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A common medicine used to relieve mild to moderate pain and reduce fever. Acetaminophen (N-acetyl-p-aminophenol), or paracetamol, is a white crystalline compound with the molecular formula C8H9NO2 (Fig. 1). It is a member of the aniline family of…

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…

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 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.…

The function and transport of the bicarbonate ion for maintaining the acid level (pH) throughout the body for stable physiological processes; disfunction of bicarbonate transport results in disease. Bicarbonate (HCO3−) is a small inorganic negative…

Any of the 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…

The portion of the nervous system composed of the brain and spinal cord in vertebrates. The division of the vertebrate nervous system comprising the brain and spinal cord is known as the central nervous system (see illustration). In general, the…

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 interaction of the behavior and ecology of hosts with the biology of pathogens as relating to the impact of diseases on populations. Although human diseases have been studied from an ecological perspective for centuries, diseases in plant and…

An international consortium of research groups dedicated to determining how the human genome functions.Many human diseases are caused by changes in the abundance or activity of proteins, and proteins are encoded by segments of DNA called genes (Fig.…

The study of heritable (stable) changes in gene expression and function that are not attributed to alterations of the DNA sequence. The epigenome represents the complete set of epigenetic modifications possessed by an organism or an individual cell.…

An interdisciplinary science area that is concerned with modeling aspects of natural evolution in order to solve real-world problems of optimization. Living organisms, as well as those that are long extinct, demonstrate optimized complex behavior at…

The upper part of the female oviduct present in humans and other higher vertebrates, also known as uterine tube. The fallopian tube extends from the ovary to the uterus and transports ova from the ovary to the cavity of the uterus. Each tube is…

The use of pollen and spore evidence in legal cases. In its broader application, the field of forensic palynology also includes legal information derived from the analysis of a broad range of microscopic organisms that can be found in both fresh and…

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…

A plant that has 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…

A commercially grown plant that has 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…

A biological organism whose genetic material has been manipulated artificially through genetic or genomic editing techniques. In contrast to the long history of artificial selection (domestication), in which humans selectively breed organisms to…

One class of several hormones that are produced in the outer part of adrenal glands, known as the cortex. Glucocorticoids (GCs) are crucial for regulating metabolism, the immune system, blood pressure, and other essential functions (see…

A simple sugar, or monosaccharide, with the chemical formula C6H12O6; also called dextrose, d-glucopyranose, cerelose, grape sugar, and corn sugar. Glucose is a monosaccharide carbohydrate with six carbon atoms. It is the most common sugar and the…

Pathologies or aberrations in structure or function of blood cells or the blood-clotting mechanism. Hematologic disorders affect the blood and blood-forming organs. Although many diseases and abnormal conditions may be reflected by the blood and its…

The iron-containing, oxygen-carrying protein molecule of red blood cells. The protein hemoglobin (Fig. 1), which carries oxygen and contains iron, represents more than 95% of the solid constituents of the red blood cell. It is found in almost all…

A mature cell that has 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 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,…

A protein that spans cellular membranes and mediates the passage of electrolytes across the hydrophobic barrier that maintains the integrity of the cell and interior compartments. Ion channels are found in both prokaryotes and eukaryotes and their…

A salt or ester of lactic acid (CH3CHOHCOOH). In lactates, the acidic hydrogen of the carboxyl group has been replaced by a metal or an organic radical. Lactates are optically active, with a chiral center at carbon 2. The two optical isomers of a…

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,…

A device that is capable of converting chemical energy into electrical energy with the use of whole bacterial cells as the catalyst. Fuels that are produced biologically are known as biofuels. Biofuel cells convert chemical energy from organic…

A portion of the nervous system that regulates and controls the contractile activity of muscles and the secretory activity of glands. The motor systems of the body are highly complex networks responsible for muscular movements and gland secretions.…

An autoimmune disease resulting from an abnormality in neuromuscular transmission, characterized by a fluctuating degree of muscle weakness. Myasthenia gravis is a chronic autoimmune neuromuscular disorder (see illustration) characterized by varying…

