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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…

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 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…

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 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…

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,…

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 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…