bacteriophages Viruses that attack and kill bacterial cells; composed only of DNA and protein. PICTURE
bark The outer layer of the stems of woody plants; composed of an outer layer of dead cells (cork) and an inner layer of phloem. PICTURE
Barr body Inactivated X-chromosome in mammalian females. Although inactivated, the Barr body is replicated prior to cell division and thus is passed on to all descendant cells of the embryonic cell that had one of its X-chromosomes inactivated. PICTURE 1 PICTURE 2
barriers to gene flow Factors, such as geographic, mechanical, and behavioral isolating mechanisms that restrict gene flow between populations, leading to populations with differing allele frequencies.
basal body A structure at the base of a cilium or flagellum; consists of nine triplet microtubules arranged in a circle with no central microtubule.
base A substance that lowers the hydrogen ion concentration in a solution.
basidia Specialized club-shaped structures on the underside of club fungi (Basidiomycetes) within which spores form (sing.: basidium).
basidiomycetes The club fungi, a major group of fungi that all produce a structure (basidium) on which basidiospores are produced. Includes mushrooms and toadstools. PICTURE
basidiospores The spores formed on the basidia of club fungi (Basidiomycetes).
B cells Type of lymphocyte responsible for antibody-mediated immunity; mature in the bone marrow and circulate in the circulatory and lymph systems where they transform into antibody-producing plasma cells when exposed to antigens.
benthic zone One of the two basic subdivisions of the marine biome; includes the sea floor and bottom-dwelling organisms.
bicarbonate ions A weak base present in saliva that helps to neutralize acids in food.
big bang theory A model for the evolution of the universe that holds that all matter and energy in the universe were concentrated in one point, which suddenly exploded. Subsequently, matter condensed to form atoms, elements, and eventually galaxies and stars.
bilateral symmetry In animals, refers to those that have a single axis of symmetry.
biliary system The bile-producing system consisting of the liver, gallbladder, and associated ducts.
binary fission The method by which bacteria reproduce. The circular DNA molecule is replicated; then the cell splits into two identical cells, each containing an exact copy of the original cell's DNA.
binding sites Areas on the ribosome within which tRNA-amino acid complexes fit during protein synthesis.
binomial system of nomenclature A system of taxonomy developed by Linnaeus in the early eighteenth century. Each species of plant and animal receives a two-term name; the first term is the genus, and the second is the species.
biochemical cycle The flow of an element through the living tissue and physical environment of an ecosystem; e. g., the carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus cycles.
biochemical reactions Specific chemical processes that occur in living things.
biochemistry Chemical processes associated with living things.
biodiversity Biological diversity; can be measured in terms of genetic, species, or ecosystem diversity.
biogeography The study of the distribution of plants and animals across the Earth.
bioluminescent Refers to organisms that emit light under certain conditions.
biomass The total weight of living tissue in a community.
biome A large-scale grouping that includes many communities of a similar nature.
biosphere All ecosystems on Earth as well as the Earth's crust, waters, and atmosphere on and in which organisms exist; also, the sum of all living matter on Earth.
birds Taxonomic class of terrestrial vertebrates that are characterized by endothermy and feathers; descended from some group of reptiles (or possibly dinosaurs).
birth rate The ratio between births and individuals in a specified population at a particular time.
bladder A hollow, distensible organ with muscular walls that stores urine and expels it through the urethra.
blastocoel The fluid-filled cavity at the center of a blastula.
blastocyst The developmental stage of the fertilized ovum by the time it is ready to implant; formed from the morula and consists of an inner cell mass, an internal cavity, and an outer layer of cells (the trophoblast).
blastula A ball of cells surrounding a fluid-filled cavity (the blastocoel) that is produced by the repeated cleavage of a zygote. PICTURE
blending Term applied to 19th century belief that parental traits "blended" in their offspring; disproven by Mendel's work.
blood group or type One of the classes into which blood can be separated on the basis of the presence or absence of certain antigens; notably, the ABO types and the Rh blood group.
B memory cells Long-lived B cells that are produced after an initial exposure to an antigen and play an important role in secondary immunity. They remain in the body and facilitate a more rapid responce if the antigen is encountered again.
body fossil The actual remains (however permineralized, compressed or otherwise post-mortem altered) of an organism; includes bones, shells, and teeth.
bolus A mass of chewed food mixed with salivary secretions that is propelled into the espohagus during the swallowing phase of digestion.
bony fish A term applied collectively to all groups of fish with bony (as opposed to cartilaginous) skeletons.
bottlenecks Drastic short-term reductions in population size caused by natural disasters, disease, or predators; can lead to random changes in the population's gene pool.
brachiopods A phylum of hinge-shelled animals that have left an excellent fossil record; brachiopods live on or in the ocean floor.
brachydactly Human genetic disorder that causes production of an extra digit; an autosomal dominant trait. Sometimes referred to as polydactly.
brain The most anterior, most highly developed portion of the central nervous system. PICTURE
brain stem The portion of the brain that is continuous with the spinal cord and consists of the medulla oblongata and pons of the hindbrain and the midbrain. PICTURE
bronchi Tubes that carry air from the trachea to the lungs (sing.: bronchus).
bronchioles Small tubes in the lungs that are formed by the branching of the bronchi; terminate in the alveoli.
bronchitis A respiratory disorder characterized by excess mucus production and swelling of the bronchioles; caused by long-term exposure to irritants such as cigarette smoke and air pollutants.
brown algae Multicellular protistans placed in the Division Phaeophyta, includes kelp.
brush border The collection of microvilli forming a border on the intestinal side of the epithelial cells of the small intestine.
bryophytes The nonvascular plants, characterized by life cycles dominated by the gametophyte phase. This group includes the mosses, liverworts, and hornworts, which lack lignified conducting tissues.
budding 1. Asexual production of new organisms; usually found in yeast; 2. the process by which HIV and similar viruses leave the cell (other than by lysing).
bud sports Buds that produce fruit that is different from the rest of the fruit on the tree; vegetatively propagated by grafting cuttings onto another plant.
buffers Chemicals that maintain pH values within narrow limits by absorbing or releasing hydrogen ions.
bulbourethral glands Glands that secrete a mucus-like substance that is added to sperm and provides lubrication during intercourse.
bursae Small sacs lined with synovial membrane and filled with synovial fluid; act as cushions to reduce friction between tendons and bones.
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