A nerve cell; the functional unit of the nervous system. A neuron is a specialized, basic nerve cell that transmits and receives information, in the form of electrical impulses, throughout the body. With the exceptions of sponges and placozoa, the…

A progressive degenerative disorder of the nervous system belonging to a group of conditions called motor system disorders. Parkinson's disease—more specifically known as idiopathic Parkinson's disease to distinguish it from the four Parkinson-plus…

A regulator of cellular lipid storage and the major surface coat protein of cytoplasmic lipid droplets. Perilipins (derived from the Greek words for “surrounding lipid”) are primary regulators of cellular lipid storage. The perilipins sequester…

One of a class of cyclic compounds in which the parent macrocycle consists of four pyrrole-type units linked together by single carbon bridges. Several porphyrins with selected peripheral substitution and metal coordination carry out vital…

Any of the small particles, present in large numbers in every living cell, whose function is to convert stored genetic information into protein molecules. Ribosomes are cellular factories that make proteins and are essential for life in organisms…

A family of nicotinamide adenine dinucleotide (NAD)–dependent protein deacetylases (enzymes that remove acetyl groups from protein molecules) and adenosine diphosphate (ADP)–ribosyltransferases (enzymes that catalyze the transfer of the ADP-ribose…

The portion of the central nervous system within the spinal canal of the vertebral column. In vertebrates, the central nervous system is composed of the brain and spinal cord (Fig. 1). The spinal cord extends from the foramen magnum at the base of…

Any of the 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…

The physiologic mechanism by which one nerve cell (neuron) influences the activity of an anatomically adjacent neuron with which it is functionally coupled. Synaptic transmission (Fig. 1) enables a neuron to communicate with a target cell across a…

A rare genetic disorder in which there is a deficiency of hexosaminidase A (a lysosomal enzyme that hydrolyzes GM2 gangliosides). Tay-Sachs disease is a severe and progressive neurodegenerative disease. Because of the deficiency of hexosaminidase A…

An aqueous solution of organic and inorganic substances—mostly waste products of metabolism—that collects in the kidneys and is removed from the body primarily through the urinary bladder and urinary tract. Urine is a liquid waste produced by the…

A sensory system that provides the information required for proper sense of balance and spatial orientation. The vestibular system detects motion of the head in space and, in turn, generates reflexes that are crucial for daily activities, such as…

Diseases that are transmitted from animals to humans. Zoonoses (the singular form is zoonosis) are also termed zoonotic diseases. They are infectious diseases passed from animals (Fig. 1) to humans. Zoonotic agents are found among all major…

A major plant hormone that plays an important role in plant growth and development. Abscisic acid (ABA), with a chemical formula of C15H20O4 (Fig. 1), is one of the most significant hormones found in plants. It is responsible for regulating various…

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…

A complex endocrine organ in proximity to the kidney. The adrenal gland (see illustration), often called simply as the adrenal, is an important endocrine organ located on top of the kidneys. Adrenal gland tissue is present in all vertebrates from…

A plasma protein produced by the liver that maintains fluid balance in the blood and transports fatty acids in the plasma and interstitial fluid. Albumin is the most abundant protein in human serum (see illustration), as well as being one of the…

Any of a number of alternative forms of a gene. An allele is a variant form of a gene (see illustration). Allele is a contraction of allelomorph, which is a term that William Bateson used to designate one of the alternative forms of a unit showing…

A glycoprotein that is normally present in significant amounts only in the serum of the fetus. Alpha fetoprotein (AFP) is an important plasma glycoprotein found in a developing fetus. This protein is also detectable in the mother's blood by passing…

Any of the plant growth forms characteristic of the upper reaches of mountain slopes. Alpine vegetation refers to the plants that grow in mountainous areas above the limit of subalpine trees and other subalpine vegetation. In such environments,…

An intergrading region on a mountain slope 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…

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 generalized or localized tissue reaction occurring within minutes of an antigen-antibody reaction. Anaphylaxis is a severe allergic reaction and immune response that can be life-threatening. Similar reactions elicited by nonimmunologic mechanisms…

