs in sub-Saharan Africa, but incursions outside Africa have been documented. Dogs may be affected by…

A variety of chalcedonic quartz that is distinguished by the presence of color banding in curved or irregular patterns (Fig. 1). Most agate used for ornamental purposes is composed of two or more tones or intensities of brownish-red, often…

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…

A branch of meteorology that examines the effects and impacts of weather and climate on crops, rangeland, livestock and various agricultural operations. The branch of agricultural meteorology dealing with atmospheric-biospheric processes occurring…

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…

A predominantly mechanical mixture of a variety of individual gases enveloping the terrestrial globe to form the Earth's atmosphere. In this sense air is one of the three basic components, air, water, and land (atmosphere, hydrosphere, and…

In meteorology, an extensive body of the atmosphere which is relatively homogeneous horizontally. An air mass may be followed on the weather map as an entity in its day-to-day movement in the general circulation of the atmosphere. The expressions…

The force per unit area that the air exerts on any surface in contact with it, arising from the collisions of the air molecules with the surface. It is equal and opposite to the pressure of the surface against the air, which for atmospheric air in…

The temperature of the atmosphere represents the average kinetic energy of the molecular motion in a small region, defined in terms of a standard or calibrated thermometer in thermal equilibrium with the air. Temperature Measurement Many different…

Visible, infrared, and ultraviolet emissions from the atoms and molecules in the atmosphere above 30 km (20 mi), generally in layers, and mostly between 70 and 300 km (45 and 200 mi).The airglow, together with the ionosphere, is found in the…

A sodium-rich plagioclase feldspar mineral whose composition extends over the range Ab100An0 to Ab90An10, where Ab (= albite) is NaAlSi3O8 and An (= anorthite) is CaAl2Si2O8 (see illustration). Albite occurs in crustal igneous rocks as a major…

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…

The repeated and/or excessive use of alcohol (ethanol) that continues despite negative physical, psychiatric, or social consequences. Alcoholism is characterized by consuming alcohol more often, in larger amounts, or over longer periods than…

Light emissions in the upper atmosphere from elemental lithium, potassium, and especially sodium. These alkali metals are present in the upper atmosphere at altitudes from about 50 to 62 mi (80 to 100 km) and are very efficient in resonant…

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…

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…

Plant growth forms characteristic of upper reaches of forests on mountain slopes. In such an environment, trees undergo gradual changes that, though subtle at first, may become dramatic beyond the dense forest as the zone of transition leads into…

Intergrading regions on mountain slopes characterized by specific plant life forms or species composition, and determined by complex environmental gradients.Along an altitudinal transect of a mountain, there are sequential changes in the physiognomy…

A mineral of composition KAl3(SO4)2(OH)6. Alunite occurs in white to gray rhombohedral crystals or in fine-grained, compact masses. It has a hardness of 4 and a specific gravity of 2.6–2.8.Alunite is produced by sulfurous vapors on acid volcanic…

An alloy of mercury. Practically all metals will form alloys, or amalgams, with mercury, with the notable exception of iron. Amalgams are used as dental materials (see illustration), in the concentration of gold and silver from their ores, and as…

Most commonly, a generic name for all fossil resins, although it has been restricted by some to refer only to succinite, the mineralogical species of fossil resin that includes most of the Baltic Coast deposits. Resins generally are complex mixtures…

A lithium aluminum phosphate mineral of basic formula LiAl(PO4)(F). The structure of amblygonite consists of phosphate (PO4) groups of tetrahedra and AlO6 groups of octahedra. Each PO4 tetrahedron is connected to an AlO6 octahedron. Corner-sharing…