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 synthetic manufactured drug 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…

The increase in size or weight of an animal. Animal growth is an increase in an animal’s size or mass over its lifetime. Growth, which is often associated with progressive development, may extend throughout the animal's life or it may be limited to…

The formation of new animal 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 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…

The application of ecological principles to the solution of human problems and the maintenance of a high quality of life. Applied ecology is a branch of ecology that applies concepts of ecology to address contemporary problems involving natural…

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…

Any of a class of plant growth substances (phytohormones or plant hormones) that have an essential role in coordination of many growth and developmental processes during the life cycle of plants, especially the ability to increase the rate of shoot…

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 intestinal infection. Rod-shaped bacteria of the…

The production of visible light (usually blue-green) by bacteria. Luminous bacteria emit visible light by converting chemical energy to light energy. The phenomenon of bacterial luminescence is seen in many species of several genera, including…

Any of several leguminous plants, or their seeds, long utilized as food by humans or livestock. There are more than 40,000 varieties of beans, with only a small number of these being mass-produced for human or animal consumption. In particular,…

The predominant orange pigment of bile. Bilirubin (see illustration) is the major metabolic breakdown product of heme, which is the prosthetic group of hemoglobin in red blood cells, and other chromoproteins, such as myoglobin, cytochrome, and…

The measurement of the energetics of biological processes, such as biochemical reactions, association of ligands to biological macromolecules, folding of proteins into their native conformations, phase transitions in biomembranes, and enzymatic…

The number of different plant and animal species living in a defined ecosystem or study area; a contraction of biological diversity. Biodiversity includes genetic diversity (variability in the genetic makeup among individuals in a single species),…

A discipline that combines biology, mathematics, and computer science to acquire, store, analyze, and disseminate complex biological data. The term bioinformatics was coined by Paulien Hogeweg and Ben Hesper in the early 1970s to describe the "the…

A biological product used to induce immunity to various infectious diseases or noxious substances of biological origin. Biological products, or simply biologicals, are usually limited to immune serums, antitoxins, vaccines (see illustration), and…

Fungi that exhibit the natural phenomenon of bioluminescence (the ability of an organism to emit visible light). Bioluminescence is the emission of photons of visible light by living organisms via the chemical reaction of molecular oxygen (O2) with…

A field that combines the disciplines of biology and engineering mechanics and utilizes the tools of physics, mathematics, and engineering to quantitatively describe the properties of biological materials. Biomechanics helps explain how the…

Generally, any technique that is used to make or modify the products of living organisms in order to improve plants or animals or to develop useful microorganisms. According to the general definition, biotechnology has actually been practiced for…

A major mental disorder in which there are lifelong episodes of both mania and depression; also known as manic-depressive illness. Bipolar disorder, also termed manic-depressive illness or manic depression, is characterized by sudden and often…

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…

Any of the tubular channels for blood transport, of which there are three principal types: arteries, capillaries, and veins. Blood circulates through the body via blood vessels, which include the arteries, veins, and capillaries. These structures…

A nutrient-poor, acid peatland. A bog is a type of peatland, which is a terrestrial wetland ecosystem. Bogs (Fig. 1) are nutrient-poor and acidic, and they contain vegetation in which peat mosses (Sphagnum species) and ericaceous dwarf shrubs play a…

The hard connective tissue that, together with cartilage, forms the skeleton of humans and other vertebrates. Bone (Fig. 1) is made of calcium phosphate crystals arranged on a protein scaffold and performs a variety of functions. For example, it has…

Malignant and benign lesions of the human mammary glands. Breast cancer and other breast disorders are potentially serious medical conditions. Benign breast disorders are often symptomatic and bothersome, but do not have malignant potential.…

A rare inflammatory lung disease affecting the small airways (bronchioles) and air sacs (alveoli) of the respiratory system. Bronchiolitis obliterans organizing pneumonia (BOOP) was first described in 1985 by Gary Epler as a disease distinct from…

Any terrestrial or aquatic animal that is capable of excavating holes in the ground for protection from adverse environmental conditions, as well as for storing food. Burrowing animals are uniquely adapted species, having the capacity to dig burrows…

A hormone, the only known secretory product of the parenchymal or C cells of the mammalian thyroid and of the ultimobranchial glands of lower animal forms. The hormone calcitonin (also referred to as thyrocalcitonin) has been isolated and…

A member of the mammalian family Camelidae in the order Artiodactyla. Camels are even-toed, hoofed mammals (ungulates). There are three extant camel species: the Bactrian camel (Camelus bactrianus; Fig. 1), the wild Bactrian camel (C. ferus), and…

The common name for a malignant neoplasm or tumor. Cancer is a serious medical condition that develops when the orderly relationship of cell division and cell differentiation becomes disordered, leading to the development of neoplasms. Neoplasms are…

Any of the group of organic compounds consisting of carbon, hydrogen, and oxygen that includes sugars, starches, and cellulose. Carbohydrate compounds consist of carbon, hydrogen, and oxygen, with the general chemical formula Cm(H2O)n [Fig. 1]; the…

A member of a class of common pigments, typically of yellow, orange, or red hues, that serve in both light absorption and the protection against too much light in photosynthesis as well as in the vision process. Carotenoids comprise a class of…

A firm, resilient connective tissue of vertebrates and some invertebrates. Cartilage (see illustration) is an important structural element found in the bodies of vertebrates and some invertebrates. It is a resistant, elastic connective tissue that…

The process whereby cells interact and attach to other cells or to inanimate surfaces. Cell adhesion is mediated by cell surface proteins and associated macromolecules. Cell adhesion is important because it forms the physical basis of…

The mechanism by which cells in a multicellular organism become specialized to perform specific functions in a variety of tissues and organs. Cell differentiation is the process through which cells are transformed into divergent types of cells…

The division of a cell into daughter cells that receive identical copies of its genetic material. Cell division is the process by which living cells multiply (Fig. 1). The cell division cycle, more commonly and simply known as the cell cycle,…

The process whereby naive embryonic cells establish their future identity or adult cell type. During the process of embryonic development, a single fertilized egg produces the many cell types found in the adult organism. This fascinating process,…

The movement of cells, the movement of materials within cells, and changes in cell shape (including cell division). Cell motility is an important facet of cell biology, encompassing the movement of individual cells, the coordinated movement of many…

The organizing and compartmentation of eukaryotic cells. Eukaryotic cells are divided into several compartments, and each compartment has a characteristic structure, biochemical composition, and function (Fig. 1). These compartments are called…

The limited capacity of all normal human and other animal cells to reproduce and function. The gradual decline in normal physiological function of cells is referred to as aging or senescence (Fig. 1). The aging process ends with the death of…

The field concerning the interactions among cells and molecules of the immune system, and how such interactions contribute to the recognition and elimination of pathogens. Humans (and vertebrates in general) possess a range of nonspecific mechanical…

Neural systems used for physiological reception of and response to chemical stimulation by excitation of specialized receptors. In vertebrates, the senses of smell (olfaction) and taste (gustation), along with the so-called common chemical sense,…

A white or colorless amorphous polysaccharide that forms a base for the hard outer integuments of crustaceans, insects, and other invertebrates. Chitin (see illustration) is an important nitrogenous polysaccharide that is found abundantly in nature.…

An illness of unknown cause and uncertain prevalence, often resulting in debilitating fatigue accompanied by diverse symptoms. Chronic fatigue syndrome (CFS) is a complicated and disabling illness. Characterized by extreme degrees of fatigue (see…

The collective processes by which metabolic materials are transported from one region of an organism to another region. The circulatory system, often termed the cardiovascular system, is responsible for the functions of circulation, allowing…

A branch of clinical pathology concerned with the role of the immune defense system in disease. The subject of clinical immunology encompasses diseases where a malfunction of the immune system itself is the basic cause, together with diseases where…